15.3 Phenotypic Methods in Diagnosing Infections

Learning Outcomes

Assess Your Progress
- Factors affecting the identification of an infectious agent from a patient sample.
- Discuss the impact of bedside health professionals, such as nurses, on the identification of disease agents.

Direct microscopic observation of fresh or stained specimens is a rapid method for determining microbial characteristics.
- Gram Stain: Essential for bacterial identification.
- Acid-Fast Stain: Used for identifying Mycobacterium species, abbreviated as AFB (Acid-Fast Bacillus stain).
Calcofluor-Potassium Hydroxide Stain (KOH Prep): Used for identifying fungi in hair, nails, skin, and bodily fluids.
- Fungal identity can sometimes be deduced by their morphology under a microscope.

Pre-selection of lab medium is made by trained lab technicians based on:
- Sample site from which it was obtained.
- Physician's suspicions regarding possible pathogens.
Enrichment of nonsterile specimens (like urine and feces) containing diverse bacterial species can be done with specialized media.
- Example: Urinary tract infections are commonly caused by Escherichia coli, thus selective media are chosen to promote its growth.
Specimens are cultured on differential media to identify characteristics, such as:
- Reactions in blood (e.g., blood agar).
- Fermentation patterns (e.g., mannitol salt and MacConkey agar).

A straightforward phenotypic identification involves combining Gram staining, growth on different media, and enzymatic tests.
- The Gram stain is usually the primary test, categorizing the specimen as either gram-negative or gram-positive.
Dichotomous Key: A graphic method, essentially a flowchart, leading to specimen identification.
- The term "dichotomous" indicates two possible outcomes at each branch.

Physiological reactions of bacteria to nutrients and substrates provide evidence of specific enzyme systems within species.
- Enzymes indicate identity: tests visualize reactions via color changes.
Microbial culture is conducted in a substrate medium and is tested for specific end products:
- Color change indicates enzyme presence.
- Absence of color indicates the enzyme is lacking for that substrate.
Automated systems can test and identify over 2,500 different bacteria, performing growth and incubation automatically.
Rapid tests can now yield results within hours instead of days.

MGIT (Mycobacterial Growth Indicator Tube): Detects slow-growing Mycobacterium tuberculosis.
- Monitors oxygen in an inoculated specimen tube (sputum, blood).
- Depletion of oxygen due to bacterial growth leads to fluorescence from a silicon chip at the bottom.
- A visual and audible signal indicates positive culture when fluorescence surpasses a predefined threshold.

Identifying microbial pathogens is crucial for determining appropriate antimicrobial treatments, especially with rising antimicrobial resistance.
Automated phenotypic systems test susceptibility while identifying pathogens, often using a tube-dilution method.
Rapid antimicrobial susceptibility testing (rAST) allows for quicker determination of susceptibility in blood cultures.

Phage typing uses bacteriophages that target bacteria in a species-specific manner, useful for identifying certain bacteria like Salmonella.
Involves creating a bacterial lawn on agar, dividing into a grid, and applying different phages into each section. Cleared areas indicate sensitivity, allowing for bacterial identification.

Rapidly determining if an isolate is clinically significant or merely normal biota is paramount:
- Number of Microbes: A few colonies of Escherichia coli in urine could indicate normal biota, while several hundred can indicate active infection.
- Presence of True Pathogens: A single colony of pathogens like Mycobacterium tuberculosis from sterile sites (e.g., cerebrospinal fluid or blood) suggests its clinical importance.
- Repeated Isolation: A relatively pure culture of any microbe may suggest it as a disease agent.
Limitations of phenotypic methods:
- Culturing requires a minimum of 18-24 hours and may not detect nonculturable organisms.

Table summarizing various phenotypic diagnostic methods (not included in transcript).