Microbial Diagnostics and Identification Methods

Categories of Microbial Identification

  • Phenotypic: Considers morphology (microscopic and macroscopic), physiology, and biochemistry.
  • Immunologic: Serological analysis of antibodies or antigens.
  • Genotypic: Nucleic acid-based techniques (DNA or RNA analysis).

Phenotypic Methods

  • Microscopic Morphology: Observation of cell shape, size, arrangement, and staining reactions (Gram stain, acid-fast). Electron microscopy is used for structures like the cell wall, flagella, pili, and fimbriae.
  • Macroscopic Morphology: Evaluation of colony traits visible to the naked eye, such as texture, pigment, size, shape, and growth patterns in various media.
  • Physiological/Biochemical Characteristics: Testing for enzyme production and metabolic activities (e.g., sugar fermentation, gas production, antibiotic sensitivity).
  • Phage Typing: Uses strain-specific bacteriophages to identify bacteria; a positive result is indicated by cleared areas of lysed cells on a bacterial lawn.

Genotypic Methods

  • Advantages: Rapid results without the need for culturing; allows identification of Viable Noncultured (VNC) microbes.
  • Polymerase Chain Reaction (PCR): Used to amplify minute quantities of DNA or RNA into numerous identical copies within hours.
  • Hybridization and Probes: Short nucleic acid segments (probes) complementary to specific microbial sequences emit signals upon base-pairing. Fluorescent In Situ Hybridization (FISH) allows visualization via fluorescence microscopy.

Immunologic Methods

  • Serology: In vitro testing of serum based on the high specificity of antibody-antigen interactions.
  • Antibody Titers: Measured by serially diluting patient serum to find the highest dilution that still produces agglutination (clumping). For example, a titer of 1:1601:160 indicates the most dilute concentration showing a positive reaction.
  • Immunofluorescence:
    • Direct: Uses a fluorescently labeled primary antibody to detect antigens.
    • Indirect: Uses a labeled secondary antibody that binds to an unlabeled primary antibody.
  • Enzyme-Linked Immunosorbent Assay (ELISA): Utilizes an enzyme-linked indicator antibody and a solid support (microtiter plate) to visualize results.

Clinical Significance of Isolates

  • Distinguishing between normal biota and true pathogens is critical.
  • A few colonies of E. coli in urine may indicate normal flora, but several hundred suggest infection.
  • A single colony of a true pathogen like Mycobacterium tuberculosis in a sputum culture is highly suggestive of disease.

Questions & Discussion

  • The session concluded with a call for questions regarding the material presented.