MLPAO Microbiology and Parasitology Diagnostic Methods Flashcards

What is the principle of the latex agglutination test used in microbiology?

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.

Which molecular method is the gold standard for detecting Chlamydia trachomatis and Neisseria gonorrhoeae?

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.

What is the purpose of performing a Gram stain on cerebrospinal fluid (CSF)?

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.

Which chromogenic medium is used for screening of MRSA in clinical laboratories?

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.

What is the India ink preparation used to detect?

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.

What is the purpose of the beta-lactamase test in clinical microbiology?

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.