Bacterial Classification, Morphology and Cell Structure

• Bacteriology is the study of bacteria, a large domain of prokaryotic microorganisms.
• It encompasses bacterial identification, classification, structure, function, and their roles in nature and disease

Bacterial Classification

• Bacteria are classified using a combination of phenotypic and genotypic methods.
• Phenotypic methods include observing morphology, staining characteristics, metabolic capabilities, and antibiotic sensitivities.
• Genotypic methods involve analyzing the bacterial genome, including 16S rRNA gene sequencing, whole-genome sequencing, and DNA hybridization.
• Bergey's Manual of Systematics of Archaea and Bacteria is a key resource for bacterial classification

Bacterial Morphology and Cell Structure

• Bacteria exhibit diverse morphologies, including cocci (spherical), bacilli (rod-shaped), spirilla (spiral), and vibrios (comma-shaped).
• The bacterial cell structure includes:
  • Cell wall: Provides shape and rigidity, distinguishing Gram-positive and Gram-negative bacteria.
  • Cell membrane: Encloses the cytoplasm and regulates the passage of substances.
  • Cytoplasm: Contains the bacterial chromosome, plasmids, ribosomes, and various enzymes.
  • Capsule: A polysaccharide layer outside the cell wall for protection and adherence.
  • Flagella: Appendages used for motility.
  • Pili (fimbriae): Hair-like structures for attachment to surfaces.
  • Endospores: Dormant, highly resistant structures formed by some bacteria.

Antibiotic Resistance

• Antibiotic resistance occurs when bacteria develop mechanisms to survive exposure to antibiotics.
• Mechanisms of resistance include:
  • Enzymatic inactivation of antibiotics (e.g., beta-lactamases).
  • Modification of the antibiotic target site.
  • Decreased permeability of the bacterial cell to the antibiotic.
  • Efflux pumps that actively remove the antibiotic from the cell.
• Resistance genes can be transferred between bacteria through horizontal gene transfer (conjugation, transduction, transformation).
• The spread of antibiotic resistance is a major global health threat.

Laboratory Techniques in Bacteriology

• Specimen Collection:
  • Proper collection is vital to prevent contamination and ensure accurate results.
  • Use sterile containers and transport media.
  • Collect samples from the actual site of infection.
• Culture Media:
  • Provide nutrients required for bacterial growth in the laboratory.
  • Selective media inhibit the growth of some bacteria while allowing others to grow.
  • Differential media allow differentiation of bacteria based on biochemical reactions.
• Microscopy:
  • Used for direct observation of bacteria.
  • Gram staining differentiates bacteria based on cell wall structure (Gram-positive vs. Gram-negative).
  • Special stains (e.g., acid-fast stain) are used for specific bacteria.
• Biochemical Tests:
  • Used to identify bacteria based on their metabolic activities.
  • Examples include catalase test, oxidase test, coagulase test, and sugar fermentation tests.
• Molecular Techniques:
  • PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences for identification and resistance gene detection.
  • Sequencing: Identifies bacteria, identifies resistance mechanisms, and characterizes virulence factors.
  • MALDI-TOF MS: Analyzes bacterial protein profiles for rapid identification.
• Antimicrobial Susceptibility Testing:
  • Determines the susceptibility of bacteria to antibiotics.
  • Methods include disk diffusion (Kirby-Bauer) and broth microdilution.
  • Results guide antibiotic selection for treatment.

Pathogenic Bacteria

• Pathogenic bacteria are those that can cause disease in humans, animals, or plants.
• Virulence factors contribute to the ability of bacteria to cause disease, including:
  • Adhesins: Enable bacteria to adhere to host cells.
  • Toxins: Damage host cells or disrupt their function (e.g., exotoxins, endotoxins).
  • Enzymes: Degrade tissues or promote bacterial spread.
  • Capsules: Inhibit phagocytosis.
  • Invasion factors: Allow bacteria to enter host cells.
• Examples of pathogenic bacteria and the diseases they cause include:
  • Staphylococcus aureus: Skin infections, pneumonia, sepsis.
  • Streptococcus pyogenes: Strep throat, scarlet fever, impetigo.
  • Escherichia coli: Urinary tract infections, gastroenteritis.
  • Salmonella species: Salmonellosis (food poisoning).
  • Shigella species: Shigellosis (dysentery).
  • Pseudomonas aeruginosa: Pneumonia, bloodstream infections, wound infections.
  • Mycobacterium tuberculosis: Tuberculosis.
  • Clostridium difficile: Antibiotic-associated diarrhea.
  • Neisseria gonorrhoeae: Gonorrhea.
  • Chlamydia trachomatis: Chlamydia.
• Emerging Pathogens: Bacteria that have recently increased in incidence or geographic range.
• Healthcare-Associated Infections (HAIs): Infections acquired in healthcare settings, often caused by antibiotic-resistant bacteria.

Explore the classification, morphology, and cell structure of bacteria. Learn about phenotypic and genotypic methods used for bacterial identification, including morphology observations and genome analysis. Understand the diverse shapes and key components of bacterial cells, such as the cell wall.