CSMLS Clinical Microbiology Techniques Questions and Answers — Questions and Answers
Question 1: A clinical microbiology laboratory adopts MALDI-TOF MS for routine bacterial identification. What is the underlying principle of this technology?
- Analysis of ribosomal protein profiles by mass spectrometry (Correct answer)
- Detection of specific nucleic acid sequences using fluorescent probes
- Measurement of enzymatic activity through colorimetric substrate changes
- Determination of fatty acid methyl esters (FAME) by gas chromatography
Correct answer: Analysis of ribosomal protein profiles by mass spectrometry
Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) identifies microorganisms by analyzing the mass of their proteins. A laser strikes a mixture of the organism and a matrix, causing the proteins (primarily ribosomal proteins) to ionize and travel through a vacuum tube. The time it takes for these ions to reach the detector is measured, which corresponds to their mass-to-charge ratio. This creates a unique proteomic fingerprint that is compared to a database for identification.
Question 2: A laboratory in Alberta is following a common two-step algorithm for the diagnosis of Clostridioides difficile infection (CDI) from a stool sample. The first step is a screening test for Glutamate Dehydrogenase (GDH) antigen, which is positive. What is the most appropriate second step to confirm active, toxin-producing CDI?
- Perform a cell culture cytotoxicity assay
- Repeat the GDH antigen test on a new sample
- Perform a nucleic acid amplification test (NAAT) for the toxin B gene (tcdB) (Correct answer)
- Culture the organism on cycloserine-cefoxitin-fructose agar (CCFA)
Correct answer: Perform a nucleic acid amplification test (NAAT) for the toxin B gene (tcdB)
In Canadian laboratories, a common algorithm for C. difficile testing involves a sensitive screening test followed by a specific confirmatory test. A positive GDH antigen test indicates the presence of the C. difficile organism but does not confirm toxin production. Therefore, a positive screen must be followed by a test that specifically detects the toxin or the gene responsible for its production. A nucleic acid amplification test (NAAT), such as PCR for the tcdB gene, is the recommended and widely used confirmatory step to detect toxigenic C. difficile.
Question 3: A medical laboratory technologist performs a Modified Hodge Test (MHT) to screen for carbapenemase production in a Klebsiella pneumoniae isolate. A distinct 'cloverleaf-like' pattern of indented growth of the indicator organism (E. coli ATCC 25922) is observed around the test streak. How should this result be interpreted?
- The test is invalid due to over-inoculation of the indicator strain
- The isolate is negative for carbapenemase production
- The isolate is positive for carbapenemase production (Correct answer)
- The isolate possesses an AmpC beta-lactamase but not a carbapenemase
Correct answer: The isolate is positive for carbapenemase production
The Modified Hodge Test (MHT) is a phenotypic method used to detect carbapenemase production. The principle is that if the test organism produces carbapenemase, it will hydrolyze the carbapenem antibiotic diffusing from the disk. This allows the carbapenem-susceptible indicator strain (E. coli ATCC 25922) to grow towards the disk in the area where the antibiotic has been inactivated. This enhanced growth creates a characteristic cloverleaf-shaped indentation, which indicates a positive result for carbapenemase production.
Question 4: According to Clinical and Laboratory Standards Institute (CLSI) guidelines, which are broadly followed in Canada for antimicrobial susceptibility testing (AST), what is the primary purpose of performing quality control using ATCC reference strains?
- To verify that patient isolates are correctly identified before testing
- To ensure the antimicrobial agents have not expired
- To monitor the precision and accuracy of the test procedure (Correct answer)
- To determine the minimum inhibitory concentration (MIC) for new antibiotics
Correct answer: To monitor the precision and accuracy of the test procedure
Quality control in antimicrobial susceptibility testing, using well-characterized reference strains from the American Type Culture Collection (ATCC), is essential for monitoring the entire test system. This includes the performance of the media, the antimicrobial disks or solutions, incubation conditions, and the technologist's technique. By testing these stable strains with known susceptibility profiles, the laboratory can ensure the precision and accuracy of the results obtained for patient isolates. This is a core principle of quality management outlined by standards bodies like CLSI.
Question 5: A technologist receives a cerebrospinal fluid (CSF) specimen in the microbiology lab from a suspected case of bacterial meningitis. The specimen is received at 3:00 PM and the Gram stain is performed immediately. The day shift ends at 4:00 PM. Which of the following actions is most critical regarding the culture of this specimen?
- Inoculate the culture plates and leave them at room temperature for the evening shift to incubate.
- Refrigerate the specimen at 4°C until the next morning to ensure a fresh subculture.
- Immediately inoculate the specimen onto appropriate media, such as Chocolate and Blood agar, and place in a CO2 incubator. (Correct answer)
- Perform an antigen detection test and wait for the results before setting up the culture.
Correct answer: Immediately inoculate the specimen onto appropriate media, such as Chocolate and Blood agar, and place in a CO2 incubator.
CSF is considered a critical, irreplaceable specimen. Potential pathogens that cause meningitis, such as Neisseria meningitidis and Streptococcus pneumoniae, are fastidious and sensitive to temperature changes. The specimen must be processed immediately upon receipt. Delay or improper storage (like refrigeration, which can kill N. meningitidis) can lead to false-negative culture results. Immediate inoculation onto appropriate enriched media (like Chocolate agar) and incubation in a CO2-enriched atmosphere is crucial for the recovery of these delicate organisms.
Question 6: Which of the following is the most appropriate method for sterilizing a heat-sensitive solution containing antibiotics for use in culture media?
- Autoclaving at 121°C for 15 minutes
- Filtration through a 0.22 µm pore size membrane (Correct answer)
- Exposure to ultraviolet (UV) radiation for 30 minutes
- Boiling at 100°C for 10 minutes
Correct answer: Filtration through a 0.22 µm pore size membrane
Heat-sensitive solutions, such as those containing antibiotics, vitamins, or certain sugars, will be degraded by the high temperatures of an autoclave. The most appropriate method for sterilizing these liquids is filtration. Using a membrane filter with a pore size of 0.22 micrometers will effectively remove bacteria and fungal spores without altering the chemical composition of the solution. The other methods involve heat or radiation which would destroy the heat-labile components.
CSMLS Clinical Microbiology Techniques Questions and Answers
A clinical microbiology laboratory adopts MALDI-TOF MS for routine bacterial identification.
What is the underlying principle of this technology?