MLPAO Microbiology and Parasitology Diagnostic Methods 1 — Questions and Answers
Question 1: What is the principle of the latex agglutination test used in microbiology?
- Measuring growth inhibition by antibiotics
- Latex particles coated with antibodies agglutinate in the presence of target antigen (Correct answer)
- Measuring enzyme activity of bacterial isolates
- Detecting nucleic acid sequences by amplification
Correct answer: Latex particles coated with antibodies agglutinate in the presence of target antigen
Latex agglutination tests use latex particles coated with specific antibodies. When mixed with a specimen containing the target antigen, the antibody-coated particles cross-link, forming visible agglutination. This rapid technique is used for identifying organisms (e.g., Cryptococcus, Group B Strep) and detecting antigens in CSF and other fluids.
Question 2: Which molecular method is the gold standard for detecting Chlamydia trachomatis and Neisseria gonorrhoeae?
- Culture on Thayer-Martin agar
- Nucleic acid amplification test (NAAT) (Correct answer)
- Gram stain of discharge
- Enzyme immunoassay
Correct answer: Nucleic acid amplification test (NAAT)
NAAT (nucleic acid amplification test) is the gold standard for detecting C. trachomatis and N. gonorrhoeae due to its superior sensitivity and specificity. It can be performed on urine, endocervical, urethral, pharyngeal, and rectal specimens. Culture is still needed for antimicrobial susceptibility testing of N. gonorrhoeae.
Question 3: What is the purpose of performing a Gram stain on cerebrospinal fluid (CSF)?
- To culture the organism
- To rapidly identify the presence and morphology of bacteria for urgent empiric antibiotic selection (Correct answer)
- To determine antibiotic susceptibility
- To measure protein levels
Correct answer: To rapidly identify the presence and morphology of bacteria for urgent empiric antibiotic selection
Gram stain of CSF provides rapid (minutes) preliminary identification of bacterial meningitis pathogens based on morphology: gram-positive diplococci (S. pneumoniae), gram-negative diplococci (N. meningitidis), or gram-positive rods (Listeria). This guides empiric antibiotic therapy before culture results are available.
Question 4: Which chromogenic medium is used for screening of MRSA in clinical laboratories?
- MacConkey agar
- CHROMagar MRSA or similar chromogenic MRSA medium (Correct answer)
- Blood agar
- Sabouraud dextrose agar
Correct answer: CHROMagar MRSA or similar chromogenic MRSA medium
Chromogenic MRSA media contain cefoxitin (selecting for MRSA) and chromogenic substrates that produce distinctive coloured colonies (typically pink/mauve for S. aureus). These media provide rapid screening results (18-24 hours) for MRSA colonization, which is important for infection prevention and control programs.
Question 5: What is the India ink preparation used to detect?
- Acid-fast bacilli
- Cryptococcus neoformans capsule in CSF (Correct answer)
- Plasmodium species
- Gram-negative diplococci
Correct answer: Cryptococcus neoformans capsule in CSF
The India ink preparation is a negative staining technique that reveals the thick polysaccharide capsule of Cryptococcus neoformans in CSF. The ink particles are excluded by the capsule, creating a clear halo around the yeast cell against a dark background. The cryptococcal antigen test (CrAg) has largely replaced India ink due to higher sensitivity.
Question 6: What is the purpose of the beta-lactamase test in clinical microbiology?
- To identify the species of a bacterial isolate
- To rapidly detect the production of beta-lactamase enzyme that inactivates penicillins (Correct answer)
- To determine the organism's Gram reaction
- To test for antimicrobial susceptibility to vancomycin
Correct answer: To rapidly detect the production of beta-lactamase enzyme that inactivates penicillins
The beta-lactamase test (using nitrocefin or acidimetric methods) rapidly detects the production of beta-lactamase enzymes that hydrolyze the beta-lactam ring of penicillins. It is particularly important for Haemophilus influenzae, Neisseria gonorrhoeae, and staphylococci to determine if penicillin-class antibiotics can be used.
MLPAO Microbiology and Parasitology Diagnostic Methods 1
What is the principle of the latex agglutination test used in microbiology?