MLT GlobeBacteriology: Culture media, specimens & AST
126 Bacteriology MCQs on Culture media, specimens & AST with answers and explanations.
A 5-day-old baby has fever, poor feeding and a bulging fontanelle. Which specimen gives the definitive laboratory diagnosis of meningitis?
Meningitis is confirmed by examining the patient's own CSF (cell count, Gram stain, culture). Maternal swabs may show a colonising source but do not diagnose the infant's meningitis.
A patient with 7 days of sustained fever is suspected to have enteric fever. Which test is the reference (gold-standard) routine laboratory diagnosis?
Culture isolates S. Typhi and allows susceptibility testing; blood culture is the routine standard (bone marrow is even more sensitive). Widal and rapid serology have poor specificity in endemic areas.
Which medium is recommended by CLSI and EUCAST for routine disc diffusion susceptibility testing of rapidly growing aerobes?
Mueller-Hinton agar is standardised for reproducibility, supports most pathogens and has low levels of sulphonamide/trimethoprim inhibitors. Nutrient agar is not standardised for AST.
A stool sample for suspected cholera will reach the lab after 24 hours. Which transport medium should be used?
Cary-Blair is a semi-solid, low-nutrient buffered medium that keeps enteric pathogens, including Vibrio, viable without overgrowth. Alkaline peptone water is used for short enrichment rather than prolonged transport.
Adding blood, serum or egg to a basal medium to support fastidious organisms produces which type of medium?
Enriched media (e.g. blood agar) add nutrients for fastidious bacteria. 'Enrichment' broth, by contrast, favours one organism over competitors.
Christensen's urea agar turns pink within hours when inoculated with Proteus. In this role the medium is best described as a(n):
Urease releases ammonia, raising pH so phenol red turns pink; the medium distinguishes organisms by a colour change rather than suppressing growth.
What is the purpose of the streak plate technique?
Each successive streak thins the inoculum until single cells are deposited separately, giving isolated colonies for pure culture.
In a sugar fermentation broth, a small inverted Durham tube is used to detect:
Gas from fermentation collects as a bubble in the inverted tube. pH change is shown separately by the broth's indicator.
Glucose broth is prepared by adding glucose to nutrient broth at what concentration?
About 0.5-1% glucose provides a fermentable sugar and extra energy; higher concentrations are not used for routine glucose broth.
In MacConkey agar, neutral red changes colour when lactose fermentation acidifies the medium. What colour does it produce?
Neutral red is colourless/yellowish above about pH 8 and pink-red below about pH 6.8, giving pink lactose-fermenting colonies.
In media containing phenol red, acid production from sugar fermentation turns the indicator:
Phenol red is red at neutral pH, yellow below about pH 6.8 and deep pink above about pH 8.4, as used in TSI and urea media.
Standard blood agar for routine bacteriology, giving reliable haemolysis patterns, is prepared with blood from which animal?
Defibrinated sheep blood gives clear haemolysis reactions and inhibits Haemophilus haemolyticus; horse blood is used for some special media.
What concentration of defibrinated blood is normally added to the agar base when preparing blood agar?
Blood agar is typically made with 5% (up to 10%) sterile defibrinated blood added to a base cooled to about 45-50 °C.
Which antibiotic is a macrolide rather than an aminoglycoside?
Erythromycin is a macrolide binding the 50S subunit. Amikacin, neomycin and gentamicin are aminoglycosides acting on the 30S subunit.
MacConkey agar is primarily used to isolate and differentiate:
Bile salts and crystal violet inhibit Gram-positives, while lactose and neutral red differentiate lactose fermenters among Gram-negative rods.
Robertson's cooked meat medium is used mainly to culture:
Meat particles contain reducing substances (glutathione) that lower the redox potential, supporting anaerobes; saccharolytic clostridia turn meat pink, proteolytic ones blacken it.
Campylobacter needs a 'microaerobic' incubation atmosphere. What does this term describe?
Microaerophiles need oxygen but only at low tension (roughly 5–10% O2). A candle jar or CO2 incubator still contains about 15–18% O2, which is too much; anaerobes need no oxygen at all.
Which penicillin keeps its activity against penicillinase-producing Staphylococcus aureus?
Isoxazolyl penicillins such as cloxacillin have a side chain that blocks staphylococcal beta-lactamase. Ampicillin, penicillin G and carbenicillin are hydrolysed.
A urine sample for culture cannot reach the laboratory within 2 hours and no boric acid container is available. Best action?
Refrigeration slows bacterial multiplication so colony counts stay meaningful for up to 24 hours. Room temperature allows overgrowth; acids kill organisms.
MacConkey agar contains crystal violet and bile salts. What is the role of the crystal violet?
Crystal violet and bile salts inhibit gram-positive organisms, making the medium selective. Neutral red is the pH indicator showing lactose fermentation.