General Bacteriology PDF

Introduction to General Bacteriology

Objectives

1. Provide an overview of bacterial structure and cell envelope.
2. Explain the spores and sporulation process.
3. List the characteristics of bacterial growth and metabolism.
4. Classify bacterial species according to gram stain and shape.

Overview

• All bacteria are prokaryotes.
• Genetic material: A single double-stranded molecule of DNA (nucleoid).

Shape and Size

• Classified by shape: Cocci (spherical), bacilli (rod-shaped), and spirochetes (spiral).
• Size range: 0.2 to 5 μm.

The Cell Envelope

• Cytoplasmic membrane: Composed of phospholipid (lipid bilayer), serving as a permeability barrier.
• Peptidoglycan: A linear polymer of alternating monosaccharides (NAG, NAM).

Differences Between Gram-Positive and Gram-Negative Bacteria

• Gram-positive:
  • Thick, multilayered peptidoglycan cell wall.
  • Embedded teichoic acids (major antigens).
• Gram-negative:
  • Complex, thin peptidoglycan layer.
  • Embedded lipopolysaccharides (LPS).
    • O-polysaccharide: Antigenic part.
    • Lipid-A: Toxic to humans.

Bacterial DNA

• Genome: Single circular chromosome located in the nucleoid.
• Plasmids: Extrachromosomal circular DNA encoding exotoxins and enzymes causing antibiotic resistance.
• Transposons: DNA segments moving between chromosomal DNA and plasmid DNA, often carrying antibiotic-resistant genes.

Bacterial Structure

• Capsule: A tightly bound and organized structure, often protective.
• Glycocalyx: A loosely bound and viscous layer, aiding in adherence and protection.
• Flagella: Long, helical structures used for movement.
• Pili (fimbriae): Short, thin structures for attachment and cell-to-cell contact.

Bacterial Classification

• Gram stain: Differentiates bacteria based on cell wall structure.
• Shape: Categorized into cocci, bacilli, and spirochetes.

Spores and Sporulation

• Endospores: Dormant cells formed by some gram-positive bacteria to survive adverse conditions.
• Resistant to: Heat, UV, desiccation, and bactericidal chemicals. Sensitive to autoclaving (120°C, high pressure).
• Medical importance: Can remain viable for many years (e.g., Bacillus cereus and Clostridium botulinum).

Bacterial Growth and Metabolism

• Cell multiplication: Occurs through binary fission.
• Generation time: Time required for a bacterium to divide into two daughter cells under optimal conditions (e.g., E. coli: 20 minutes).

Factors Affecting Growth of Bacteria

• Oxygen:
  • Aerobic bacteria: Require oxygen.
    • Obligate aerobes: Only grow in oxygen (e.g., Pseudomonas aeruginosa).
    • Facultative aerobes: Grow with or without oxygen (e.g., E. coli).
    • Microaerophilic bacteria: Grow in low oxygen and low carbon dioxide (e.g., Campylobacter jejuni).
  • Anaerobic bacteria: Grow only without oxygen (e.g., Clostridium botulinum).
• Temperature: Optimal for pathogenic bacteria is 37°C.
• pH: Most grow between pH 7.2 and 7.6.
• Osmotic pressure: Affects growth based on the salt concentration.

Normal Flora (Human Microbiome)

• Permanent residents: Various bacteria and fungi in specific body sites.
• Functions:
  • Host defense: Colonization resistance.
  • Nutritional: Production of B vitamins and vitamin K.
  • Pathogenic potential: Can cause disease in immunocompromised individuals or unusual locations.

References

• Lippincott’s Illustrated Reviews, Microbiology. 4th edition, pages 49-55.
• Review of Medical Microbiology and Immunology. 14th edition by Warren E. Levinson, Chapters 2 & 3.