Review of Medical Microbiology and Immunology, Fourteenth Edition PDF

mebooksfree.com  a LANGE medical book Review of Medical Microbiology and Immunology Fourteenth Edition Warren Levinson, MD, PhD Professor of Microbiology Department of Microbiology and Immunology University of Ca...

mebooksfree.com  a LANGE medical book Review of Medical Microbiology and Immunology Fourteenth Edition Warren Levinson, MD, PhD Professor of Microbiology Department of Microbiology and Immunology University of California, San Francisco San Francisco, California New York Chicago San Francisco Athens London Madrid Mexico City Milan New Delhi Singapore Sydney Toronto mebooksfree.com Review of Medical Microbiology and Immunology, Fourteenth Edition Copyright © 2016 by McGraw-Hill Education. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. Previous editions copyright © 2014, 2012, 2010, 2008, 2006, 2004, 2002, 2000 by The McGraw-Hill Companies, Inc., and copyright © 1998, 1996, 1994, 1992, 1989 by Appleton & Lange. 1 2 3 4 5 6 7 8 9 0 DOW/DOW 19 18 17 16 ISBN 978-0-07-184574-8 MHID 0-07-184574-7 ISSN 1042-8070 Notice Medicine is an ever-changing science. As new research and clinical experience broaden our knowledge, changes in treatment and drug therapy are required. The authors and the publisher of this work have checked with sources believed to be reliable in their efforts to provide information that is complete and generally in accord with the standards accepted at the time of publication. 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To contact a representative, please visit the Contact Us pages at www.mhprofessional.com. mebooksfree.com Contents Preface v Acknowledgments vii How to Use This Book ix PA R T I BASIC BACTERIOLOGY 1 Bacteria Compared with 1.  Mycoplasmas 197 23.  Other Microorganisms 1 Spirochetes 199 24.  Structure of Bacterial Cells 4 2.  Chlamydiae 208 25.  Growth 15 3.  Rickettsiae 212 26.  Genetics 18 4.  Minor Bacterial Pathogens 217 27.  Classification of Medically Important 5.  Bacteria 24 Normal Flora 26 6.  PA R T III Pathogenesis 31 7.  BASIC VIROLOGY 225 Host Defenses 52 8.  Laboratory Diagnosis 61 9.  Structure 226 28.  Antimicrobial Drugs: 10.  Replication 232 29.  Mechanism of Action 69 Genetics & Gene Therapy 244 30.  Antimicrobial Drugs: Resistance 87 11.  Classification of Medically Important 31.  Bacterial Vaccines 96 12.  Viruses 248 Sterilization & Disinfection 100 13.  Pathogenesis 253 32.  Host Defenses 260 33.  PA R T II Laboratory Diagnosis 266 34.  Antiviral Drugs 269 35.  CLINICAL BACTERIOLOGY 105 Viral Vaccines 280 36.  Overview of the Major Pathogens & Introduction 14.  to Anaerobic Bacteria 105 PA R T IV Gram-Positive Cocci 109 15.  CLINICAL VIROLOGY 285 Gram-Negative Cocci 128 16.  Gram-Positive Rods 135 17.  DNA Enveloped Viruses 288 37.  Gram-Negative Rods Related to the 18.  DNA Nonenveloped Viruses 304 38.  Enteric Tract 148 RNA Enveloped Viruses 311 39.  Gram-Negative Rods Related to the 19.  RNA Nonenveloped Viruses 333 40.  Respiratory Tract 171 Hepatitis Viruses 342 41.  Gram-Negative Rods 20.  Arboviruses 354 42.  Related to Animal Sources Tumor Viruses 361 43.  (Zoonotic Organisms) 177 Slow Viruses & Prions 372 44.  Mycobacteria 183 21.  Human Immunodeficiency Virus 378 45.  Actinomycetes 194 22.  Minor Viral Pathogens 391 46.  iii mebooksfree.com iv    Contents PA R T V Central Nervous System 72.  Infections 605 MYCOLOGY 397 Gastrointestinal Tract Infections 614 73.  Contributed by Peter Chin-Hong, MD Basic Mycology 397 47.  Pelvic Infections 622 74.  Cutaneous & Subcutaneous Mycoses 403 48.  Contributed by Peter Chin-Hong, MD Systemic Mycoses 407 49.  Upper Respiratory Tract Infections 629 75.  Opportunistic Mycoses 414 50.  Contributed by Peter Chin-Hong, MD Lower Respiratory Tract Infections 635 76.  PA R T VI Contributed by Peter Chin-Hong, MD Skin and Soft Tissue Infections 641 77.  PARASITOLOGY 423 Contributed by Brian S. Schwartz, MD Urinary Tract Infections 649 78.  Intestinal & Urogenital—Protozoa 424 51.  Contributed by Brian S. Schwartz, MD Blood & Tissue Protozoa 433 52.  Minor Protozoan Pathogens 450 53.  Cestodes 453 54.  PA R T X Trematodes 462 55.   RIEF SUMMARIES OF B Nematodes 469 56.  MEDICALLY IMPORTANT ORGANISMS 653 PA R T VII IMMUNOLOGY 489 PA R T XI Immunity 489 57.  CLINICAL CASES 691 Cellular Basis of the Immune Response 500 58.  Antibodies 522 59.  Humoral Immunity 531 60.  PA R T XII Cell-Mediated Immunity 534 61.  PEARLS FOR THE USMLE 701 Major Histocompatibility Complex & 62.  Transplantation 536 Complement 542 63.  PA R T XIII Antigen–Antibody Reactions in the 64.  Laboratory 546  SMLE (NATIONAL BOARD) U Hypersensitivity (Allergy) 557 65.  PRACTICE QUESTIONS 709 Tolerance & Autoimmune Disease 567 66.  Tumor Immunity 576 67.  Immunodeficiency 578 68.  PA R T XIV  SMLE (NATIONAL BOARD) U PA R T VIII PRACTICE EXAMINATION 751 ECTOPARASITES 585 INDEX 761 Ectoparasites That Cause Human Disease 585 69.  PA R T IX INFECTIOUS DISEASES 593 Bone and Joint Infections 593 70.  Cardiac Infections 598 71.  Contributed by Brian S. Schwartz, MD mebooksfree.com Preface This book is a concise review of the medically important aspects The following features are included to promote a successful of microbiology and immunology. It covers both the basic and learning experience for students using this book: clinical aspects of bacteriology, virology, mycology, parasitol- 1. The information is presented succinctly, with stress on ogy, and immunology. It also discusses important infectious making it clear, interesting, and up to date. diseases using an organ system approach. 2. There is strong emphasis in the text on the clinical appli- Its two major aims are (1) to assist those who are preparing cation of microbiology and immunology to infectious- for the USMLE (National Boards) and (2) to provide students diseases. who are currently taking medical microbiology courses with a 3. In the clinical bacteriology and virology sections, the brief and up-to-date source of information. The goal is to pro- organisms are separated into major and minor pathogens. vide the reader with an accurate source of clinically relevant This allows the student to focus on the most important information at a level appropriate for those beginning their clinically relevant microorganisms. medical education. 4. Key information is summarized in useful review tables. This new edition presents current, medically important Important concepts are illustrated by figures using color. information in the rapidly changing fields of microbiology and 5. Important facts called “Pearls” are listed at the end of each immunology. It contains many color micrographs of stained basic science chapter. microorganisms as well as images of important laboratory tests. 6. Self-assessment questions with answers are included at the It also includes many images of clinical lesions and highlights end of the chapters. current information on antimicrobial drugs and vaccines. 7. The 654 USMLE (National Board) practice questions cover These aims are achieved by using several different formats, the important aspects of each of the subdisciplines on the which should make the book useful to students with varying USMLE: Bacteriology, Virology, Mycology, Parasitology, study objectives and learning styles: and Immunology. A separate section containing extended 1. A narrative text for complete information. matching questions is included. In view of the emphasis 2. A separate section containing summaries of important placed on clinical relevance in the USMLE, another section microorganisms for rapid review of the high-yield essentials. provides questions set in a clinical case context. 3. Sample questions in the USMLE (National Board) style, with 8. Brief summaries of medically important microorganisms answers provided after each group of questions. are presented together in a separate section to facilitate 4. A USMLE (National Board) practice examination consisting rapid access to the information and to encourage compari- of 80 microbiology and immunology questions. The ques- son of one organism with another. tions are written in a clinical case format and simulate the 9. Fifty clinical cases are presented as unknowns for the reader computer-based examination. Answers are provided at the to analyze in a brief, problem-solving format. These cases end of each block of 40 questions. illustrate the importance of basic science information in 5. Self-assessment questions at the end of the chapters so you clinical diagnosis. can evaluate whether the important information has been 10. Color images depicting clinically important findings, such mastered. Answers are provided. as infectious disease lesions, Gram stains of bacteria, elec- 6. Clinical case vignettes to provide both clinical information tron micrographs of viruses, and microscopic images of and practice for the USMLE. fungi, protozoa, and worms, are included in the text. 7. A section titled “Pearls for the USMLE” describing impor- 11. There are nine chapters on infectious diseases from an tant epidemiologic information helpful in answering ques- organ system perspective. They are written concisely and tions on the USMLE. are appropriate for a medical student’s introduction to 8. Many images of clinically important lesions seen in patients this subject. These chapters include Bone and Joint Infec- with infectious diseases described in this book are available tions, Cardiac Infections, Central Nervous System Infec- on the McGraw-Hill Online Learning Center’s Web site tions, Gastrointestinal Tract Infections, Pelvic Infections, (www.langetextbooks.com). Upper Respiratory Tract Infections, Lower Respiratory v mebooksfree.com vi    Preface Tract Infections, Skin and Soft Tissue Infections, and readable, interesting, and varied format. I hope you find that Urinary Tract Infections. this book meets those criteria. After teaching both medical microbiology and clinical Warren Levinson, MD, PhD infectious disease for many years, I believe that students San Francisco, California appreciate a book that presents the essential information in a January 2016 mebooksfree.com Acknowledgments The author welcomes Brian S. Schwartz, MD, as a contributor to to the editor of the seventh and ninth editions, Jennifer the thirteenth and fourteenth editions of this book. Brian is an Bernstein; to the editor of the eighth edition, Linda Conheady; Associate Professor of Clinical Medicine in the School of to the editor of the tenth and eleventh editions, Sunita Dogra; Medicine, University of California, San Francisco, specializing to the editor of the twelfth edition, Rebecca Kerins; to the editor in infectious diseases. He contributed three excellent chapters in of the thirteenth edition, Caroline Define; and to the editor of the new infectious diseases part of this book. the fourteenth edition, Nupur Mehra; all of whom ensured The author also welcomes Peter Chin-Hong, MD, as a con- that the highest standards of grammar, spelling, and style tributor to the thirteenth and fourteenth editions of this book. were met. Peter is a Professor of Clinical Medicine in the School of The invaluable assistance of my wife, Barbara, in making this Medicine, University of California, San Francisco, specializing book a reality is also gratefully acknowledged. in infectious diseases. He contributed four outstanding chapters I dedicate this book to my father and mother, who instilled a in the new infectious diseases part of this book. love of scholarship, the joy of teaching, and the value of being I am indebted to the editor of the first five editions, Yvonne organized. Strong; to the editor of the sixth edition, Cara Lyn Coffey; vii mebooksfree.com mebooksfree.com How to Use This Book 1. CHAPTER CONTENTS: The main headings in each chap- 4. SELF-ASSESSMENT QUESTIONS: USMLE-style ques- ter are listed so the reader can determine, at a glance, the tions with answers are included at the end of the chapters. topics discussed in the chapter. 5. PEARLS FOR THE USMLE: Eleven tables containing 2. TEXT: A concise, complete description of medically impor- important clinical and epidemiologic information that tant information for the professional student. Includes basic will be useful for answering questions on the USMLE and clinical bacteriology (pages 1-225), basic and clinical (pages 701-708). virology (pages 226-396), mycology (fungi) (pages 397-422), 6. USMLE-TYPE QUESTIONS: 654 practice questions that parasitology (pages 423-448), immunology (pages 489-584), can be used to review for the USMLE and class examina- and ectoparasites (pages 585-592). tions (pages 709-750). The text also includes nine chapters on infectious dis- 7. USMLE PRACTICE EXAM: Two 40-question practice eases. These chapters include Bone and Joint Infections examinations in USMLE format (pages 751-760). (pages 593-597), Cardiac Infections (pages 598-604), 8. PEARLS: Summary points at the end of each basic science Central Nervous System Infections (pages 605-613), chapter. Gastrointestinal Tract Infections (pages 614-621), Pelvic 9. CLINICAL CASES: 50 cases describing important infec- Infections (pages 622-628), Upper Respiratory Tract Infections tious diseases with emphasis on diagnostic information (pages 629-634), Lower Respiratory Tract Infections (pages (pages 691-700). 635-640), Skin and Soft Tissue Infections (pages 641-648), and 10. CLINICAL IMAGES: More than 100 images of clinically Urinary Tract Infections (pages 649-652). important lesions illustrate the text. Additional clinical lesions 3. SUMMARIES OF ORGANISMS: A quick review for can be seen on the McGraw-Hill Online Learning Center’s examinations describing the important characteristics of Web site (www.langetextbooks.com/levinson/gallery/). the organisms (pages 653-690). ix mebooksfree.com mebooksfree.com PART I BASIC BACTERIOLOGY 1 C H A P T E R Bacteria Compared with Other Microorganisms CHAPTER CONTENTS Microbes That Cause Infectious Diseases Pearls Important Features of Microbes Self-Assessment Questions Eukaryotes & Prokaryotes Practice Questions: USMLE & Course Examinations Terminology MICROBES THAT CAUSE fungi, protozoa, and helminths are cellular, whereas viruses are not. This distinction is based primarily on three INFECTIOUS DISEASES criteria: The agents of human infectious diseases belong to five (1) Structure. Cells have a nucleus or nucleoid (see major groups of organisms: bacteria, fungi, protozoa, hel- below), which contains DNA; this is surrounded by cyto- minths, and viruses. Bacteria belong to the prokaryote plasm, within which proteins are synthesized and energy is kingdom, fungi (yeasts and molds) belong to the kingdom generated. Viruses have an inner core of genetic material of fungi, and protozoa are members of the kingdom of pro- (either DNA or RNA) but no cytoplasm, and so they tists. Helminths (worms) are classified in the animal king- depend on host cells to provide the machinery for protein dom (Table 1–1). Protists and fungi are distinguished from synthesis and energy generation. animals and plants by being either unicellular or relatively simple multicellular organisms. In contrast, helminths are complex multicellular organisms. Taken together, the hel- minths and the protozoa are commonly called parasites. TABLE 1–1 Biologic Relationships of Pathogenic Viruses are quite distinct from other organisms—they are Microorganisms not cells but can replicate only within cells. Kingdom Pathogenic Microorganisms Type of Cells Animal Helminths (worms) Eukaryotic IMPORTANT FEATURES OF Protists Protozoa Eukaryotic MICROBES Fungi Fungi (yeasts and molds) Eukaryotic Many of the essential characteristics of these organisms are Prokaryote Bacteria Prokaryotic Viruses Noncellular described in Table 1–2. One salient feature is that bacteria, 1 mebooksfree.com 2 PART I Basic Bacteriology TABLE 1–2 Comparison of Medically Important Organisms Characteristic Viruses Bacteria Fungi Protozoa and Helminths Cells No Yes Yes Yes Approximate diameter (μm)1 0.02–0.2 1–5 3–10 (yeasts) 15–25 (trophozoites) Nucleic acid Either DNA or RNA Both DNA and RNA Both DNA and RNA Both DNA and RNA Type of nucleus None Prokaryotic Eukaryotic Eukaryotic Ribosomes Absent 70S 80S 80S Mitochondria Absent Absent Present Present Nature of outer surface Protein capsid and Rigid wall containing Rigid wall containing chitin Flexible membrane lipoprotein envelope peptidoglycan Motility None Some None Most 2 Method of replication Not binary fission Binary fission Budding or mitosis Mitosis3 1 For comparison, a human red blood cell has a diameter of 7 μm. 2 Yeasts divide by budding, whereas molds divide by mitosis. 3 Helminth cells divide by mitosis, but the organism reproduces itself by complex, sexual life cycles. (2) Method of replication. Cells replicate either by on the basis of their structure and the complexity of their binary fission or by mitosis, during which one parent cell organization. Fungi, protozoa, and helminths are eukary- divides to make two progeny cells while retaining its cellu- otic, whereas bacteria are prokaryotic. lar structure. Prokaryotic cells (e.g., bacteria) replicate by binary fission, whereas eukaryotic cells replicate by mitosis. (1) The eukaryotic cell has a true nucleus with multiple In contrast, viruses disassemble, produce many copies of chromosomes surrounded by a nuclear membrane and their nucleic acid and protein, and then reassemble into uses a mitotic apparatus to ensure equal allocation of the multiple progeny viruses. Furthermore, viruses must repli- chromosomes to progeny cells. cate within host cells because, as mentioned previously, (2) The nucleoid of a prokaryotic cell consists of a sin- they lack protein-synthesizing and energy-generating sys- gle circular molecule of loosely organized DNA, lacking a tems. With the exception of rickettsiae and chlamydiae, nuclear membrane and mitotic apparatus (Table 1–3). which also require living host cells for growth, bacteria can In addition to the different types of nuclei, the two replicate extracellularly. classes of cells are distinguished by several other (3) Nature of the nucleic acid. Cells contain both DNA characteristics: and RNA, whereas viruses contain either DNA or RNA, but not both. (1) Eukaryotic cells contain organelles, such as mito- chondria and lysosomes, and larger (80S) ribosomes, whereas prokaryotes contain no organelles and smaller (70S) ribosomes. EUKARYOTES & PROKARYOTES (2) Most prokaryotes have a rigid external cell wall that Cells have evolved into two fundamentally different types, contains peptidoglycan, a polymer of amino acids and eukaryotic and prokaryotic, which can be distinguished sugars, as its unique structural component. Eukaryotes, on TABLE 1–3 Characteristics of Prokaryotic and Eukaryotic Cells Characteristic Prokaryotic Bacterial Cells Eukaryotic Human Cells DNA within a nuclear membrane No Yes Mitotic division No Yes DNA associated with histones No Yes Chromosome number One More than one Membrane-bound organelles, such as mitochondria and lysosomes No Yes Size of ribosome 70S 80S Cell wall containing peptidoglycan Yes No mebooksfree.com  CHAPTER 1 Bacteria Compared with Other Microorganisms 3 the other hand, do not contain peptidoglycan. Either they SELF-ASSESSMENT QUESTIONS are bound by a flexible cell membrane, or, in the case of fungi, they have a rigid cell wall with chitin, a homopoly- 1. You’re watching a television program that is discussing viruses called bacteriophages that can kill bacteria. Your roommate says, mer of N-acetylglucosamine, typically forming the “Wow, maybe viruses can be used to kill the bacteria that infect framework. people! You’re taking the Microbiology course now; what’s the (3) The eukaryotic cell membrane contains sterols, difference between viruses and bacteria?” Which one of the fol- whereas no prokaryote, except the wall-less Mycoplasma, lowing would be the most accurate statement to make? has sterols in its membranes. (A) Viruses do not have mitochondria, whereas bacteria do. Motility is another characteristic by which these organ- (B) Viruses do not have a nucleolus, whereas bacteria do. isms can be distinguished. Most protozoa and some bacte- (C) Viruses do not have ribosomes, whereas bacteria do. (D) Viruses replicate by binary fission, whereas bacteria replicate ria are motile, whereas fungi and viruses are nonmotile. by mitosis. The protozoa are a heterogeneous group that possess three (E) Viruses are prokaryotic, whereas bacteria are eukaryotic. different organs of locomotion: flagella, cilia, and pseudo- 2. Bacteria, fungi (yeasts and molds), viruses, and protozoa are pods. The motile bacteria move only by means of flagella. important causes of human disease. Which one of the following microbes contains either DNA or RNA but not both? (A) Bacteria TERMINOLOGY (B) Molds Bacteria, fungi, protozoa, and helminths are named accord- (C) Protozoa ing to the binomial Linnean system that uses genus and (D) Viruses species, but viruses are not so named. For example, regard- (E) Yeasts 3. Which one of the following contains DNA that is not surrounded ing the name of the well-known bacteria Escherichia coli, by a nuclear membrane? Escherichia is the genus and coli is the species name. Simi- (A) Bacteria larly, the name of the yeast Candida albicans consists of (B) Molds Candida as the genus and albicans as the species. But (C) Protozoa viruses typically have a single name, such as poliovirus, (D) Yeasts measles virus, or rabies virus. Some viruses have names with two words, such as herpes simplex virus, but those do not represent genus and species. ANSWERS (1) (C) PEARLS (2) (D) (3) (A) The agents of human infectious diseases are bacteria, fungi (yeasts and molds), protozoa, helminths (worms), and viruses. PRACTICE QUESTIONS: USMLE & Bacterial cells have a prokaryotic nucleus, whereas human, fungal, protozoan, and helminth cells have a eukaryotic COURSE EXAMINATIONS nucleus. Viruses are not cells and do not have a nucleus. Questions on the topics discussed in this chapter can be found All cells contain both DNA and RNA, whereas viruses con- in the Basic Bacteriology section of Part XIII: USMLE (National tain either DNA or RNA, but not both. Board) Practice Questions starting on page 709. Also see Part Bacterial and fungal cells are surrounded by a rigid cell wall, XIV: USMLE (National Board) Practice Examination starting whereas human, protozoan, and helminth cells have a flex- on page 751. ible cell membrane. The bacterial cell wall contains peptidoglycan, whereas the fungal cell wall contains chitin. mebooksfree.com  2 C H A P T E R Structure of Bacterial Cells CHAPTER CONTENTS Shape & Size of Bacteria Structures Outside the Cell Wall Structure of Bacteria Bacterial Spores Cell Wall Pearls Cytoplasmic Membrane Self-Assessment Questions Cytoplasm Practice Questions: USMLE & Course Examinations SHAPE & SIZE OF BACTERIA Cell Wall Bacteria are classified by shape into three basic groups: The cell wall is the outermost component common to all cocci, bacilli, and spirochetes (Figure 2–1). The cocci are bacteria (except Mycoplasma species, which are bounded round, the bacilli are rods, and the spirochetes are spiral- by a cell membrane, not a cell wall). Some bacteria have shaped. Some bacteria are variable in shape and are said to surface features external to the cell wall, such as a capsule, be pleomorphic (many-shaped). The shape of a bacterium is determined by its rigid cell wall. The microscopic appearance of a bacterium is one of the most important criteria used in its identification. In addition to their characteristic shapes, the arrange- ment of bacteria is important. For example, certain cocci A-1 A-2 A-3 A-4 occur in pairs (diplococci), some in chains (streptococci), and others in grapelike clusters (staphylococci). These arrangements are determined by the orientation and degree of attachment of the bacteria at the time of cell division. The arrangement of rods and spirochetes is medically less important and is not described in this introductory B-1 B-2 B-3 B-4 B-5 chapter. Bacteria range in size from about 0.2 to 5 μm (Figure 2–2). The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside a host. The longest C-1 C-2 bacteria rods are the size of some yeasts and human red FIGURE 2–1 Bacterial morphology. A: Cocci in clusters (e.g., blood cells (7 μm). Staphylococcus; A-1); chains (e.g., Streptococcus; A-2); in pairs with pointed ends (e.g., Streptococcus pneumoniae; A-3); in pairs with kid- ney bean shape (e.g., Neisseria; A-4). B: Rods (bacilli): with square ends (e.g., Bacillus; B-1); with rounded ends (e.g., Salmonella; B-2); STRUCTURE OF BACTERIA club-shaped (e.g., Corynebacterium; B-3); fusiform (e.g., Fusobacte- rium; B-4); comma-shaped (e.g., Vibrio; B-5). C: Spirochetes: relaxed The structure of a typical bacterium is illustrated in Figure 2–3, coil (e.g., Borrelia; C-1); tightly coiled (e.g., Treponema; C-2). and the important features of each component are pre- (Reproduced with permission from Joklik WK et al. Zinsser Microbiology. 20th ed. sented in Table 2–1. Originally published by Appleton & Lange. Copyright 1992, McGraw-Hill.) 4 mebooksfree.com  CHAPTER 2 Structure of Bacterial Cells    5 Range of Red Range of Lower limit electron blood optical of microscope cell microscope human vision Escherichia coli Mycoplasma Bacillus Hepatitis Haemophilus anthracis B virus influenzae Candida Poliovirus HIV Poxvirus albicans Protozoa 0.005 0.01 0.03 0.05 0.1 0.3 0.5 1 3 5 10 30 50 100 300 Scale (µm) FIGURE 2–2 Sizes of representative bacteria, viruses, yeasts, protozoa, and human red cells. The bacteria range in size from Mycoplasma, the smallest, to Bacillus anthracis, one of the largest. The viruses range from poliovirus, one of the smallest, to poxviruses, the largest. Yeasts, such as Candida albicans, are generally larger than bacteria. Protozoa have many different forms and a broad size range. HIV, human immunode- ficiency virus. (Reproduced with permission from Joklik WK et al. Zinsser Microbiology. 20th ed. Originally published by Appleton & Lange. Copyright 1992, McGraw-Hill.) flagella, and pili, which are less common components and (1) The peptidoglycan layer is much thicker in gram- are discussed next. positive than in gram-negative bacteria. Many gram-posi- The cell wall is located external to the cytoplasmic tive bacteria also have fibers of teichoic acid that protrude membrane and is composed of peptidoglycan (see page 6). outside the peptidoglycan, whereas gram-negative bacteria The peptidoglycan provides structural support and main- do not have teichoic acids. tains the characteristic shape of the cell. (2) In contrast, the gram-negative bacteria have a com- plex outer layer consisting of lipopolysaccharide, lipopro- Cell Walls of Gram-Positive and tein, and phospholipid. Lying between the outer-membrane Gram-Negative Bacteria layer and the cytoplasmic membrane in gram-negative The structure, chemical composition, and thickness of the bacteria is the periplasmic space, which is the site, in some cell wall differ in gram-positive and gram-negative bacteria species, of enzymes called β-lactamases that degrade peni- (Table 2–2, Figure 2–4A, and “Gram Stain” box). cillins and other β-lactam drugs. Cytoplasm Ribosomes Nucleoid DNA Flagella Cell membrane Attachment pili Plasmid Cell wall Capsule Sex pilus FIGURE 2–3 Bacterial structure. (Reproduced with permission from Ryan K et al. Sherris Medical Microbiology. 4th ed. Copyright 2004, McGraw-Hill.) mebooksfree.com 6 PART I Basic Bacteriology TABLE 2–1 Bacterial Structures Structure Chemical Composition Function Essential components Cell wall Peptidoglycan Glycan (sugar) backbone with peptide Gives rigid support, protects against osmotic pressure, is the site of side chains that are cross-linked action of penicillins and cephalosporins, and is degraded by lysozyme Outer membrane of gram- Lipid A Toxic component of endotoxin negative bacteria Polysaccharide Major surface antigen used frequently in laboratory diagnosis Surface fibers of gram-positive Teichoic acid Major surface antigen but rarely used in laboratory diagnosis bacteria Plasma membrane Lipoprotein bilayer without sterols Site of oxidative and transport enzymes Ribosome RNA and protein in 50S and 30S Protein synthesis; site of action of aminoglycosides, erythromycin, subunits tetracyclines, and chloramphenicol Nucleoid DNA Genetic material Mesosome Invagination of plasma membrane Participates in cell division and secretion Periplasm Space between plasma membrane Contains many hydrolytic enzymes, including β-lactamases and outer membrane Nonessential components Capsule Polysaccharide1 Protects against phagocytosis Pilus or fimbria Glycoprotein Two types: (1) mediates attachment to cell surfaces; (2) sex pilus mediates attachment of two bacteria during conjugation Flagellum Protein Motility Spore Keratinlike coat, dipicolinic acid Provides resistance to dehydration, heat, and chemicals Plasmid DNA Contains a variety of genes for antibiotic resistance and toxins Granule Glycogen, lipids, polyphosphates Site of nutrients in cytoplasm Glycocalyx Polysaccharide Mediates adherence to surfaces 1 Except in Bacillus anthracis, in which it is a polypeptide of d-glutamic acid. The cell wall has several other important properties: Gram-stained (Figure 2–4B). These bacteria are said to be acid-fast because they resist decolorization with acid–alco- (1) In gram-negative bacteria, it contains endotoxin, a hol after being stained with carbolfuchsin. This property is lipopolysaccharide (see pages 9 and 44). related to the high concentration of lipids, called mycolic (2) Its polysaccharides and proteins are antigens that are acids, in the cell wall of mycobacteria. useful in laboratory identification. Note that Nocardia asteroides is weakly acid-fast. The (3) Its porin proteins play a role in facilitating the pas- meaning of the term “weakly” is that if the acid-fast stain- sage of small, hydrophilic molecules into the cell. Porin ing process uses a weaker solution of hydrochloric acid to proteins in the outer membrane of gram-negative bacteria decolorize than that used in the stain for Mycobacteria, act as a channel to allow the entry of essential substances then N. asteroides will not decolorize. However, if the regular- such as sugars, amino acids, vitamins, and metals as well as strength hydrochloric acid is used, N. asteroides will many antimicrobial drugs such as penicillins. decolorize. In view of their importance, three components of the Cell Walls of Acid-Fast Bacteria cell wall (i.e., peptidoglycan, lipopolysaccharide, and tei- Mycobacteria (e.g., Mycobacterium tuberculosis) have an choic acid) are discussed in detail here. unusual cell wall, resulting in their inability to be Peptidoglycan Peptidoglycan is a complex, interwoven network that sur- TABLE 2–2 Comparison of Cell Walls of Gram-Positive rounds the entire cell and is composed of a single cova- and Gram-Negative Bacteria lently linked macromolecule. It is found only in bacterial Component Gram-Positive Cells Gram-Negative Cells cell walls. It provides rigid support for the cell, is important in maintaining the characteristic shape of the cell, and Peptidoglycan Thicker; multilayer Thinner; single layer allows the cell to withstand media of low osmotic pressure, Teichoic acids Yes No such as water. A representative segment of the peptidogly- Lipopolysaccharide No Yes can layer is shown in Figure 2–5. The term peptidoglycan (endotoxin) is derived from the peptides and the sugars (glycan) that mebooksfree.com  CHAPTER 2 Structure of Bacterial Cells    7 Flagellum Teichoic acid Pilus Capsule Outer membrane ~8 nm Peptidoglycan 15–80 nm ~2 nm Periplasmic space Cytoplasmic ~8 nm membrane ~8 nm Gram-positive Gram-negative A Mycolic acid Arabinoglycan Peptidoglycan Cytoplasmic membrane B FIGURE 2–4 A: Cell walls of gram-positive and gram-negative bacteria. Note that the peptidoglycan in gram-positive bacteria is much thicker than in gram-negative bacteria. Note also that only gram-negative bacteria have an outer membrane containing endotoxin (lipopoly- saccharide [LPS]) and have a periplasmic space where β-lactamases are found. Several important gram-positive bacteria, such as staphylococci and streptococci, have teichoic acids. (Reproduced with permission from Ingraham JL, Maaløe O, Neidhardt FC. Growth of the Bacterial Cell. Sinauer Associates; 1983.) B: Cell wall of Mycobacterium tuberculosis: Note the layers of mycolic acid and arabinoglycan that are present in members of the genus Mycobacterium but not in most other genera of bacteria. NAG NAM NAG NAM Glycan chain Sugar Peptide backbone NAM NAG interbridge Peptidoglycan Tetra- (Gram-positive peptide cells) chain (amino acids) Tetrapeptide chain Tetrapeptide (amino acids) chains Glycan NAM NAG NAM NAG chain Tetrapeptide chain Peptide interbridge (amino acids) A B FIGURE 2–5 Peptidoglycan structure. A: Peptidoglycan is composed of a glycan chain (NAM and NAG), a tetrapeptide chain, and a cross- link (peptide interbridge). B: In the cell wall, the peptidoglycan forms a multilayered, three-dimensional structure. NAG, N-acetylglucosamine; NAM, N-acetylmuramic acid. (Reproduced with permission from Nester EW et al. Microbiology: A Human Perspective. 6th ed. Copyright 2009, McGraw-Hill.) mebooksfree.com 8 PART I Basic Bacteriology GRAM STAIN This staining procedure, developed in 1884 by the Danish The Gram stain is useful in two ways: physician Christian Gram, is the most important procedure (1) In the identification of many bacteria. in microbiology. It separates most bacteria into two groups: (2) In influencing the choice of antibiotic because, in gen- the gram-positive bacteria, which stain blue, and the gram- eral, gram-positive bacteria are more susceptible to penicillin negative bacteria, which stain red. The Gram stain involves G than are gram-negative bacteria. the following four-step procedure: However, not all bacteria can be seen in the Gram stain. (1) The crystal violet dye stains all cells blue/purple. Table 2–3 lists the medically important bacteria that cannot (2) The iodine solution (a mordant) is added to form a be seen and describes the reason why. The alternative micro- crystal violet–iodine complex; all cells continue to appear blue. scopic approach to the Gram stain is also described. (3) The organic solvent, such as acetone or ethanol, Note that it takes approximately 100,000 bacteria/mL to see extracts the blue dye complex from the lipid-rich, thin- 1 bacterium per microscopic field using the oil immersion walled gram-negative bacteria to a greater degree than from (100×) lens. So the sensitivity of the Gram stain procedure is the lipid-poor, thick-walled gram-positive bacteria. The low. This explains why a patient’s blood is rarely stained gram-negative organisms appear colorless; the gram-positive immediately but rather is incubated in blood cultures over- bacteria remain blue. night to allow the bacteria to multiply. One important excep- (4) The red dye safranin stains the decolorized gram- tion to this is meningococcemia in which very high negative cells red/pink; the gram-positive bacteria remain concentrations of Neisseria meningitidis can occur in the blue. blood. make up the molecule. Synonyms for peptidoglycan are Staphylococcus aureus, for example, five glycines link the murein and mucopeptide. terminal D-alanine to the penultimate L-lysine. Figure 2–5 illustrates the carbohydrate backbone, which Because peptidoglycan is present in bacteria but not in is composed of alternating N-acetylmuramic acid and human cells, it is a good target for antibacterial drugs. Sev- N-acetylglucosamine molecules. Attached to each of the eral of these drugs, such as penicillins, cephalosporins, and muramic acid molecules is a tetrapeptide consisting of both vancomycin, inhibit the synthesis of peptidoglycan by D- and L-amino acids, the precise composition of which inhibiting the transpeptidase that makes the cross-links differs from one bacterium to another. Two of these amino between the two adjacent tetrapeptides (see Chapter 10). acids are worthy of special mention: diaminopimelic acid, Lysozyme, an enzyme present in human tears, mucus, which is unique to bacterial cell walls, and d-alanine, which and saliva, can cleave the peptidoglycan backbone by is involved in the cross-links between the tetrapeptides and breaking its glycosyl bonds, thereby contributing to the in the action of penicillin. Note that this tetrapeptide con- natural resistance of the host to microbial infection. Lyso- tains the rare d-isomers of amino acids; most proteins zyme-treated bacteria may swell and rupture as a result of contain the l-isomer. The other important component in the entry of water into the cells, which have a high internal this network is the peptide cross-link between the two tet- osmotic pressure. However, if the lysozyme-treated cells are rapeptides. The cross-links vary among species; in in a solution with the same osmotic pressure as that of the TABLE 2–3 Medically Important Bacteria That Cannot Be Seen in the Gram Stain Name Reason Alternative Microscopic Approach Mycobacteria, including M. tuberculosis Too much lipid in cell wall so dye cannot Acid-fast stain penetrate Treponema pallidum Too thin to see Dark-field microscopy or fluorescent antibody Mycoplasma pneumoniae No cell wall; very small None Legionella pneumophila Poor uptake of red counterstain Prolong time of counterstain Chlamydiae, including C. trachomatis Intracellular; very small Inclusion bodies in cytoplasm Rickettsiae Intracellular; very small Giemsa or other tissue stains mebooksfree.com  CHAPTER 2 Structure of Bacterial Cells    9 bacterial interior, they will survive as spherical forms, are called lipoteichoic acid; others anchor to the muramic called protoplasts, surrounded only by a cytoplasmic acid of the peptidoglycan. membrane. The medical importance of teichoic acids lies in their ability to induce inflammation and septic shock when Lipopolysaccharide caused by certain gram-positive bacteria; that is, they The lipopolysaccharide (LPS) of the outer membrane of the activate the same pathways as does endotoxin (LPS) in cell wall of gram-negative bacteria is endotoxin. It is respon- gram-negative bacteria. Teichoic acids also mediate the sible for many of the features of disease, such as fever and attachment of staphylococci to mucosal cells. Gram-nega- shock (especially hypotension), caused by these organisms tive bacteria do not have teichoic acids. (see page 44). It is called endotoxin because it is an integral part of the cell wall, in contrast to exotoxins, which are Cytoplasmic Membrane actively secreted from the bacteria. The constellation of Just inside the peptidoglycan layer of the cell wall lies the symptoms caused by the endotoxin of one gram-negative cytoplasmic membrane, which is composed of a phospho- bacterium is similar to another, but the severity of the symp- lipid bilayer similar in microscopic appearance to that in toms can differ greatly. In contrast, the symptoms caused by eukaryotic cells. They are chemically similar, but eukary- exotoxins of different bacteria are usually quite different. otic membranes contain sterols, whereas prokaryotes gen- The LPS is composed of three distinct units (Figure 2–6): erally do not. The only prokaryotes that have sterols in their (1) A phospholipid called lipid A, which is responsible membranes are members of the genus Mycoplasma. The for the toxic effects. membrane has four important functions: (1) active trans- (2) A core polysaccharide of five sugars linked through port of molecules into the cell, (2) energy generation by ketodeoxyoctulonate (KDO) to lipid A. oxidative phosphorylation, (3) synthesis of precursors of (3) An outer polysaccharide consisting of up to 25 the cell wall, and (4) secretion of enzymes and toxins. repeating units of three to five sugars. This outer polymer is the important somatic, or O, antigen of several gram- Cytoplasm negative bacteria that is used to identify certain organisms The cytoplasm has two distinct areas when seen in the in the clinical laboratory. Some bacteria, notably members electron microscope: of the genus Neisseria, have an outer lipooligosaccharide (1) An amorphous matrix that contains ribosomes, (LOS) containing very few repeating units of sugars. nutrient granules, metabolites, and plasmids. (2) An inner, nucleoid region composed of DNA. Teichoic Acid Teichoic acids are fibers located in the outer layer of the Ribosomes gram-positive cell wall and extend from it. They are com- Bacterial ribosomes are the site of protein synthesis as in posed of polymers of either glycerol phosphate or ribitol eukaryotic cells, but they differ from eukaryotic ribosomes phosphate. Some polymers of glycerol teichoic acid pene- in size and chemical composition. Bacterial ribosomes are trate the peptidoglycan layer and are covalently linked to 70S in size, with 50S and 30S subunits, whereas eukaryotic the lipid in the cytoplasmic membrane, in which case they ribosomes are 80S in size, with 60S and 40S subunits. The differences in both the ribosomal RNAs and proteins con- stitute the basis of the selective action of several antibiotics Poly- Somatic that inhibit bacterial, but not human, protein synthesis (see or "O" saccharide antigen Chapter 10). Core Granules The cytoplasm contains several different types of granules Disaccharide- P P diphosphate that serve as storage areas for nutrients and stain character- istically with certain dyes. For example, volutin is a reserve of Lipid A high energy stored in the form of polymerized metaphos- Fatty acids phate. It appears as a “metachromatic” granule since it stains red with methylene blue dye instead of blue as one would expect. Metachromatic granules are a characteristic feature of Corynebacterium diphtheriae, the cause of diphtheria. FIGURE 2–6 Endotoxin (lipopolysaccharide [LPS]) structure. The Nucleoid O-antigen polysaccharide is exposed on the exterior of the cell, whereas the lipid A faces the interior. (Reproduced with permission from The nucleoid is the area of the cytoplasm in which DNA is Brooks GF et al. Medical Microbiology. 19th ed. Originally published by Appleton & located. The DNA of prokaryotes is a single, circular mol- Lange. Copyright 1991, McGraw-Hill.) ecule that has a molecular weight (MW) of approximately mebooksfree.com 10 PART I Basic Bacteriology 2 × 109 and contains about 2000 genes. (By contrast, human importance of bacteriocins is that they may be useful in DNA has approximately 100,000 genes.) Because the nucle- treating infections caused by antibiotic-resistant bacteria. oid contains no nuclear membrane, no nucleolus, no (2) Nitrogen fixation enzymes in Rhizobium in the root mitotic spindle, and no histones, there is little resemblance nodules of legumes. to the eukaryotic nucleus. One major difference between (3) Tumors caused by Agrobacterium in plants. bacterial DNA and eukaryotic DNA is that bacterial DNA (4) Several antibiotics produced by Streptomyces. has no introns, whereas eukaryotic DNA does. (5) A variety of degradative enzymes that are produced by Pseudomonas and are capable of cleaning up environ- Plasmids mental hazards such as oil spills and toxic chemical waste Plasmids are extrachromosomal, double-stranded, circular sites. DNA molecules that are capable of replicating indepen- dently of the bacterial chromosome. Although plasmids are Transposons usually extrachromosomal, they can be integrated into the Transposons are pieces of DNA that move readily from one bacterial chromosome. Plasmids occur in both gram-posi- site to another either within or between the DNAs of bac- tive and gram-negative bacteria, and several different types teria, plasmids, and bacteriophages. Because of their of plasmids can exist in one cell: unusual ability to move, they are nicknamed “jumping genes.” Some transposons move by replicating their DNA (1) Transmissible plasmids can be transferred from cell and inserting the new copy into another site (replicative to cell by conjugation (see Chapter 4 for a discussion of transposition), whereas others are excised from the site conjugation). They are large (MW 40–100 million), since without replicating and then inserted into the new site they contain about a dozen genes responsible for synthesis (direct transposition). Transposons can code for drug- of the sex pilus and for the enzymes required for transfer. resistant enzymes, toxins, or a variety of metabolic enzymes They are usually present in a few (1–3) copies per cell. and can either cause mutations in the gene into which they (2) Nontransmissible plasmids are small (MW 3–20 insert or alter the expression of nearby genes. million), since they do not contain the transfer genes; they Transposons typically have four identifiable domains. are frequently present in many (10–60) copies per cell. On each end is a short DNA sequence of inverted repeats, Plasmids carry the genes for the following functions and which are involved in the integration of the transposon into structures of medical importance: the recipient DNA. The second domain is the gene for the transposase, which is the enzyme that mediates the exci- (1) Antibiotic resistance, which is mediated by a variety sion and integration processes. The third region is the gene of enzymes, such as the beta-lactamase of S. aureus, for the repressor that regulates the synthesis of both the Escherichia coli, and Klebsiella pneumoniae. transposase and the protein encoded by the fourth domain, (2) Exotoxins, such as the enterotoxins of E. coli, anthrax which, in many cases, is an enzyme mediating antibiotic toxin of Bacillus anthracis, exfoliative toxin of S. aureus and resistance (Figure 2–7). Note that for simplicity, the repres- tetanus toxin of Clostridium tetani. sor gene is not shown in Figure 2–7. (3) Pili (fimbriae), which mediate the adherence of bac- Antibiotic resistance genes are transferred from one teria to epithelial cells. bacterium to another primarily by conjugation (see Chap- (4) Resistance to heavy metals, such as mercury, the ter 4). This transfer is mediated primarily by plasmids, but active component of some antiseptics (e.g., merthiolate and some transposons, called conjugative transposons, are mercurochrome), and silver, which is mediated by a reduc- capable of transferring antibiotic resistance as well. tase enzyme. In contrast to plasmids or bacterial viruses, transposons (5) Resistance to ultraviolet light, which is mediated by are not capable of independent replication; they replicate as DNA repair enzymes. part of the DNA in which they are integrated. More than one Other plasmid-encoded products of interest are as transposon can be located in the DNA; for example, a plas- follows: mid can contain several transposons carrying drug-resistant (1) Bacteriocins are toxic proteins produced by certain bacteria that are lethal for other bacteria. Two common mechanisms of action of bacteriocins are (i) degradation of bacterial cell membranes by producing pores in the mem- IR Transposase IR Antibiotic- IR IR brane and (ii) degradation of bacterial DNA by DNAse. gene resistance Examples of bacteriocins produced by medically important gene bacteria are colicins made by E. coli and pyocins made by FIGURE 2–7 Transposon genes. This transposon is carrying a Pseudomonas aeruginosa. Bacteria that produce bacterio- drug-resistance gene. IR, inverted repeat. (Reproduced with permission cins have a selective advantage in the competition for food from Willey JM et al. Prescott’s Principles of Microbiology. New York: McGraw-Hill, sources over those that do not. However, the medical 2009.) mebooksfree.com  CHAPTER 2 Structure of Bacterial Cells    11 genes. Insertion sequences are a type of transposon that has and are therefore nonmotile. Spirochetes move by using a fewer bases (800–1500 base pairs), since they do not code for flagellumlike structure called the axial filament, which their own integration enzymes. They can cause mutations at wraps around the spiral-shaped cell to produce an undulat- their site of integration and can be found in multiple copies ing motion. at the ends of larger transposon units. Flagella are medically important for two reasons: (1) Some species of motile bacteria (e.g., E. coli and Pro- Structures Outside the Cell Wall teus species) are common causes of urinary tract infections. Capsule Flagella may play a role in pathogenesis by propelling the bacteria up the urethra into the bladder. The capsule is a gelatinous layer covering the entire bacte- (2) Some species of bacteria (e.g., Salmonella species) rium. It is composed of polysaccharide, except in the are identified in the clinical laboratory by the use of spe- anthrax bacillus, which has a capsule of polymerized cific antibodies against flagellar proteins. d-glutamic acid. The sugar components of the polysaccharide vary from one species of bacteria to another and frequently determine the serologic type (serotype) within a species. For Pili (Fimbriae) example, there are 84 different serotypes of Streptococcus Pili are hairlike filaments that extend from the cell sur- pneumoniae, which are distinguished by the antigenic differ- face. They are shorter and straighter than flagella and are ences of the sugars in the polysaccharide capsule. composed of subunits of pilin, a protein arranged in heli- The capsule is important for four reasons: cal strands. They are found mainly on gram-negative (1) It is a determinant of virulence of many bacteria organisms. since it limits the ability of phagocytes to engulf the bacte- Pili have two important roles: ria. Negative charges on the capsular polysaccharide repel (1) They mediate the attachment of bacteria to specific the negatively charged cell membrane of the neutrophil and receptors on the human cell surface, which is a necessary prevent it from ingesting the bacteria. Variants of encapsu- step in the initiation of infection for some organisms. lated bacteria that have lost the ability to produce a capsule Mutants of Neisseria gonorrhoeae that do not form pili are are usually nonpathogenic. nonpathogens. (2) Specific identification of an organism can be made (2) A specialized kind of pilus, the sex pilus, forms the by using antiserum against the capsular polysaccharide. In attachment between the male (donor) and the female the presence of the homologous antibody, the capsule will (recipient) bacteria during conjugation (see Chapter 4). swell greatly. This swelling phenomenon, which is used in the clinical laboratory to identify certain organisms, is Glycocalyx (Slime Layer) called the quellung reaction. (3) Capsular polysaccharides are used as the antigens in The glycocalyx is a polysaccharide coating that is secreted certain vaccines because they are capable of eliciting pro- by many bacteria. It covers surfaces like a film and allows tective antibodies. For example, the purified capsular poly- the bacteria to adhere firmly to various structures (e.g., skin, saccharides of 23 types of S. pneumoniae are present in the heart valves, prosthetic joints, and catheters). The glycocalyx current vaccine. is an important component of biofilms (see page 37). The (4) The capsule may play a role in the adherence of bac- medical importance of the glycocalyx is illustrated by the teria to human tissues, which is an important initial step in finding that it is the glycocalyx-producing strains of causing infection. P. aeruginosa that cause respiratory tract infections in cystic fibrosis patients, and it is the glycocalyx-producing strains Flagella of Staphylococcus epidermidis and viridans streptococci that cause endocarditis. The glycocalyx also mediates adherence Flagella are long, whiplike appendages that move the bacte- of certain bacteria, such as Streptococcus mutans, to the ria toward nutrients and other attractants, a process called surface of teeth. This plays an important role in the forma- chemotaxis. The long filament, which acts as a propeller, is tion of plaque, the precursor of dental caries. composed of many subunits of a single protein, flagellin, arranged in several intertwined chains. The energy for movement, the proton motive force, is provided by ade- Bacterial Spores nosine triphosphate (ATP), derived from the passage of These highly resistant structures are formed in response to ions across the membrane. adverse conditions by two genera of medically important Flagellated bacteria have a characteristic number and gram-positive rods: the genus Bacillus, which includes the location of flagella: some bacteria have one, and others have agent of anthrax, and the genus Clostridium, which includes many; in some, the flagella are located at one end, and in the agents of tetanus and botulism. Spore formation (spor- others, they are all over the outer surface. Only certain ulation) occurs when nutrients, such as sources of carbon bacteria have flagella. Many rods do, but most cocci do not and nitrogen, are depleted (Figure 2–8). The spore forms mebooksfree.com 12 PART I Basic Bacteriology Plasma membrane Septum Peptidoglycan Keratin coat Cell wall Nucleoid DNA Free spore FIGURE 2–8 Bacterial spores. The spore contains the entire DNA genome of the bacterium surrounded by a thick, resistant coat. inside the cell and contains bacterial DNA, a small amount reproduction since one cell produces one spore that germi- of cytoplasm, cell membrane, peptidoglycan, very little nates into one cell. water, and most importantly, a thick, keratinlike coat that is The medical importance of spores lies in their extraor- responsible for the remarkable resistance of the spore to dinary resistance to heat and chemicals. As a result of heat, dehydration, radiation, and chemicals. This resistance their resistance to heat, sterilization cannot be achieved by may be mediated by dipicolinic acid, a calcium ion chela- boiling. Steam heating under pressure (autoclaving) at tor found only in spores. 121°C, for at least 15 minutes, is required to ensure the Once formed, the spore has no metabolic activity and sterility of products for medical use. Spores are often not can remain dormant for many years. Upon exposure to seen in clinical specimens recovered from patients infected water and the appropriate nutrients, specific enzymes by spore-forming organisms because the supply of nutri- degrade the coat, water and nutrients enter, and germina- ents is adequate. tion into a potentially pathogenic bacterial cell occurs. Table 2–4 describes the medically important features of Note that this differentiation process is not a means of bacterial spores. TABLE 2–4 Important Features of Spores and Their Medical Implications Important Features of Spores Medical Implications Highly resistant to heating; spores are not killed by boiling (100°C), but Medical supplies must be heated to 121°C for at least 15 minutes to be are killed at 121°C. sterilized. Highly resistant to many chemicals, including most disinfectants, due to Only solutions designated as sporicidal will kill spores. the thick, keratinlike coat of the spore. They can survive for many years, especially in the soil. Wounds contaminated with soil can be infected with spores and cause diseases such as tetanus (C. tetani) and gas gangrene (C. perfringens). They exhibit no measurable metabolic activity. Antibiotics are ineffective against spores because antibiotics act by inhibiting certain metabolic pathways of bacteria. Also, spore coat is impermeable to antibiotics. Spores form when nutrients are insufficient but then germinate to form Spores are not often found at the site of infections because nutrients are bacteria when nutrients become available. not limiting. Bacteria rather than spores are usually seen in Gram-stained smears. Spores are produced by members of only two genera of bacteria of Infections transmitted by spores are caused by species of either Bacillus medical importance, Bacillus and Clostridium, both of which are gram- or Clostridium. positive rods. PEARLS Shape & Size Bacterial Cell Wall Bacteria have three shapes: cocci (spheres), bacilli (rods), and All bacteria have a cell wall composed of peptidoglycan except spirochetes (spirals). Mycoplasma, which are surrounded only by a cell membrane. Cocci are arranged in three patterns: pairs (diplococci), chains Gram-negative bacteria have a thin peptidoglycan covered by an (streptococci), and clusters (staphylococci). outer lipid-containing membrane, whereas gram-positive bacteria The size of most bacteria ranges from 1 to 3 μm. Mycoplasma, have a thick peptidoglycan and no outer membrane. These differ- the smallest bacteria (and therefore the smallest cells), are 0.2 ences explain why gram-negative bacteria lose the stain when μm. Some bacteria, such as Borrelia, are as long as 10 μm; that exposed to a lipid solvent in the Gram stain process, whereas gram- is, they are longer than a human red blood cell, which is 7 μm positive bacteria retain the stain and remain purple. in diameter. mebooksfree.com  CHAPTER 2 Structure of Bacterial Cells    13 The outer membrane of gram-negative bacteria contains endo- Bacterial DNA toxin (lipopolysaccharide, LPS), the main inducer of septic The bacterial genome consists of a single chromosome of shock. Endotoxin consists of lipid A, which causes the fever and circular DNA located in the nucleoid. hypotension seen in septic shock, and a polysaccharide called O antigen, which is useful in laboratory identification. Plasmids are extrachromosomal pieces of circular DNA that encode both exotoxins and many enzymes that cause antibi- Between the inner cell membrane and the outer membrane of otic resistance. gram-negative bacteria lies the periplasmic space, which is the location of β-lactamases—the enzymes that degrade Transposons are small pieces of DNA that move frequently β-lactam antibiotics, such as penicillins and cephalosporins. between chromosomal DNA and plasmid DNA. They carry antibiotic-resistant genes. Peptidoglycan is found only in bacterial cells. It is a network that covers the entire bacterium and gives the organism its shape. It is composed of a sugar backbone (glycan) and pep- Structures External to the Cell Wall tide side chains (peptido). The side chains are cross-linked by Capsules are antiphagocytic; that is, they limit the ability of neu- transpeptidase—the enzyme that is inhibited by penicillins trophils to engulf the bacteria. Almost all capsules are composed and cephalosporins. of polysaccharide; the polypeptide capsule of anthrax bacillus is The cell wall of mycobacteria (e.g., M. tuberculosis) has more the only exception. Capsules are also the antigens in several vac- lipid than either the gram-positive or gram-negative bacteria. cines, such as the pneumococcal vaccine. Antibodies against the As a result, the dyes used in the Gram stain do not penetrate capsule neutralize the antiphagocytic effect and allow the bacte- into (do not stain) mycobacteria. The acid-fast stain does stain ria to be engulfed by neutrophils. Opsonization is the process by mycobacteria, and these bacteria are often called acid-fast which antibodies enhance the phagocytosis of bacteria. bacilli (acid-fast rods). Pili are filaments of protein that extend from the bacterial sur- Lysozymes kill bacteria by cleaving the glycan backbone of face and mediate attachment of bacteria to the surface of peptidoglycan. human cells. A different kind of pilus, the sex pilus, functions in The cytoplasmic membrane of bacteria consists of a phospho- conjugation (see Chapter 4). lipid bilayer (without sterols) located just inside the peptido- The glycocalyx is a polysaccharide “slime layer” secreted by glycan. It regulates active transport of nutrients into the cell certain bacteria. It attaches bacteria firmly to the surface of and the secretion of toxins out of the cell. human cells and to the surface of catheters, prosthetic heart valves, and prosthetic hip joints. Gram Stain Bacterial Spores Gram stain is the most important staining procedure. Gram- positive bacteria stain purple, whereas gram-negative bacteria Spores are medically i