MLPAO Microbiology and Parasitology Diagnostic Methods 2 — Questions and Answers
Question 1: Which staining method is used to identify Pneumocystis jirovecii in respiratory specimens?
- Gram stain
- Gomori methenamine silver (GMS) stain or immunofluorescence (Correct answer)
- Acid-fast stain
- Wright-Giemsa stain only
Correct answer: Gomori methenamine silver (GMS) stain or immunofluorescence
Gomori methenamine silver (GMS) stain highlights the cyst wall of Pneumocystis jirovecii as dark brown/black against a green background. Direct fluorescent antibody (DFA) testing is more sensitive and specific. PCR is increasingly used as it can detect both cystic and trophic forms. P. jirovecii causes pneumonia in immunocompromised patients.
Question 2: What is the principle of blood culture gram stain and how should results be communicated?
- Results are communicated within 24 hours as routine
- Gram stain is performed immediately when the instrument flags positive, and results are communicated urgently to the physician (Correct answer)
- Gram stain is only performed after subculture growth
- Results are only communicated if the organism is identified to species
Correct answer: Gram stain is performed immediately when the instrument flags positive, and results are communicated urgently to the physician
When an automated blood culture system flags a bottle as positive, the Gram stain is performed immediately to provide preliminary organism identification. This result must be communicated urgently (STAT) to the physician as it guides empiric antibiotic therapy. The Gram stain morphology (e.g., gram-positive cocci in clusters) is critical for initial treatment decisions.
Question 3: Which serological test is used for the diagnosis of syphilis and what is the screening algorithm used in Canada?
- RPR only; no confirmation needed
- Reverse algorithm: treponemal test (EIA/CIA) screen followed by RPR and confirmatory treponemal test (Correct answer)
- FTA-ABS as the sole test
- Dark-field microscopy for all stages
Correct answer: Reverse algorithm: treponemal test (EIA/CIA) screen followed by RPR and confirmatory treponemal test
Many Canadian laboratories use the reverse syphilis screening algorithm: a treponemal-specific immunoassay (EIA or CIA) is used for screening, followed by RPR (a non-treponemal test to assess disease activity) and a second treponemal test (TP-PA or FTA-ABS) for confirmation. This approach has higher sensitivity for detecting latent syphilis.
Question 4: What is the KOH (potassium hydroxide) preparation used for in clinical microbiology?
- Identifying bacteria in blood cultures
- Dissolving keratin and cellular debris to reveal fungal elements in clinical specimens (Correct answer)
- Staining parasites in stool
- Detecting virus-infected cells
Correct answer: Dissolving keratin and cellular debris to reveal fungal elements in clinical specimens
KOH preparation dissolves keratin, cellular material, and debris in specimens (skin scrapings, nail clippings, hair) while leaving fungal cell walls intact. This allows visualization of fungal hyphae, pseudohyphae, and yeast cells under microscopy. Adding calcofluor white and viewing under fluorescence microscopy enhances sensitivity.
Question 5: Which test differentiates Enterococcus from Group D Streptococcus?
- Catalase test
- Growth in 6.5% NaCl broth (Correct answer)
- Coagulase test
- Optochin susceptibility
Correct answer: Growth in 6.5% NaCl broth
Enterococci grow in 6.5% NaCl broth while Group D streptococci (S. bovis group) cannot. Both are bile esculin-positive. This salt tolerance test is important because enterococci have different antibiotic resistance patterns and treatment implications than Group D streptococci.
Question 6: What is the purpose of the modified Kinyoun stain (modified acid-fast stain) in parasitology?
- Detecting malaria parasites
- Detecting Cryptosporidium, Cyclospora, and Isospora oocysts in stool specimens (Correct answer)
- Identifying hookworm larvae
- Staining Giardia cysts
Correct answer: Detecting Cryptosporidium, Cyclospora, and Isospora oocysts in stool specimens
The modified acid-fast stain (using a weaker decolorizer than the standard Ziehl-Neelsen) detects partially acid-fast parasites including Cryptosporidium (4-6 µm, round, pink), Cyclospora (8-10 µm, variable staining), and Cystoisospora (Isospora) oocysts in stool. These parasites are not visible on routine ova and parasite examination.
MLPAO Microbiology and Parasitology Diagnostic Methods 2
Which staining method is used to identify Pneumocystis jirovecii in respiratory specimens?