MLT GlobeBacteriology: Non-fermenters & other GNR – page 2
76 Bacteriology MCQs on Non-fermenters & other GNR with answers and explanations.
Very mucoid colonies of Pseudomonas aeruginosa from the sputum of a patient with cystic fibrosis are caused by overproduction of:
Mucoid strains overproduce alginate, an exopolysaccharide that forms biofilm and protects the organism in the CF lung. Pyocyanin is a pigment and does not cause mucoid colonies.
Which incubation conditions are recommended for isolating Campylobacter jejuni from stool?
C. jejuni is thermophilic and microaerophilic; about 5% O2, 10% CO2 at 42 °C on selective media favours it and suppresses normal stool flora. It does not grow in ambient air or strict anaerobic conditions.
A 30-year-old develops ascending flaccid paralysis two weeks after an episode of diarrhoea. Which enteric pathogen is most strongly linked to this complication?
Guillain-Barré syndrome follows C. jejuni infection because antibodies against its lipo-oligosaccharide cross-react with nerve gangliosides (molecular mimicry). Yersinia is linked with reactive arthritis rather than GBS.
A neutropenic patient develops painless skin nodules that become necrotic ulcers with black centres. This lesion, ecthyma gangrenosum, is classically caused by:
Ecthyma gangrenosum results from P. aeruginosa invading blood vessel walls in the skin, usually in neutropenic patients with bacteremia.
For enrichment of Vibrio cholerae from stool, the specimen is inoculated into:
Alkaline peptone water (pH about 8.4) favours vibrios, which tolerate alkaline pH, and suppresses most other stool flora. It is subcultured to TCBS after 6–8 hours.
On a Gram stain of stool from a patient with Campylobacter enteritis, the organisms typically appear as:
Campylobacter are thin, curved, S-shaped or seagull-wing rods. They stain faintly with safranin, so carbol fuchsin or basic fuchsin counterstain improves detection.
A false-positive oxidase test is most likely when colonies are picked with:
Iron in nichrome wire can oxidize the oxidase reagent and give a false-positive colour. Platinum, wood or plastic tools are recommended.
Several people develop itchy red papules and pustules on the trunk 1–2 days after using a poorly chlorinated hot tub. The most likely organism is:
Hot tub folliculitis is caused by P. aeruginosa, which survives in warm water with low chlorine and infects hair follicles.
Helicobacter pylori is a major cause of which condition?
H. pylori causes chronic gastritis and most duodenal and gastric ulcers.
In a hanging-drop preparation of fresh stool, Vibrio cholerae shows which type of motility?
A single polar flagellum gives V. cholerae rapid darting motility, which is stopped by specific antiserum.
Which test for syphilis detects antibodies against Treponema pallidum antigens (a treponemal test)?
TPHA detects specific antibody to T. pallidum; VDRL and RPR are non-treponemal tests using cardiolipin antigen.
Chlamydia trachomatis cannot be grown on ordinary agar media because it:
Chlamydiae depend on host-cell energy, so they are grown only in cell culture or detected by NAAT.
Epidemic typhus caused by Rickettsia prowazekii is transmitted by the:
R. prowazekii is spread by the body louse, typically in crowded, unhygienic conditions.
Scrub typhus (Orientia tsutsugamushi) is transmitted by:
Scrub typhus is spread by the bite of larval trombiculid mites, which often leaves an eschar.
Lyme disease, caused by Borrelia burgdorferi, is transmitted by:
Hard ticks of the genus Ixodes transmit B. burgdorferi; louse-borne relapsing fever is caused by B. recurrentis.
Spirochetes such as Treponema and Leptospira differ from most bacteria in their motility. What gives them movement?
Endoflagella lie in the periplasm and rotate to produce corkscrew motility, letting spirochetes move through viscous tissue.
When performing the Kovács oxidase test on filter paper, a positive result should be read within:
A true positive turns deep purple within about 10 seconds. Later colour change can be due to reagent auto-oxidation and should be ignored.
Which of these gram-negative rods is motile?
P. aeruginosa has a polar flagellum and is motile. H. influenzae, B. pertussis and Y. pestis are non-motile.
Which organism is NOT a sexually transmitted pathogen?
H. influenzae biogroup aegyptius causes conjunctivitis. H. ducreyi (chancroid), K. granulomatis (donovanosis) and T. pallidum (syphilis) are sexually transmitted.
Which Brucella species is associated mainly with goats and sheep and causes the most severe human brucellosis?
B. melitensis (goats, sheep) is the most virulent for humans. B. abortus is from cattle, B. suis from pigs, B. canis from dogs.