MLT GlobeBacteriology: Non-fermenters & other GNR – page 4
76 Bacteriology MCQs on Non-fermenters & other GNR with answers and explanations.
When Burkholderia pseudomallei is suspected, culture plates should be kept closed and not sniffed, and work should be done in a biological safety cabinet because:
B. pseudomallei is a Tier 1 select agent and can infect laboratory staff through aerosols. Suspected isolates should be handled in a biosafety cabinet and referred to a reference laboratory.
Selective media for Campylobacter, such as charcoal cefoperazone deoxycholate agar (CCDA), suppress stool flora mainly by using:
Campylobacter media contain antibiotics, such as cefoperazone with other agents, to inhibit normal stool flora. Charcoal removes toxic oxygen products. High salt and alkaline pH are used for Vibrio media.
When a colony of Vibrio cholerae is mixed with 0.5% sodium deoxycholate on a slide, the suspension becomes viscous and forms a thread when lifted with a loop. This is the:
Vibrio cells lyse in deoxycholate, releasing DNA that forms a mucoid string; Aeromonas is usually string negative.
The yellow-green pigment of Pseudomonas aeruginosa that fluoresces under ultraviolet light is:
Pyoverdin (fluorescein) is fluorescent; pyocyanin is the blue non-fluorescent pigment, and prodigiosin is the red pigment of Serratia.
Which of these Gram-negative non-fermenters is oxidase positive?
P. aeruginosa is oxidase positive; Acinetobacter and Stenotrophomonas are characteristically oxidase negative.
Epidemic cholera is caused mainly by Vibrio cholerae of which serogroups?
Only toxigenic O1 and O139 strains cause epidemic cholera; O157 is a serogroup of E. coli.
A motile, oxidase-negative non-fermenter grows from blood of a patient receiving imipenem. It is DNase positive and lysine decarboxylase positive. Which agent is the usual first choice for testing and treatment?
These features fit Stenotrophomonas maltophilia, which carries a chromosomal metallo-beta-lactamase and is intrinsically resistant to carbapenems. Trimethoprim-sulfamethoxazole is the drug of choice. Aminoglycosides are also intrinsically inactive.
A rice farmer from Thailand has pneumonia and liver abscesses. Blood cultures grow an oxidase-positive, motile gram-negative rod with bipolar staining and dry, wrinkled colonies at 48 hours. It is resistant to gentamicin and colistin but susceptible to amoxicillin-clavulanate. The most likely organism is:
This is melioidosis. B. pseudomallei is typically gentamicin and colistin resistant but amoxicillin-clavulanate susceptible, the reverse of most P. aeruginosa. B. mallei is non-motile. Work should be done in a biosafety cabinet.
Carbapenem resistance in Acinetobacter baumannii worldwide is most often caused by:
Acquired class D OXA carbapenemases (OXA-23, OXA-24/40, OXA-58) are the main cause of carbapenem resistance in A. baumannii. Loss of OprD porin is the classic mechanism in P. aeruginosa, not Acinetobacter.
Several neonates in an intensive care unit develop meningitis. The CSF grows an oxidase-positive, indole-positive non-fermenter with pale yellow colonies. It is resistant to colistin and carbapenems. The most likely organism is:
Elizabethkingia meningoseptica causes neonatal meningitis outbreaks. It is oxidase and indole positive, often yellowish, and intrinsically resistant to colistin and carbapenems. Acinetobacter and Stenotrophomonas are oxidase negative.
A leg ulcer after seawater exposure grows an oxidase-positive non-fermenter. On TSI it gives alkaline slant/alkaline butt with blackening from H2S. The most likely organism is:
Shewanella is an unusual non-fermenter because it produces H2S in TSI. It is oxidase positive and found in marine environments. Salmonella and Proteus are oxidase-negative fermenters.
An isolate from an outbreak of cholera-like diarrhoea is biochemically Vibrio cholerae but does not agglutinate in O1 antiserum. What is the most appropriate next step?
Epidemic cholera is caused by serogroups O1 and O139. An O1-negative isolate from a cholera-like outbreak should be tested with O139 antiserum before being called non-O1/non-O139.
A man without a spleen develops sepsis with DIC after a dog bite. Blood cultures grow thin, fusiform gram-negative rods that need CO2, show gliding motility and are oxidase and catalase positive. The most likely organism is:
Capnocytophaga canimorsus from dog saliva causes fulminant sepsis in asplenic or alcoholic patients. It forms fusiform rods, needs CO2 and shows gliding motility; Pasteurella is a short coccobacillus.
During imipenem therapy, a Pseudomonas aeruginosa becomes imipenem resistant but stays susceptible to ceftazidime and cefepime, and carbapenemase tests are negative. The most likely mechanism is:
Imipenem enters P. aeruginosa mainly through the OprD porin. Loss of OprD causes imipenem resistance without affecting cephalosporins, and carbapenemase tests stay negative.
Blood cultures from an elderly patient grow a curved, oxidase-positive gram-negative rod. It grows at 25 °C but not at 42 °C, is hippurate negative and is cephalothin susceptible. The most likely organism is:
C. fetus mainly causes bloodstream and systemic infection in elderly or immunocompromised patients. Unlike C. jejuni and C. coli, it grows at 25 °C, not at 42 °C, and is cephalothin susceptible.
Which transport medium should not be used for stool when Vibrio infection is suspected?
Glycerol is toxic to vibrios, so buffered glycerol saline is unsuitable, although it can be used for Shigella. Cary-Blair is the preferred transport medium for Vibrio.