Introduction to Medical Microbiology

Introduction to Microbiology

• Microbiology is the study of very small living organisms (microorganisms) that are viewed with the help of a microscope.
• Microorganisms include bacteria, viruses, fungi, algae, parasites, and protozoa.

Branches of Microbiology

• Medical microbiology: studies pathogens, their pathogenicity, and body defenses against microbes.
  • Subdivided into medical bacteriology and medical virology.
• Food microbiology: concerned with microbes in soil, air, water, sewage, and food.
• Industrial microbiology: uses microbes to produce beer, wine, antibiotics, and vitamins.
• Sanitary microbiology: deals with processing and disposal of garbage and sewage wastes.

Milestones of Microbiology

• Robert Hooke (1665): proposed the Cell Theory.
• Anton van Leeuwenhoek (1667): described bacteria using a simple microscope.
• Louis Pasteur (1822-1895): developed the germ theory of disease, proposed aseptic techniques, and developed vaccines.
• Joseph Lister (1867): used phenol to disinfect wounds.
• Robert Koch (1876): postulated the Germ Theory, identified microbes that cause diseases, and developed microbiological media.

Eukaryotic and Prokaryotic Cells

• Eukaryotic cells: have a true nucleus and nuclear membrane.
• Prokaryotic cells: lack a true nucleus and nuclear membrane, are smaller than eukaryotic cells.

Structure of the Bacterial Cell

• Bacterial cells are small, prokaryotic, and unicellular.
• Shapes of bacteria: bacilli, cocci, and spirals.
• Bacterial cell wall: protects the cell, maintains shape, and expresses receptors.
• Gram-positive and gram-negative bacteria: differ in cell wall composition.

Bacterial Growth and Metabolism

• Bacterial reproduction: occurs through binary fission.
• Bacterial growth curve: includes lag, logarithmic, stationary, and decline phases.
• Bacterial metabolism: involves breakdown of organic compounds to produce energy.
  • Catabolism: breaks down organic compounds to produce energy.
  • Anabolism: uses energy to synthesize protoplasmic material.

Classification of Bacteria

• Based on phenotypic, genotypic, and medical characteristics.
• Phenotypic classification: includes shape, staining reaction, ways of respiration, arrangement, biochemical reactions, motility, presence of capsule, and spore production.

Bacterial Sporulation

• Process by which bacteria produce spores to survive unfavorable conditions.
• Steps: DNA division, separation of DNA and cytoplasm, formation of cortex and protein coat, and release of endospore.### Antigenic Structure of Bacteria
• Most bacteria are antigenic, meaning they can stimulate a host's immune response to produce antibodies.
• Antigenic determinants are the parts of the bacteria that stimulate the immune response.

Classification of Bacteria

• Bacteria can be classified according to the place of existence:
  • Extracellular bacteria: live outside the host cell
  • Intracellular bacteria: live inside the host cell
• Medical classification of bacteria:
  • According to the mode of transmission:
    • Airborne infection
    • Food and waterborne infection
    • Sexually transmitted infection
    • Through animal bites infection
    • Direct contact infection
  • According to the site of pathogenesis:
    • Bacteria infecting the urinary system
    • Bacteria infecting the digestive system
    • Bacteria infecting the nervous system
    • Bacteria infecting the genital system
    • Bacteria infecting the respiratory system
    • Bacteria infecting the cardiovascular system
    • Bacteria infecting the musculoskeletal system
    • Bacteria infecting the blood and lymph
    • Bacteria infecting the skin
• Genotypic classification of bacteria:
  • Based on the analysis and sequencing of bacterial DNA

Virulence Factors of Bacteria

• Virulence is the degree of pathogenicity of an organism, or the ability of an organism to cause disease.
• Virulence factors are the properties or characteristics that enable a microorganism to establish itself on or within a host and enhance its potential to cause disease.
• Types of virulence factors:
  • Adhesion factors:
    • Flagella (fimbriae): help bacteria adhere to the host cell mucous membrane
    • Pili: help bacteria adhere to the urinary tract epithelium
    • Glycocalyx: increases the ability of bacteria to adhere to the teeth and mucous membrane
  • Protection factors (anti-phagocytic factors):
    • M-proteins: help bacteria adhere and act as anti-phagocytic function
    • Capsule: protects bacteria from phagocytosis
    • Protein A: protects bacteria from phagocytosis
  • Invasion factors (extracellular enzymes):
    • Coagulase: enables bacteria to clot plasma and persist
    • Collagenase and hyaluronidase: help bacteria spread through subcutaneous tissue
    • Lecithinase: breaks down lecithin
    • Streptokinase: breaks down fibrin clot
    • Immunoglobulin A (IgA) protease: breaks down secretory IgA
    • Deoxyribonuclease: breaks down DNA

Bacterial Toxins

• Bacterial toxins are proteins that have a direct harmful effect on tissue cells.
• Types of toxins:
  • Endotoxins: integral part of the cell wall of Gram-negative bacteria
    • Examples: Escherichia coli, meningococci
  • Exotoxins: protein toxins, secreted by living bacteria
    • Examples: Clostridium tetani, Clostridium botulinum, salmonella, staphylococcus aureus

Infection and Epidemiology

• Pathogen: a microorganism capable of causing disease
• Types of bacteria:
  • Saprophytic bacteria: live freely in nature, in soil or water, and on decaying organic matter
  • Parasitic bacteria: live in or on a living host
  • Normal flora (commensals): live externally or internally on the surface of the body without causing disease
  • Opportunistic bacteria: can cause disease under certain conditions

Stages of Bacterial Pathogenesis

• Transmission from an external source into the portal of entry
• Evasion of primary host defenses
• Adherence to mucous membranes
• Colonization by growth of the bacteria
• Disease symptoms caused by toxin production or invasion
• Host responses, both nonspecific and specific immunity

Carriers and Normal Flora

• Carrier: a person who harbors and transmits a pathogen without showing clinical manifestations
• Types of carriers:
  • Incubatory carriers
  • Convalescent carriers
  • Healthy carriers
  • Active carriers
• Normal flora (microbial flora):
  • Resident flora
  • Transient flora
  • Opportunistic pathogens
• Functions of resident flora:
  • Help in the synthesis of vitamins
  • Help in the absorption of nutrients
  • Inhibit colonization of microorganisms
  • Maintain inhibitory pH in the vagina### Complications of Antibiotics
• Development of drug resistance strains due to:
  • Prolonged treatment or usage of inadequate dose
  • Inappropriate use of antibiotics
  • Inadequate diagnosis
• Super infection due to suppression of normal flora by antibiotics
• Drug toxicity leading to:
  • Deafness
  • Depression
• Hypersensitivity reactions, where some drugs act as haptens, binding to tissue proteins and stimulating immune responses, leading to tissue damage
• Bone marrow depression caused by certain drugs, such as chloramphenicol

Antimicrobial Drug Resistance

• Definition: ability of microorganisms to grow in the presence of drugs that would normally kill or limit their growth
• Causes of drug resistance:
  • Mutation (genetic change or chromosomal mutation)
  • Gene transfer (microbes acquiring genes that code for drug resistance)
  • Plasmids carrying genes that code for the production of enzymes that convert active drugs to inactive drugs
  • Inappropriate use of antibiotics
  • Inadequate diagnosis
• Types of multidrug resistance bacteria:
  • Methicillin resistance Staphylococcus aureus (MRSA)
  • Vancomycin resistance Staphylococcus aureus (VRSA)
  • Vancomycin resistance Enterococci

Control of Bacterial Growth

• By sterilization and disinfection:
  • Sterilization: complete destruction or elimination of all viable organisms
  • Disinfection: procedures that kill most microorganisms, except endospores and viruses
  • Decontamination: procedures that reduce pathogenic microorganisms to a safe level
• Antiseptics: microbicidal agents harmless enough to be applied to the skin and mucous membranes
• Examples of antiseptics:
  • Alcohols
  • Mercurials
  • Silver nitrate
  • Iodine solution
  • Detergents
• Methods of sterilization:
  • Sterilization by heat (physical methods)
  • Sterilization by radiation:
    • Ultraviolet light (UV)
    • Ionizing radiation (e.g. gamma rays)
  • Sterilization by filtration (mechanical methods)
  • Sterilization by gaseous chemicals:
    • Ethylene oxide (EO)
    • Formaldehyde
• Chemical disinfectants and antiseptics:
  • Tincture of iodine (2% of K iodine in ethanol) as a skin antiseptic
  • Ethyl alcohol (70% concentration) as a skin antiseptic
  • Hydrogen peroxide (H2O2) as an antiseptic to clean wounds

Bacterial Genetics

• Definition: study of the inheritance of different characters that are transmitted from parent to offspring
• Definition of genes: units of heredity, segments of the chromosome (DNA) that carry information for a specific character or structure
• Prokaryotic genomes:
  • Bacterial genome: made of DNA, floats freely in the cytoplasm
  • Chemically composed of a backbone of phosphate and sugar (deoxyribose) attached to purine and pyrimidine bases
  • Genes essential for bacterial growth are carried on the chromosome
  • Genes associated with specialized functions are carried on plasmids
• Replication, transcription, and translation:
  • Replication: making a copy of the genetic material
  • Transcription: process of transferring genetic information from DNA to RNA
  • Translation: process of transferring genetic information from RNA to protein

Bacterial Variation

• Phenotypic variation: change in bacterial characters under the influence of the environment, with no genetic change
• Genotypic variation: change in bacterial characters due to changes in the genetic constitution
• Mutation: change in the sequence of bases in the DNA that occur due to:
  • Substitution
  • Deletion
  • Insertion of new bases
• Gene transfer:
  • Transformation: transfer of genetic material from one bacterium to another
  • Transduction: transfer of genetic material from one bacterium to another through a bacteriophage
  • Conjugation: direct transfer of genetic material from one bacterium to another

Nosocomial Infection

• Definition: infection acquired in a medical setting in the course of medical treatment
• Factors that favor nosocomial infection:
  • Elderly
  • Children
  • Underlying illness
  • Immunosuppressed patients
  • Instrumentation
• Microorganisms involved in nosocomial infection:
  • Staphylococcus aureus
  • E. coli
  • Pseudomonas
  • Klebsiella
  • Proteus
• Prevention measures:
  • Isolation
  • Handwashing and gloving
  • Education of healthcare workers
  • Vaccination
  • Proper disposal of infectious hospital waste

Introduction to Virology

• Definition: viruses are the smallest infectious agents that can infect man, animal, insects, plants, and bacteria
• General properties of viruses:
  • Small in size
  • Contain one kind of nucleic acid (DNA or RNA)
  • Obligate intracellular parasites
  • Cannot be grown on artificial media
  • Depend on the host cell's ribosomes and enzymes for protein synthesis
• Discovery of viruses:
  • Dmitri I. Winooski discovered the tobacco mosaic virus (TMV) in 1892
• Virus size and structure:
  • Variable in size (10nm to 500nm)
  • Can pass through bacterial filters
  • Require high speeds for sedimentation
  • Structure consists of a protein coat (capsid) and a nucleic acid core (nucleocapsid)

Viral Genome

• Function: carries the genetic information for replication, virulence, and antigenic specificity
• Structure: linear or circular, single-stranded or double-stranded
• Segmented genome: some RNA viruses have a segmented genome, such as Rotavirus and influenza virus

Learn the basics of microbiology, including the meaning of the term and the study of very small living organisms. This introduction is part of a medical microbiology course.