MLT GlobeBacteriology: Culture media, specimens & AST – page 2
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
A stool sample from a suspected cholera case should be plated on which selective medium?
Thiosulphate-citrate-bile salts-sucrose agar is alkaline and selective for vibrios. Blood and nutrient agar are non-selective; CLED is for urine.
Beta-lactam antibiotics such as penicillins and cephalosporins act by inhibiting:
Beta-lactams bind penicillin-binding proteins and block peptidoglycan cross-linking, causing cell lysis.
Ciprofloxacin and other fluoroquinolones kill bacteria by inhibiting:
Fluoroquinolones inhibit DNA gyrase (topoisomerase II) and topoisomerase IV. Rifampicin inhibits RNA polymerase.
Peptone water, containing only peptone and sodium chloride, is an example of a:
Basal media support non-fastidious organisms without additions. Enriched media add blood or serum; defined media have known chemical composition.
A stab culture shows diffuse cloudiness spreading from the stab line in a soft medium. The medium used for this motility test is:
Motility media contain low agar (semisolid) so motile bacteria can migrate. Solid agar would restrict growth to the stab line.
Which medium is commonly used to culture urine because it supports urinary pathogens and prevents Proteus swarming?
Cystine-lactose-electrolyte-deficient agar lacks electrolytes, stopping swarming, and shows lactose fermentation.
Prepared agar plates are best stored:
Cold storage in sealed bags prevents drying and contamination, keeping plates usable for weeks. Light-sensitive media also need darkness.
To recover Staphylococcus aureus from faeces with mixed flora, the best medium is:
7.5% NaCl suppresses most other bacteria, and S. aureus ferments mannitol giving yellow colonies. EMB and MacConkey select gram-negatives.
Mycobacterium tuberculosis on Löwenstein–Jensen medium usually takes how long to show visible colonies?
M. tuberculosis divides every 15–20 hours, so colonies take 3–6 weeks on LJ; LJ is held up to 8 weeks. Liquid MGIT systems are faster.
Which antibiotic is bactericidal by acting on the bacterial cell wall?
Amoxicillin is a beta-lactam blocking peptidoglycan cross-linking. Gentamicin, tetracycline and erythromycin act on ribosomes.
Among Enterobacterales, which route accounts for most horizontal spread of acquired antimicrobial resistance genes between cells?
Conjugation of resistance plasmids (often carrying transposons and integrons) is the main vehicle of horizontal resistance transfer. Point mutation is vertical, not transfer; transformation and transduction occur but are less important in Gram-negative rods.
For Kirby-Bauer disk diffusion, the inoculum is adjusted to a turbidity of:
CLSI disk diffusion uses a 0.5 McFarland suspension swabbed onto Mueller-Hinton agar. A heavier inoculum gives falsely small zones (false resistance); a lighter one gives false susceptibility.
Thayer-Martin agar is used to isolate:
Thayer-Martin is chocolate agar with vancomycin, colistin and nystatin (plus trimethoprim in modified forms). These inhibit the normal flora of genital sites so pathogenic Neisseria can grow.
A CSF sample for bacterial culture from a patient with suspected meningitis should be:
Organisms such as Neisseria meningitidis and Haemophilus influenzae are killed by cold. CSF for bacterial culture must never be refrigerated and should be processed immediately as a stat specimen.
In most continuously monitored blood culture systems, routine bottles are incubated for how long before being reported as no growth?
Five days of incubation detects almost all clinically significant bacteria and yeasts, including HACEK organisms, in modern systems. Longer incubation is only needed in special situations.
Chocolate agar supports the growth of Haemophilus and Neisseria because heating the blood:
Gentle heating lyses red cells, releasing hemin and NAD, and inactivates NADases in the blood. This makes chocolate agar enriched, not selective.
Which components of MacConkey agar inhibit gram-positive bacteria?
Crystal violet and bile salts make MacConkey selective for gram-negative bacteria. Lactose with neutral red is the differential system showing lactose fermentation as pink colonies.
A medium that contains an indicator so that organisms can be told apart by a biochemical reaction, without inhibiting their growth, is called:
A differential medium shows different colony reactions (for example lactose fermentation) through an indicator. A selective medium inhibits some organisms, and an enriched medium adds nutrients for fastidious ones.
The minimum inhibitory concentration (MIC) is defined as the:
The MIC is the lowest concentration in a dilution series that inhibits visible growth after standard incubation. Killing 99.9% of the inoculum defines the minimum bactericidal concentration (MBC).
A rapid beta-lactamase test on Haemophilus influenzae uses nitrocefin. A positive result is shown by:
Nitrocefin is a chromogenic cephalosporin; when its beta-lactam ring is broken, it changes from yellow to red. Bubbling is the catalase reaction.