Intro to med micro Pt. 2 Microbiology Lab: Specimen Processing and Candida albicans Identification

Specimen Analysis and Culture Identification Principles

• Sterile vs. Non-Sterile Specimens

  • Sterile Specimens: These include blood, cerebrospinal fluid (CSF), and bone marrow. Any organism recovered from a sterile specimen is considered a pathogen. Significance is attached to even a single "inkling" of an organism found in these samples.

  • Non-Sterile/Superficial Specimens: These include urine or superficial wound specimens. If a mold or yeast grows from these samples, it is not always classified as a pathogen; differentiation is required to determine its significance.

  • Pathogen Recovery Balance: In highly colonized areas like the mouth, care must be taken not to allow normal flora to overgrow, as this might mask a single, significant pathogen.

• Growth Modification through Temperature

  • Refrigeration: This is the most common method used to slow the growth of organisms to prevent overgrowth before analysis.

  • Encouraging Growth: For fungal cultures, specific temperatures are used to stimulate growth.

    • Room Temperature: Approximately $25\,^\circ\text{C}$. At this temperature, many molds will grow at a standard rate.

    • Optimal Fungal Growth: $30\,^\circ\text{C}$ is considered the best temperature to encourage growth.

    • Accelerated Growth: Incubators can be used to speed up growth beyond the standard room temperature rate.

• Microscopic Preparation Techniques

  • Wet Preps: These are temporary mounts.

    • Clinical Application: Urinalysis is a foundational type of wet prep, where liquid urine is examined under a microscope.

    • Lighting and Optics: Wet preps require lowering the light intensity on the microscope and using specific objectives compared to permanent smears.

  • Permanent Stains: These are fixed slides used for detailed examination of red blood cells or bacteria.

• Specialized Stains and their Applications

  • Calcofluor White: A special fluorescent stain that specifically targets and binds to cellulose and chitin in fungal cell walls.

  • Lactophenol Cotton Blue (LPCB): Used for both yeast and mold, this stain enhances the visualization of morphology and fungal structures.

  • India Ink: Classified as a negative stain.

    • Mechanism: The ink darkens the background, leaving the organism visible. It is specifically used to visualize the capsule of certain yeasts, which appears as a clear "halo" around the cell.

• Morphological Terminology

  • Blastoconidia: The technical term for a budding yeast cell.

Mycological Media and Growth Selection

• Primary Isolation Media

  • Sabouraud Dextrose Agar (SAB Dex): The standard primary media used to invite all organisms to grow.

  • SAB Dex with Selective Inhibitors (SAB Dex CC): Uses antimicrobial "bouncers" to prevent overgrowth.

    • Chloramphenicol: An antibiotic added to keep bacteria out of the fungal culture.

    • Cycloheximide: An antifungal agent used to inhibit the growth of environmental molds or certain fungi.

• Enriched and Differential Media

  • Brain Heart Infusion (BHI): An enriched media used to ensure more fastidious organisms grow; it serves as a highly nutritious environment.

  • Cornmeal Tween 80 Agar: A specialized media used to induce specific morphological features in yeast.

    • Function: It induces the formation of pseudohyphae, blastoconidia, and chlamydospores, which are critical for the final identification of genus and species.

  • Birdseed Agar (Staib Agar/Niger Seed Agar): Specifically used for the identification of Cryptococcus neoformans.

Candida albicans: Pathogenesis and Clinical Significance

• Taxonomy and Prevalence

  • Genus: Candida.

  • Species: albicans.

  • It is the most commonly isolated fungus in the microbiology lab.

• Ecological Niche and Opportunism

  • Endogenous Nature: Candida albicans is part of the natural human biome (normal flora). It is native to the mouth, skin, and vagina.

  • Opportunistic Infection: It becomes a pathogen (Candidiasis) when the host becomes compromised due to radiation, chemotherapy, or antibiotic use. Antibiotics decrease normal bacterial flora, removing the biological balance and allowing the yeast to overgrow.

• Clinical Manifestations of Candidiasis

  • Thrush (Oral Candidiasis): A thick, white layer on the tongue.

  • Genital Infections: Vaginitis in females and Balanitis (infection of the area between the scrotum and penis) in males.

  • Onychomycosis: Infection of the nail bed and paronychia (area around the nail).

  • Intertriginous Candidiasis: Infections occurring between digits (fingers and toes) or in skin folds where moisture is trapped.

  • Urinary Tract Infection (UTI): Yeast can be the causative agent of UTIs.

  • Pneumonia: Colonization of the lungs, often diagnosed via sputum samples and visible on chest X-rays.

  • Systemic Infections: These are severe and represent the organism entering body systems.

    • Meningitis: Recovery from cerebrospinal fluid (CSF).

    • Septicemia: Recovery from the blood, which carries the organism throughout the body.

    • Endocarditis: Infection of the heart valves.

Laboratory Identification of Candida albicans

• Diagnostic Algorithm

  1. Specimen Collection: Sputum, tissue, or sterile fluids are received.

  2. Direct Smear Microscopy: Looking for budding yeast and pseudohyphae directly from the specimen.

  3. Primary Culturing: Inoculating onto SAB Dex and SAB Dex CC.

  4. Growth Period: Growth typically occurs within $18\text{--}24\,hr$, though plates are monitored for $24\text{--}72\,hr$.

  5. Subculturing to Cornmeal Tween Agar: Used to confirm species-specific morphology.

• Morphological Markers

  • Pseudohyphae: "False hyphae" formed by elongated blastoconidia that have not fully pinched off. They have visible constrictions at the septa.

  • Chlamydospores: Thick-walled terminal spores. Among Candida species, only Candida albicans produces chlamydospores on Cornmeal Tween agar. Identifying these allows for a final positive diagnosis.

  • Colony Morphology: On primary media, C. albicans appears off-white, shiny, buttery, or waxy.

• The Germ Tube Test

  • Procedure: Inoculate the yeast into rabbit plasma or rabbit serum and incubate at $35\text{--}37\,^\circ\text{C}$. Historically, rabbit systems were chosen because they serve as strong growth mechanisms.

  • Timeframe: Must be read within $1\text{--}3\,hr$. Reading after $4\,hr$ can lead to false positives.

  • Interpretation: A true germ tube is a straight outgrowth from the yeast cell with no constriction at the base.

  • False Positive: If the outgrowth involves “pinching” or constriction, it is representative of pseudohyphae, not a true germ tube.

  • Quality Control (QC):

    • Positive Control: Candida albicans (should produce germ tubes).

    • Negative Control: Candida tropicalis (should remain negative/pinched).

• Rapid Enzymatic Detection

  • Instead of waiting hours for a germ tube, labs use enzymatic kits to detect specific enzymes produced by C. albicans.

  • Enzymes Tested: Prolyl aminopeptidase (Prolase) and $\beta$-D-galactosaminidase (Inglase).

  • Procedure: Yeast is added to a substrate in a tube and incubated for $30\,\text{minutes}$ at $35\text{--}37\,^\circ\text{C}$. Reagents are added to induce a color change (e.g., yellow or pink).

  • Advantages: Reduces subjectivity involved in microscopic interpretation of "pinching" and provides faster results to clinicians.

Questions & Discussion

• Question: Why is it important to differentiate between a sterile and non-sterile specimen?

• Answer: In a sterile specimen like blood or CSF, any organism found is a pathogen. In non-sterile specimens like urine or mouth swabs, the organism might just be normal flora, and we must avoid letting normal flora overgrow so we don't miss a pathogen.

• Question: What is the best temperature to grow fungus?

• Answer: While room temperature is $25\,^\circ\text{C}$, $30\,^\circ\text{C}$ is the optimal temperature for encouraging growth.

• Question: What is the unique characteristic of Calcofluor White?

• Answer: It is a fluorescent stain that targets cellulose and chitin.

• Question: What do we call the “bouncers” in the media and what do they do?

• Answer: Chloramphenicol and Cycloheximide. Chloramphenicol keeps out bacteria; Cycloheximide keeps out environmental molds/fungus.

• Question: How does the germ tube of Candida albicans look under the microscope?

• Answer: It is a straight tube with no constriction (no pinching) where it joins the yeast cell.

• Question: What happens if you read a germ tube test after four hours?

• Answer: You may get a false positive because other species like Candida tropicalis may start to produce structures that look like germ tubes.