MLT GlobeBacteriology: Culture media, specimens & AST – page 6
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
Disk diffusion testing of Streptococcus pneumoniae is performed on:
Pneumococci need blood supplementation and CO2 for adequate growth, and the incubation time is extended to 20–24 hours. Plain MH in air is used for Enterobacterales.
An Enterococcus faecalis from urine shows a large zone around a cefazolin disk. How should cephalosporins be reported?
Enterococci have low-affinity penicillin-binding proteins and are intrinsically resistant to cephalosporins, which can appear active in vitro. Reporting them as susceptible could lead to treatment failure.
A laboratory reports carbapenem results for an E. coli only when the isolate is resistant to third-generation cephalosporins. This practice is called:
Cascade reporting shows broad-spectrum agents only when narrower agents are resistant, supporting antimicrobial stewardship. A cumulative antibiogram summarises many isolates over time.
During verification of a new AST system, an isolate is reported 'susceptible' by the new system but is 'resistant' by the reference method. This is classified as:
A false-susceptible result is a very major error because it may lead to ineffective therapy. A false-resistant result is a major error; a minor error involves the intermediate category.
Cystine-lactose-electrolyte-deficient (CLED) agar is useful for urine cultures because it:
The lack of electrolytes prevents Proteus from swarming, so other colonies can be counted. CLED supports both gram-positive and gram-negative uropathogens.
An Enterobacter cloacae isolate tests susceptible to ceftriaxone. CLSI advises that resistance may develop during therapy with third-generation cephalosporins because this organism:
Enterobacter cloacae has a chromosomal AmpC that can be induced or selected (derepressed mutants) during therapy. PBP2a is the staphylococcal mecA product.
For prenatal screening of group B streptococcus colonisation, the recommended specimen and processing is:
Swabbing both the lower vagina and rectum and using selective enrichment broth (e.g. LIM broth) gives the highest recovery. Cervical swabs and MacConkey agar are not suitable.
Blood culture bottles are collected at night, and loading onto the continuous monitoring system will be delayed. How should they be held?
Bottles should stay at room temperature; refrigeration can kill fastidious organisms. Long pre-incubation outside the instrument can also cause missed positives, so delay should be minimised.
Antimicrobial disk cartridges are taken from the freezer. Why should the closed container be allowed to reach room temperature before opening?
Opening a cold container lets moisture condense on the disks, and moisture degrades labile drugs such as beta-lactams. Disks are already active and sterile.
In disk diffusion testing, several distinct colonies grow inside an otherwise clear zone. The correct first action is to:
Colonies within a zone may be a contaminant or a resistant subpopulation. Purity is checked; if the culture is pure, the colony-free inner zone is measured.
A wound culture has Proteus swarming over the whole blood agar plate, hiding a possible Staphylococcus aureus. Which medium best helps recover the gram-positive coccus?
Phenylethyl alcohol inhibits gram-negative rods and prevents Proteus swarming, allowing gram-positive cocci to grow as isolated colonies. MacConkey and Hektoen inhibit gram-positive organisms.
Aminoglycosides such as gentamicin inhibit protein synthesis by binding to which bacterial ribosomal subunit?
Aminoglycosides bind the 30S subunit and cause misreading of mRNA; 40S and 60S are eukaryotic subunits.
Which quality control strain is routinely used by CLSI for disk diffusion testing of Enterobacterales?
E. coli ATCC 25922 is the standard QC strain for disk diffusion of Enterobacterales; S. aureus ATCC 29213 is used for MIC testing of staphylococci.
A penicillin-non-susceptible Streptococcus pneumoniae is isolated from a patient with pneumonia. Which beta-lactam is usually still effective?
Altered penicillin-binding proteins reduce aminopenicillin activity, but third-generation cephalosporins like ceftriaxone usually remain active; vancomycin is added for meningitis.
A continuously monitored blood culture bottle signals positive, but the Gram stain shows no organisms. What is the most appropriate next step?
Low numbers of organisms, or organisms that stain poorly, may be missed on Gram stain; acridine orange is more sensitive, and subculture confirms growth. Very high white cell counts can also give false signals.
Enterococcus faecalis ATCC 29212 gives a very small zone around trimethoprim-sulfamethoxazole on a new lot of Mueller-Hinton agar. The most likely problem is:
Thymidine lets bacteria bypass folate inhibition, falsely lowering trimethoprim-sulfonamide activity. E. faecalis ATCC 29212 is used to check that thymidine content is acceptably low. pH 7.2–7.4 is the normal range.
Pseudomonas aeruginosa ATCC 27853 shows aminoglycoside zones below the QC range, while other drugs are in range. The most likely medium problem is:
High divalent cation levels reduce aminoglycoside uptake by P. aeruginosa, giving smaller zones (false resistance). Low cations have the opposite effect. Low zinc affects carbapenem testing, not aminoglycosides.
On a gradient diffusion strip, the edge of the ellipse of inhibition meets the strip between the 0.19 and 0.25 µg/mL marks. What MIC should be reported?
Gradient strips have values between standard two-fold dilutions. The reading is taken where the ellipse meets the strip and is rounded up to the next two-fold dilution, 0.25 µg/mL, before interpretation.
A laboratory wants to change routine AST QC from daily to weekly testing. According to CLSI, which result supports this change?
CLSI allows weekly QC after a 15-replicate plan (or the 20/30-day plan) with acceptable results for each drug and QC strain. Five days is too few, and proficiency testing does not replace QC.
A Staphylococcus aureus has a penicillin zone of 30 mm. The edge of the zone is sharp and clear-cut ('cliff') rather than fuzzy ('beach'). How should penicillin be reported?
CLSI recommends the penicillin zone-edge test for S. aureus with large zones. A sharp 'cliff' edge shows beta-lactamase production, so the isolate is reported resistant despite the large zone. A fuzzy 'beach' edge suggests no beta-lactamase.