MLT GlobeBacteriology: Culture media, specimens & AST – page 3
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
When a broth microdilution panel is inoculated, a small amount of the inoculum is also streaked onto a blood agar plate. The main purpose of this 'purity plate' is to:
The purity plate shows whether the inoculum was a pure culture; a mixed culture gives unreliable MICs. The growth control well checks viability, not purity.
Before collecting a blood culture, the venipuncture site is scrubbed with chlorhexidine in alcohol. What is the most important next step?
The antiseptic needs contact time and must dry (about 30 seconds for chlorhexidine) to reduce skin flora. Re-palpating the cleaned site with an ungloved finger recontaminates it.
A formed (solid) stool is received for Clostridioides difficile toxin testing. The correct action is to:
Testing is limited to unformed stool (taking the shape of the container) to avoid detecting colonised patients without disease. Anaerobic culture does not solve this problem.
A urine is plated with a 0.001 mL calibrated loop. After overnight incubation, 120 colonies of one organism are counted. The colony count is:
With a 0.001 mL loop, multiply the colony count by 1000: 120 × 1000 = 120,000 CFU/mL (1.2 × 10^5). A 0.01 mL loop would use a factor of 100.
A suprapubic bladder aspirate from an infant grows 500 CFU/mL of E. coli. How should this be interpreted?
Suprapubic aspiration bypasses the urethra, so any growth of a uropathogen is significant. The 10^5 CFU/mL rule applies to voided midstream specimens.
Which transport medium is most suitable for a stool sample for Salmonella, Shigella and Campylobacter culture that cannot be plated within 2 hours?
Cary-Blair is a low-nutrient, semisolid transport medium with a high pH that keeps enteric pathogens alive without overgrowth. Buffered glycerol saline is poor for Campylobacter.
A tissue biopsy for bacterial culture arrives in 10% formalin. What should the laboratory do?
Formalin kills bacteria, so a fixed specimen cannot give a valid culture result. Tissue for culture should be sent sterile, unfixed, in a small amount of saline.
Which medium does CLSI recommend for disk diffusion testing of Haemophilus influenzae?
HTM is Mueller-Hinton base with haematin, NAD and yeast extract, and it is transparent for reading zones. Chocolate agar is not standardised for AST.
Mannitol salt agar is both selective and differential for Staphylococcus aureus because it contains:
The 7.5% salt inhibits most other bacteria, and S. aureus ferments mannitol, turning phenol red yellow. Crystal violet, lactose and neutral red are MacConkey components.
Which substance is used to solidify most bacteriological culture media?
Agar, a seaweed polysaccharide, melts near 100 °C, sets at about 40 °C and is not broken down by most bacteria, so it is the standard solidifying agent.
At what temperature are most routine clinical bacterial cultures incubated?
Most human pathogens grow best near body temperature, so routine cultures are incubated at 35–37 °C.
Which urine specimen is preferred for routine culture from an adult who can collect it?
A clean-catch midstream sample reduces contamination by urethral and skin flora; bag urine and first-void drops are heavily contaminated.
Partial haemolysis with a greenish zone around colonies on blood agar is called:
Alpha haemolysis is incomplete lysis giving green discolouration; beta is complete clearing, and gamma means no haemolysis.
In a disk diffusion test, a larger zone of inhibition around an antibiotic disk generally means the organism is:
The drug diffuses outward from the disk; the more susceptible the organism, the wider the area in which it cannot grow.
For culture of a deep wound infection, which specimen is generally best?
Aspirated pus or tissue contains the true pathogens in larger numbers, while surface swabs mainly pick up colonising skin flora.
When is the best time to collect an expectorated sputum sample for bacterial culture?
An early-morning deep-cough sample contains secretions pooled overnight in the lower airways; saliva is not suitable.
Blood cultures should ideally be collected at what point in patient care?
Antibiotics already in the blood can prevent organisms from growing in the bottles, so cultures should be drawn before therapy begins.
Trimethoprim-sulfamethoxazole (co-trimoxazole) kills or inhibits bacteria by blocking:
Sulfamethoxazole and trimethoprim block two sequential steps in bacterial folate synthesis, which is needed for nucleotide production.
Which of these antibiotics is a carbapenem?
Meropenem, imipenem and ertapenem are carbapenems; ceftriaxone is a cephalosporin, aztreonam a monobactam and amoxicillin a penicillin.
Which pair of antibiotics acts by blocking bacterial ribosomal protein synthesis?
Aminoglycosides bind the 30S subunit and chloramphenicol the 50S subunit. Beta-lactams and vancomycin target cell wall; rifampicin blocks RNA polymerase and nalidixic acid DNA gyrase.