Microbiology Lecture Notes

Introduction

• Presenter: Rainey Woo
• Overview of the lecture: General information about bacteria, fungi, parasites, viruses, and antimicrobial therapy.

General Information About Bacteria

• Bacteria: Single-celled prokaryotic organisms.
• Structural Elements: Differ from eukaryotic cells.
• Differences: Gram-positive vs. Gram-negative bacteria.

Gram-Positive vs. Gram-Negative Bacteria

• Gram-Positive Bacteria:
  • Thick cell walls with high peptidoglycan and lipoteichoic acids.
  • Important in bacterial stereotypes, interactions with human cells, and immune response initiation.
• Gram-Negative Bacteria:
  • Thin peptidoglycan layers, outer membranes, and periplasmic spaces.
  • Contain enzymes for virulence: proteases, phosphatases, lipases, beta-lactamases.
  • Lipopolysaccharide endotoxins in outer membranes cause immune responses leading to septic shock.

Unique Bacteria Wall Properties

• Mycoplasma: No cell wall.
• Mycobacterium: Cell wall with mycolic acid, challenging to Gram stain.

Gram Staining Technique

• Purpose: Differentiates bacterial strains by cell wall characteristics.
• Process: Crystal violet dye, iodine, decolorizer, safranin counterstain.
• Gram-Positive: Stain purple due to thick peptidoglycan walls.
• Gram-Negative: Stain pink due to thin walls and lack of CV-iodine complex.

Bacteria Not Identified by Gram Stain

• Mycoplasma: Lacks cell wall.
• Mycobacterium: Acid-fast method used.
• Treponema: Visualized by dark-field microscopy or fluorescent antibody staining.
• Legionella, Chlamydia: Visualized by immunofluorescent staining.

Specific Staining Techniques

• Giemsa Stain: Identifies Borrelia, Plasmodium, Trypanosomes, Chlamydia.
• Periodic Acid-Schiff (PAS): Stains glycogen/mucopolysaccharides; identifies Tropheryma whipplei.
• Ziehl-Neelsen Stain: Detects acid-fast organisms.
• India Ink Stain: Stains Cryptococcus neoformans.
• Silver Stain: Visualizes fungi and Legionella.

Bacterial Culture

• Growing bacteria on agar plates with selective or differential media.
• Selective Media: Contains antibiotics to prevent contamination.
• Differential Media: Induces color change for identification.

Oxygen Requirements of Bacteria

• Obligate Aerobes: Require oxygen (e.g., Nocardia, Pseudomonas, Mycobacterium).
• Obligate Anaerobes: Survive without oxygen; lack enzymes to neutralize superoxide (e.g., Clostridium).

Intracellular Bacteria

• Obligate Intracellular: Rickettsia, Chlamydia.
• Facultative Intracellular: Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, Yersinia.

Encapsulated Bacteria

• Polysaccharide capsules protect from immune responses.
• Mnemonic: 'SHIN SKiS'
  • Streptococcus pneumoniae
  • Haemophilus influenzae type B
  • Neisseria meningitidis
  • Salmonella
  • Klebsiella pneumoniae
  • Group B Streptococcus

Catalase-Positive Organisms

• Breakdown hydrogen peroxide, counteracting host defense mechanisms.
• Important for patients with Chronic Granulomatous Disease (CGD).
• Examples: Staphylococcus aureus, Serratia marcescens, Nocardia, Candida, E. coli.