Lectures 2,3 Bacterial Cell Structure (Medical Microbiology) PDF

Dr. Sozan Mohamed Fadl Head of Microbiology & Immunology Unit  Structure of Bacteria: From outside to inside: A. Cell appendages: Flagella, Fimbriae (pili), Axial filaments (endoflagella, in Spirochetes). B. Cell envelope: outside to inside...

Dr. Sozan Mohamed Fadl Head of Microbiology & Immunology Unit  Structure of Bacteria: From outside to inside: A. Cell appendages: Flagella, Fimbriae (pili), Axial filaments (endoflagella, in Spirochetes). B. Cell envelope: outside to inside Capsule (in some bacteria). Cell wall, Cytoplasmic membrane. C. Cytoplasmic contents: Nucleic acid, Plasmids, Ribosomes. Glycocalyx  Cell Appendages  1- Flagella: - Origin: cytoplasm - Structure: protein - Long, thick and twisted - In Gram-positive and Gram-negative bacteria - Can be one, two, or numerous per cell. - Function: Motility (locomotion) - Examples of motile bacteria: Salmonella Vibrio cholerae Escherichia coli (E. coli)  Monotrichous flagellum Peritrichous flagella Vibrio cholerae motility  2. Axial filaments (Endoflagella): - Present in periplasmic space. - In spirochetes (as Treponema) - Run along the axis of the bacteria→ rotation motility  3. Pili & Fimbriae - Origin: cytoplasmic membrane - Structure: Protein Common pili: Short, thin In all gram-negative bacteria & some gram-positives. Numerous up to 1000 per bacterial cell. Functions: - Adherence to the host cells. - Adherence of bacteria to each other. Sex pili: Long and thick, 1-10 per bacterial cell. Only in gram negatives Function: gene transfer by conjugation.  Sex pilus flagella Pili Pili  Flagella Fimbria or pili Sex pili Structure - Long, thick, - Short, thin Long and- and twisted thick number - Numerous - Few - 1-10 per bacterial cell. Type of Gram positive All gram Only in gram bacteria & negative negative and negative some gram bacteria positive Function Motility or Adherence to Gene transfer locomotion host cells and by conjugation. to other bacteria  Cell Envelope  1. Capsule: The outermost viscid layer formed mainly in vivo. Present in most pathogenic bacteria. Composition: o Polysaccharides e.g. Streptococcus pneumoniae, Neisseria meningitides. o Exception: Bacillus anthracis capsule which is made of polypeptides. Demonstration of the capsule: Appears as an unstained halo around bacteria by: Capsule stain India-ink. Capsule stain India ink wet mount  Importance of the Capsule: Virulence factor Anti-phagocytic (prevent phagocytosis; protect the bacteria from the immune cells). Antigenic: The immune system produce antibodies against it. Clinical use: A. Diagnosis of infections as meningitis: by detecting the capsular antigen in CSF samples. B. Vaccine preparation: e.g. Neisseria meningitidis, Haemophilus influenza type b (Hib) and pneumococcal vaccine.  2. Bacterial Cell Wall: - Outside the cytoplasmic membrane. - Determines the shape of the cell Structure: Peptidoglycan Backbone: Chains of two alternating sugars: 1. Glycan: a) N-acetyl Glucosamine (NAG) and b) N-acetyl Muramic acid (NAM) with 2. Peptido: Four amino acids side chains are attached to NAM & peptide bond cross-link backbones with each other. Transpeptidase enzymes (Penicillin binding enzymes) are responsible for crosslinking the chains. Peptidoglycan backbone is thick in Gram-positive bacteria and thin in Gram-negative bacteria  Cell Wall of Gram-Positive Bacteria: o Thick peptidoglycan layer. o Lipoteichoic acid: for attachment. Cell Wall of Gram-Negative Bacteria: o Thin peptidoglycan layer in the periplasmic space. Also contain stored enzymes as penicillinase. o Outer membrane (endotoxin): o Lipopolysaccharide (LPS). o Lipid A. Gram Positive Gram Negative Bacteria Bacteria The outer membrane of gram negatives is composed of lipopolysaccharide (LPS)  3. Cytoplasmic Membrane: Consists of phospholipid bilayers and proteins. Functions: Selective permeability and transport of solutes. Energy production by electron transport chain. Excretion of hydrolytic exoenzymes & toxic waste. Biosynthesis of cell wall. Mesosomes: Invaginations of cytoplasmic membrane into the cytoplasm: Increase the surface area and functions of the cytoplasmic membrane. Role in cell division. Cytoplasmic Membrane and Mesosome The Cytoplasm  The Cytoplasm: Contains: 1. Chromosome: One DNA, double stranded (helix). 2. Plasmid: - Circular extra double stranded DNA segment,. - In some bacteria. - Capable of autonomous replication - Function: carry genes of antibiotic resistance. 3. Ribosome (70s: 50s + 30s): - Formed of rRNA & protein - Function: protein synthesis Bacterial Spores  Spores and Sporulation: Spores: -Resistant structures formed under adverse environmental conditions (chemicals, radiations, freezing and heat) -They can remain viable for many years in a dormant form. -Formed by certain types of bacteria (spore-forming) as the genus Bacillus and Clostridium. Sporulation: - Process of production of inactive bacterial form from active vegetative bacterial cells. -Spores contain the DNA enclosed in a multi-layered rigid envelope strengthened by keratin + calcium dipicolinate. Spore Germination: In suitable environmental conditions as in vivo, the destruction of the spore coat produces active vegetative bacterial cells which begin binary fission. Example: spores enter through a wound or by inhalation→ infection and disease.  Antimicrobial Chemotherapy Antimicrobials: Chemicals that kill or inhibit the growth of microorganisms (bacteria, fungi, viruses and parasites). Classification: According to the source: natural, synthetic or semisynthetic. According to the action: Bactericidal antibiotics: lethal to the bacteria. Bacteriostatic antibiotics: inhibit bacterial growth. Broad spectrum active against many types of bacteria. Narrow spectrum: active against limited types of bacteria. Classification According to the Targets: DNA Ribosome Protein Synthesis Inhibitors  Target Sites for Antibiotics in The Bacteria Cell 1. Cell wall synthesis inhibitors: A. Beta-lactams Penicillins Cephalosporins B. Glycopeptides e.g., vancomycin 2. Cell membrane disrupters. 3. Protein synthesis inhibitors 4. Nucleic acid inhibitors. Target Sites for Antibiotics in The Bacteria Cell 2. Cell membrane inhibitors: Polymyxins. Active against Gram-negative bacteria only Nephrotoxicity and neurotoxicity 1. Protein synthesis inhibitors: Bind to the 50s or 30s ribosome and inhibit protein synthesis. Macrolides Aminoglycosides Tetracyclines e.g. erythromycin, e.g. gentamycin e.g., tetracycline, azithromycin Doxycycline 2. Nucleic acid inhibitors: Sulfonamides and Quinolones Rifampicin trimethoprim e.g. ciprofloxacin as co-trimoxazole Inhibit folic acid Inhibit DNA gyrase Inhibit RNA pathway for purines or topoisomerase polymerase (RNA synthesis transcription). Reference Lippincott Illustrated Microbiology 4th edition, Chapters: 5 & 6