Bacterial identification methods

Stains & Cultures - Seeing is Believing

• Gram Stain: Differentiates bacteria based on cell wall peptidoglycan content.

• Other Key Stains:
  • Ziehl-Neelsen: Acid-fast stain for Mycobacterium (pink/red on blue background).
  • India Ink: Negative stain for cryptococcal capsules.
  • Giemsa: For intracellular organisms. 📌 Giemsa helps you Get inside the cell (Chlamydia, Rickettsia).

⭐ Certain bacteria, like Legionella pneumophila, stain poorly with Gram stain and require specialized silver stains or growth on specific media like Buffered Charcoal Yeast Extract (BCYE) agar.

Biochemical Tests - What Bacteria Eat

• Catalase: Differentiates Staphylococcus (CAT-positive) from Streptococcus (CAT-negative).
  • Degrades $H_2O_2$ into $H_2O$ and $O_2$ (bubbles).
• Coagulase: Differentiates S. aureus (coagulase-positive) from other staphylococci.
• Urease: Identifies urease-producers like Proteus, Klebsiella, and H. pylori.
  • 📌 Pee CHUNKSS: Proteus, Cryptococcus, H. pylori, Ureaplasma, Nocardia, Klebsiella, S. epidermidis, S. saprophyticus.
• Oxidase: Identifies organisms with cytochrome c oxidase, e.g., Pseudomonas, Neisseria.
• Indole: Differentiates E. coli (positive) from other lactose-fermenting enterics.

⭐ The CAMP test identifies Streptococcus agalactiae (Group B Strep). It shows an arrowhead-shaped zone of enhanced hemolysis when grown near a streak of S. aureus.

Immunologic Methods - Antibody Detectives

Utilizes the high specificity of antibody-antigen binding for identification. Can detect either bacterial antigens or host antibodies.

• ELISA (Enzyme-Linked Immunosorbent Assay):

  • Highly sensitive method to detect antigens or antibodies.
  • Commonly used for HIV, Lyme disease, and viral hepatitis serology.
  • 📌 Sandwich ELISA: Traps the antigen between two antibodies.

• Agglutination:

  • Visible clumping of antibody-coated particles (latex) or bacteria.
  • Used in rapid strep tests and for serotyping (e.g., Lancefield grouping, Salmonella serovars).

• Immunofluorescence (IF):

  • Antibodies labeled with fluorescent dyes visualize bacteria under a microscope.
  • Direct (DFA) or Indirect (IFA) methods.

⭐ Serotyping is critical for epidemiology. E. coli O157:H7 is a key example; the O-antigen is from the lipopolysaccharide (LPS) and the H-antigen is from the flagella.

Molecular Methods - DNA Fingerprinting

• Principle: Exploits variations in DNA sequences to create a unique genetic "fingerprint" for each bacterial strain, essential for epidemiological tracking.
• Core Techniques:
  • Restriction Fragment Length Polymorphism (RFLP):
    • Bacterial DNA is digested by restriction enzymes.
    • Resulting fragments are separated by gel electrophoresis, creating a specific banding pattern.
  • Pulsed-Field Gel Electrophoresis (PFGE):
    • A specialized RFLP for separating very large DNA fragments using a periodically changing electric field.
  • Ribotyping:
    • An RFLP method targeting the highly conserved ribosomal RNA (rRNA) genes.

⭐ High-Yield: PFGE is considered the gold standard for molecular subtyping in many epidemiological investigations, crucial for tracking outbreaks of pathogens like Listeria monocytogenes, Salmonella, and E. coli O157:H7.

High‑Yield Points - ⚡ Biggest Takeaways

• Gram stain is the crucial first step, dividing bacteria into Gram-positive (purple) and Gram-negative (pink) based on peptidoglycan wall structure.
• Catalase test differentiates Staphylococcus (+) from Streptococcus (-).
• Coagulase test separates S. aureus (+) from coagulase-negative staphylococci.
• Lactose fermentation on MacConkey agar distinguishes fermenters like E. coli from non-fermenters like Salmonella.
• Hemolysis patterns (α, β, γ) are key for classifying Streptococcus species.
• Acid-fast stain is essential to identify Mycobacterium species.