Residency · Residency · Pathology
Mycobacterial and Fungal Infections: Laboratory Diagnosis
Introduction
Mycobacterial and fungal infections remain significant causes of morbidity and mortality worldwide, particularly among immunocompromised patients. The laboratory plays a central role in the timely and accurate diagnosis of these infections, employing a combination of traditional and molecular methods.
Mycobacterial Infections
Classification and Clinical Significance
The Mycobacterium tuberculosis complex (MTB) includes M. tuberculosis, M. bovis, and M. africanum. Nontuberculous mycobacteria (NTM) encompass organisms such as M. avium complex, M. kansasii, M. abscessus, and the rapidly growing mycobacteria. All acid-fast bacilli share a unique mycolic acid-rich cell wall that resists decolorization during staining.
Specimen Collection and Processing
Respiratory specimens include sputum (ideally 3 consecutive morning samples), bronchoalveolar lavage, and tissue biopsies. Non-respiratory specimens include urine, CSF, blood (collected by lysis-centrifugation or in mycobacterial blood culture bottles), and bone marrow. Decontamination using the N-acetyl-L-cysteine-sodium hydroxide (NALC-NaOH) method is necessary for non-sterile sites, and concentration by centrifugation improves sensitivity.
Staining Methods
Ziehl-Neelsen staining uses carbol fuchsin with heat fixation and acid-alcohol decolorization; AFB appear red against a blue background. The Kinyoun cold stain uses a similar principle without heat. Auramine-rhodamine fluorochrome staining offers higher sensitivity for screening, and positive smears are confirmed with Ziehl-Neelsen. Smear sensitivity is approximately 5,000-10,000 organisms/mL.
Culture Methods
Solid media such as Lowenstein-Jensen (egg-based) and Middlebrook 7H10/7H11 (agar-based) support growth in 2-8 weeks. Liquid media like the BACTEC MGIT 960 (modified Middlebrook 7H9) detect growth via fluorescence quenching with faster recovery in 1-3 weeks. Culture remains the gold standard with a sensitivity of 10-100 organisms/mL.
Molecular Diagnostics
GeneXpert MTB/RIF provides rapid PCR-based detection of MTB and rifampin resistance via the rpoB gene, with results in approximately 2 hours. Line probe assays such as the Hain GenoType detect MTB complex and common resistance mutations. 16S rRNA sequencing provides definitive identification of NTM species. Nucleic acid amplification tests (NAATs) are recommended on at least one respiratory specimen in suspected TB.
Fungal Infections
Specimen Handling and Direct Examination
KOH preparation using 10-20% KOH dissolves keratin and reveals hyphae, yeast forms, and pseudohyphae. Calcofluor white is a fluorescent stain that binds chitin in fungal cell walls and is viewed under UV microscopy. India ink provides negative staining for the capsule of Cryptococcus, though it has largely been replaced by antigen testing. In histopathology, GMS (Grocott methenamine silver) and PAS stains highlight fungal elements in tissue sections.
Culture Methods
Sabouraud dextrose agar (SDA) is the standard fungal medium incubated at 25-30 degrees C. Brain-heart infusion agar supports dimorphic fungi and is incubated at both 25 and 37 degrees C. Chromogenic media like CHROMagar Candida provide species-level identification of common Candida species by colony color. Incubation periods vary: yeasts require 2-5 days, molds 1-4 weeks, and dimorphic fungi up to 6-8 weeks.
Antigen and Antibody Detection
Cryptococcal antigen (CrAg) by lateral flow assay is highly sensitive and specific for Cryptococcus neoformans and C. gattii in CSF and serum. Galactomannan (GM) detects an Aspergillus cell wall component and is useful in neutropenic patients when followed with serial levels. (1,3)-beta-D-glucan (BDG) is a pan-fungal marker elevated in candidiasis, aspergillosis, and Pneumocystis but lacks specificity. Histoplasma antigen in urine and serum by EIA has cross-reactivity with Blastomyces and Coccidioides. Coccidioides antibodies by complement fixation and immunodiffusion correlate with disease severity through titer levels.
Dimorphic Fungi: Key Features
Histoplasma capsulatum appears as small intracellular yeast (2-4 micrometers) with tuberculate macroconidia in the mold phase. Blastomyces dermatitidis is a broad-based budding yeast measuring 8-15 micrometers. Coccidioides immitis/posadasii forms spherules with endospores in tissue and arthroconidia in culture, requiring biosafety level 3 handling. Talaromyces (Penicillium) marneffei is endemic in Southeast Asia and produces a characteristic red pigment on SDA.
| Dimorphic Fungus | Tissue Form | Size | Key Diagnostic Feature |
|---|---|---|---|
| Histoplasma capsulatum | Small intracellular yeast | 2–4 μm | Within macrophages; narrow-based budding |
| Blastomyces dermatitidis | Broad-based budding yeast | 8–15 μm | Thick refractile cell wall; broad-based bud |
| Coccidioides immitis | Spherules with endospores | 20–60 μm | Endospores released on rupture |
| Talaromyces marneffei | Yeast with central septum | 2–5 μm | Transverse septation (not budding); red pigment on SDA |
MALDI-TOF Mass Spectrometry
MALDI-TOF enables rapid identification of yeasts and some molds from culture by matching protein profiles against reference spectral databases. It reduces time to identification from days to minutes, though limited database coverage for rare molds remains a challenge.
Biosafety Considerations
MTB and dimorphic fungi require BSL-3 containment for culture manipulation. All suspected AFB and fungal cultures should be handled in a biological safety cabinet. Laboratory-acquired infections are well documented with Coccidioides and MTB.
Clinical Pearls
A negative AFB smear does not exclude tuberculosis, and culture along with molecular testing must always be performed. Galactomannan testing can yield false positives with piperacillin-tazobactam and certain dietary exposures. Dimorphic fungi must be handled under BSL-3 conditions, and the laboratory should be notified when these organisms are suspected clinically. MALDI-TOF has revolutionized fungal identification from culture but cannot replace histopathology for tissue-invasive disease.
References
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2022.
- Procop GW, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Wolters Kluwer; 2020.
- Lewinsohn DM, et al. Official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention Clinical Practice Guidelines: Diagnosis of Tuberculosis in Adults and Children. Clin Infect Dis. 2017;64(2):e1-e33.
- Ostrosky-Zeichner L, et al. An insight into the antifungal pipeline: selected new molecules and beyond. Nat Rev Drug Discov. 2010;9(9):719-727.