Introduction to Microbial Diagnostics and Identification Methods

Categories of Microbial Identification

• Phenotypic: Analysis of morphology, physiology, and biochemical properties.
• Genotypic: Analysis of microbial DNA or RNA.
• Immunologic: Analysis of microbes using antibodies or patient serum using prepackaged antigens.

Phenotypic Methods

• Microscopic Morphology: Observation of cell shape, size, arrangement, Gram stain and acid-fast reactions, endospores, granules, and capsules. Electron microscopy identifies cell wall, flagella, pili, and fimbriae.
• Macroscopic Morphology: Colony traits including texture, size, shape, pigment, growth speed, and growth patterns in media.
• Physiological/Biochemical Characteristics: Detection of specific enzymes and metabolic activities such as sugar fermentation, gas production, and sensitivity to antibiotics.
• Phage Typing: Identifying cleared areas of lysed cells on a bacterial lawn exposed to species-specific bacteriophages.

Genotypic Methods

• Advantages: Rapid results; does not require culturing; identifies Viable noncultured ($VNC$) microbes.
• Disadvantages: Potential to detect nucleic acid fragments rather than viable organisms.
• Polymerase Chain Reaction ($PCR$): Produces identical copies of DNA or RNA to amplify minute quantities.
• Hybridization: Uses small DNA or RNA probes complementary to microbial sequences. Fluorescent in situ hybridization ($FISH$) uses fluorescent probes for microscopic visualization.

Immunologic Methods and Serology

• Serology: $In\,vitro$ testing of serum based on high specificity between antigens and antibodies.
• Antibody Titer: The concentration of antibodies determined by the highest dilution of serum (e.g., $1:160$) that still produces clumping or agglutination.
• Immunofluorescence:
  • Direct: Fluorescently labeled antibody binds directly to the antigen.
  • Indirect: A labeled secondary antibody binds to a primary antibody (e.g., anti-rabbit antibody binding to a rabbit antibody).
• Enzyme-Linked Immunosorbent Assay ($ELISA$): Uses enzyme-linked indicator antibodies on a solid support (microtiter plate) to visualize reactions.

Diagnostic Questions and Clinical Significance

• Diagnostic Inquiries:
  • Does the patient make antibodies toward the microbe?
  • What is the identity of the microbe?
• Clinical Interpretation: Distinguishing between normal biota and active infection.
  • A few colonies of opportunist $E.\,coli$ in urine may be normal biota, whereas several hundred indicate infection.
  • A single colony of a true pathogen like $Mycobacterium\,tuberculosis$ in sputum is highly suggestive of disease.