MLT GlobeBacteriology: Enterobacterales – page 4
99 Bacteriology MCQs on Enterobacterales with answers and explanations.
A bubo aspirate from a patient with fever after rodent exposure shows gram-negative bacilli with bipolar 'safety-pin' staining. The laboratory should suspect:
Y. pestis shows bipolar staining with Wayson or Giemsa stains. It is a select agent, so work is done in a biosafety cabinet and referred to a reference laboratory.
Two swarming, H2S-positive, urease-positive isolates differ in one test: isolate 1 is indole positive, isolate 2 is indole negative. Isolate 1 is most likely:
P. vulgaris is indole positive; P. mirabilis is indole negative. Morganella and Providencia do not swarm.
A green colour after adding 10% ferric chloride to a phenylalanine agar slant indicates deamination. A positive result is characteristic of:
Phenylalanine deaminase converts phenylalanine to phenylpyruvic acid, which gives a green colour with ferric chloride. This is a key trait of the Proteeae tribe.
The ONPG test is useful to identify 'late' lactose fermenters because these organisms:
Late fermenters have beta-galactosidase but lack permease, so lactose enters slowly. ONPG enters the cell without permease and is split to yellow o-nitrophenol.
A TSI slant shows alkaline slant/alkaline butt (K/K) with no H2S after 24 hours. This suggests the isolate:
All Enterobacterales ferment glucose and give at least an acid butt. A K/K reaction indicates a non-fermenter such as Pseudomonas or Alcaligenes.
A known Shigella control tested in TSI shows an acid slant/acid butt when read after 8 hours. The most likely explanation is:
Early in incubation glucose fermentation acidifies the whole tube. The slant becomes alkaline only after glucose is used and peptones are oxidized, so TSI must be read at 18–24 hours.
An ESBL-producing Klebsiella pneumoniae typically remains susceptible in vitro to:
ESBLs hydrolyze third-generation cephalosporins and aztreonam but not cephamycins such as cefoxitin (and not carbapenems). Cefoxitin resistance suggests AmpC or porin loss.
Which carbapenemase requires zinc at its active site?
NDM, VIM and IMP are Ambler class B metallo-beta-lactamases that require zinc. KPC and OXA-48 are serine enzymes; CTX-M is an ESBL.
A K. pneumoniae from a community-acquired liver abscess forms a viscous string longer than 5 mm when a colony is lifted with a loop. This suggests:
A positive string test marks the hypermucoviscous phenotype linked to hypervirulent K. pneumoniae causing liver abscess and metastatic infection. It does not indicate resistance.
A motile, lactose-fermenting gram-negative rod from blood is lysine decarboxylase positive, ornithine decarboxylase positive and arginine dihydrolase negative. The most likely organism is:
K. aerogenes (formerly Enterobacter aerogenes) is motile, LDC and ODC positive and ADH negative. E. cloacae is LDC negative and ADH positive; K. pneumoniae is non-motile and ODC negative.
Which Shigella species is ornithine decarboxylase positive and a late lactose fermenter (ONPG positive)?
S. sonnei (serogroup D) is typically ODC positive and ONPG positive, fermenting lactose slowly. The other species are usually ODC negative.
A suspected Salmonella Typhi isolate agglutinates strongly in Vi antiserum but not in group D (O) antiserum. What should be done next?
The Vi capsular antigen can mask the O antigen and block O agglutination. Heating (boiling) the suspension removes Vi, after which group D O agglutination should appear.
Which TSI reaction is typical of Salmonella Typhi after 18–24 hours?
S. Typhi ferments glucose without gas and makes only a small wisp of H2S near the slant–butt junction. Most other Salmonella serotypes produce gas and heavy H2S.
In a typhoid-endemic area, a patient with fever has a single Widal test with an O agglutinin titre of 1:80. The best interpretation is:
A single low Widal titre is unreliable because of background antibodies, vaccination and cross-reactions. A fourfold rise between acute and convalescent sera, or culture, is needed.
A neonate fed powdered infant formula develops meningitis. The CSF grows a lactose-fermenting gram-negative rod that forms yellow-pigmented colonies at 25 °C. The most likely organism is:
Cronobacter sakazakii contaminates powdered infant formula and causes neonatal meningitis and sepsis. It produces a yellow pigment, best at 25 °C. Serratia produces red pigment.
Enteropathogenic E. coli (EPEC) causes infant diarrhoea mainly by:
EPEC attaches to small-bowel enterocytes with bundle-forming pili and intimin, destroying microvilli (attaching and effacing lesions). It does not produce Shiga toxin or LT/ST enterotoxins.
In a Moeller decarboxylase test, both the control tube (no amino acid) and the lysine tube are purple after 24 hours under oil. The correct interpretation is:
The control tube must turn yellow to show glucose fermentation lowered the pH, which activates decarboxylases. A purple control means the test cannot be interpreted, often because the organism is a non-fermenter.
A gram-negative rod is colourless on MacConkey agar and ONPG negative, but gives acid slant/acid butt with gas in TSI read at 20 hours. The best explanation is that the organism:
TSI contains sucrose as well as lactose. A lactose-negative, ONPG-negative organism that gives an acid slant after 18–24 hours must be fermenting sucrose, as some Proteus vulgaris and Yersinia strains do.
A patient has painless, beefy-red genital ulcers that bleed easily. A tissue smear shows Donovan bodies inside macrophages. The causative organism is:
Granuloma inguinale (donovanosis) is caused by Klebsiella granulomatis, seen as intracellular Donovan bodies with Giemsa or Wright stain. H. ducreyi causes painful soft ulcers (chancroid).
In the CLSI ESBL confirmatory disk test for an E. coli isolate, the ceftazidime zone is 14 mm and the ceftazidime-clavulanate zone is 22 mm. The result indicates:
An increase of ≥5 mm in zone size with clavulanate compared with the cephalosporin alone confirms ESBL production. Here the increase is 8 mm. AmpC enzymes are not inhibited by clavulanate.