MLT GlobeBacteriology: Culture media, specimens & AST – page 5
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
An E. coli from blood has a ceftriaxone MIC of 2 µg/mL. The CLSI breakpoints are: susceptible ≤1, intermediate 2, resistant ≥4 µg/mL. How should the result be reported?
An MIC of 2 µg/mL falls exactly on the intermediate category for ceftriaxone in Enterobacterales. Ceftriaxone has no susceptible-dose dependent category in these breakpoints.
In CLSI reports, the category 'susceptible-dose dependent' (SDD) means that:
SDD means the isolate can be treated successfully if a dosing regimen giving higher drug exposure is used, such as higher-dose cefepime for Enterobacterales. It is not a technical error flag.
In an MBC test, broth from clear MIC wells is subcultured. The minimum bactericidal concentration is the lowest concentration that:
MBC is the lowest concentration that produces a 3-log (99.9%) or greater reduction in viable count on subculture. No visible turbidity only defines the MIC.
Which Staphylococcus aureus QC strain is recommended by CLSI for broth microdilution MIC testing?
S. aureus ATCC 29213 is the MIC QC strain, while ATCC 25923 is used for disk diffusion. ATCC 43300 is an MRSA control strain, and ATCC 700603 is an ESBL-producing K. pneumoniae.
Why does CLSI state that disk diffusion should not be used to test vancomycin against Staphylococcus aureus?
Vancomycin is a large molecule that diffuses poorly, so VISA strains give zones similar to susceptible strains. An MIC method is required for S. aureus and vancomycin.
An Enterococcus faecalis from endocarditis shows no zone (6 mm) around a 120 µg gentamicin disk. This result means:
The high-content gentamicin disk screens for high-level aminoglycoside resistance. A 6 mm zone means the aminoglycoside will not act synergistically with ampicillin or vancomycin. The standard 10 µg disk is not used for this purpose.
Why is disk diffusion not recommended for colistin susceptibility testing of gram-negative rods?
Colistin is a large cationic molecule that diffuses poorly, so zones are small and do not separate susceptible from resistant strains. CLSI recommends broth microdilution, broth disk elution or agar test methods.
A Streptococcus pneumoniae isolate from sputum shows a 23 mm zone around a 1 µg oxacillin disk. How should this be interpreted for penicillin?
An oxacillin zone of 20 mm or more predicts penicillin susceptibility in pneumococci. Zones below 20 mm need a penicillin MIC, because resistance is due to altered penicillin-binding proteins, not beta-lactamase.
When preparing a cumulative antibiogram according to CLSI M39, the laboratory should include:
M39 uses the first isolate per patient per period to avoid bias from repeat cultures of resistant organisms. It also recommends at least 30 isolates of a species for reliable percentages.
In disk diffusion testing, plates should go into the incubator within 15 minutes after the disks are applied. If they stand for an hour at room temperature, the zones will most likely be:
Drug keeps diffusing at room temperature while bacterial growth is delayed, so the zone boundary forms farther out. Inactivation of the drug is not the main effect.
For Staphylococcus aureus, CLSI recommends the cefoxitin disk rather than the oxacillin disk for disk diffusion because cefoxitin:
Cefoxitin strongly induces mecA/PBP2a expression and gives more reliable, easier-to-read results; a resistant cefoxitin result is reported as oxacillin resistant. It is a surrogate, not a treatment choice for MRSA.
In the Carba NP test, a carbapenemase-producing isolate changes the phenol red indicator from red to yellow because:
Hydrolysis of the beta-lactam ring of imipenem produces a carboxylic acid, which lowers the pH and turns phenol red yellow. Ammonia would raise the pH, not lower it.
The CLSI agar screen for vancomycin resistance in enterococci uses:
BHI agar containing 6 µg/mL vancomycin is spot-inoculated; growth of more than one colony suggests resistance. The salt-oxacillin agar screens staphylococci for oxacillin resistance.
A nasopharyngeal specimen for culture of Bordetella pertussis should be inoculated onto:
Regan-Lowe (charcoal-horse blood with cephalexin) neutralises inhibitory fatty acids and supports B. pertussis. Cystine-tellurite agar is used for Corynebacterium diphtheriae.
Blood culture bottles are designed so that blood is diluted in broth at a ratio of about:
A 1:5 to 1:10 dilution reduces the effect of natural bactericidal factors and antibiotics in blood while keeping enough blood for detection.
Some blood culture bottles contain resins or activated charcoal. Their main purpose is to:
Resins and charcoal adsorb antibiotics, improving recovery from patients already on therapy. Growth detection is by separate CO2 sensors.
An intravenous catheter tip is rolled across a blood agar plate (semiquantitative method). Which result suggests catheter-related infection?
In the roll-plate (Maki) method, 15 or more colonies suggest the catheter is colonised and a likely source of infection. Fewer colonies usually reflect contamination.
When a throat swab is plated on sheep blood agar for group A streptococci, the agar is stabbed several times with the loop. This is done to:
Streptolysin O is inactivated by oxygen, so haemolysis is best seen in the stab where oxygen is reduced. CAMP testing needs a S. aureus streak, not stabs.
A urethral swab for Neisseria gonorrhoeae culture will be delayed before plating. Which handling is correct?
N. gonorrhoeae is very sensitive to cold and drying; charcoal-containing transport medium at room temperature helps survival. Refrigeration kills the organism.
For disk diffusion testing, which organism must be prepared by the direct colony suspension method rather than the growth (broth culture) method?
CLSI requires direct colony suspension for fastidious organisms such as Haemophilus, streptococci and Neisseria, and for staphylococci when testing oxacillin. Non-fastidious gram-negative rods can use either method.