Enterobacterales

29 DOH Abu Dhabi (HAAD) lab licensing practice questions on Enterobacterales, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A 46-year-old man with type 2 diabetes cleans and sells freshwater fish at a market. He cuts his hand on a fin, and 2 days later the wound is swollen and draining pus. A Gram stain of the pus shows neutrophils and gram-negative rods, and culture grows a pure, lactose-non-fermenting, oxidase-negative gram-negative rod. Results: TSI K/A with gas and a heavy black butt; indole positive; citrate negative; urease negative; phenylalanine deaminase negative; lysine decarboxylase positive; ornithine decarboxylase positive; motile. Which organism fits best?

   1. AEdwardsiella tarda
   2. BNon-typhoidal Salmonella
   3. CCitrobacter freundii
   4. DProteus vulgaris

   Show answerHide answer

   Answer: A · Edwardsiella tarda

   The isolate is an H2S-producing lactose non-fermenter that is also indole positive and lysine positive, a combination that fits Edwardsiella tarda. E. tarda lives in freshwater and marine fish and in reptiles, and causes wound infections and gastroenteritis in people who handle them. Salmonella is H2S and lysine positive too, but it is indole negative and usually citrate positive. Proteus vulgaris is indole and H2S positive but urease and phenylalanine deaminase positive and ornithine negative, and Citrobacter freundii is lysine negative and citrate positive. A Salmonella-like TSI with a positive indole is the clue to think of Edwardsiella.

   • B: Salmonella is indole negative and usually citrate positive.
   • C: C. freundii is lysine decarboxylase negative and citrate positive.
   • D: P. vulgaris is urease and phenylalanine deaminase positive and ornithine negative.

2. Question 2 · Hard

   A 3-week-old girl, born at term after an uneventful pregnancy, has fever and seizures, and brain imaging shows several abscesses. The CSF Gram stain shows gram-negative rods. CSF and blood cultures grow the same oxidase-negative gram-negative rod, which forms pale pink colonies on MacConkey agar only after 48 hours. Results: indole positive; methyl red positive; Voges-Proskauer negative; citrate positive; H2S negative on TSI; lysine decarboxylase negative; ornithine decarboxylase positive; malonate positive; motile. The organism that fits best is:

   1. AEscherichia coli
   2. BKlebsiella oxytoca
   3. CCitrobacter freundii
   4. DCitrobacter koseri

   Show answerHide answer

   Answer: D · Citrobacter koseri

   The isolate is a late lactose fermenter that is indole and citrate positive, H2S negative, lysine negative and ornithine positive: the profile of Citrobacter koseri. C. koseri is well known for neonatal meningitis complicated by brain abscesses. E. coli is citrate and malonate negative and usually lysine positive. K. oxytoca is non-motile and Voges-Proskauer and lysine positive, and C. freundii is usually indole negative, ornithine negative and H2S positive. Because abscesses are so common, C. koseri in neonatal CSF should prompt brain imaging.

   • A: E. coli is citrate and malonate negative and usually lysine decarboxylase positive.
   • B: K. oxytoca is non-motile, Voges-Proskauer positive, lysine positive and ornithine negative.
   • C: C. freundii is usually indole negative, ornithine negative and H2S positive.

3. Question 3 · Hard

   Within 1 week, three patients on a surgical ward develop fever and rigors during infusions from the same lot of intravenous fluid. Blood cultures from all three grow an oxidase-negative, glucose-fermenting gram-negative rod with yellow-pigmented colonies on blood agar; it does not ferment lactose on MacConkey agar. Results: indole negative; Voges-Proskauer positive; citrate positive; urease negative; H2S negative; lysine decarboxylase negative; arginine dihydrolase negative; ornithine decarboxylase negative; motile. The organism that fits best is:

   1. AKlebsiella aerogenes
   2. BEnterobacter cloacae complex
   3. CCronobacter sakazakii
   4. DPantoea agglomerans

   Show answerHide answer

   Answer: D · Pantoea agglomerans

   A yellow-pigmented member of the Enterobacterales that is negative for lysine, arginine and ornithine (the 'triple decarboxylase-negative' pattern) is typical of Pantoea agglomerans. Pantoea lives on plants and in the environment and is a classic contaminant of intravenous fluids, so a cluster linked to one fluid lot points to a contaminated product. Cronobacter sakazakii is also yellow but is arginine and ornithine positive, as is the Enterobacter cloacae complex. Klebsiella aerogenes is lysine and ornithine positive. The fluid lot should be withdrawn and cultured, and infection control told.

   • A: K. aerogenes is lysine and ornithine decarboxylase positive.
   • B: The E. cloacae complex is arginine and ornithine positive and is not yellow-pigmented.
   • C: Cronobacter is yellow but arginine dihydrolase and ornithine decarboxylase positive.

4. Question 4 · Medium

   Salmonella and Citrobacter freundii are split at the dashed step of the scheme shown. That step is:

   

   EnlargeClose

   

   1. AIndole production
   2. BCitrate utilization
   3. CLysine decarboxylase (LDC)
   4. DONPG (β-galactosidase)

   Show answerHide answer

   Answer: C · Lysine decarboxylase (LDC)

   Once a rapid urease test has set Proteus aside, the H2S-positive, urease-negative group still holds Salmonella and Citrobacter freundii. Most Salmonella decarboxylate lysine, while C. freundii is lysine decarboxylase negative, so a positive result points to Salmonella. ONPG separates them in the opposite direction, being positive in C. freundii, and both genera are usually indole negative and citrate positive. A presumptive Salmonella is then confirmed by serological typing.

   • A: Both are usually indole negative, so indole does not separate them.
   • B: Both usually use citrate, so citrate does not separate them.
   • D: ONPG is positive in C. freundii, the reverse of the branches shown.

5. Question 5 · Easy

   Which tube shows the reaction expected from Shigella sonnei after overnight incubation?

   

   EnlargeClose

   

   1. ATube C
   2. BTube B
   3. CTube A
   4. DTube D

   Show answerHide answer

   Answer: D · Tube D

   Shigella ferments glucose but not lactose or sucrose within 24 hours, so the little acid formed in the slant is reversed by peptone breakdown while the butt stays acid: a red slant over a yellow butt (K/A). Shigella does not produce H2S, and S. sonnei forms no gas. Tube B (K/A with H2S) fits Salmonella, tube C (A/A with gas) a lactose or sucrose fermenter such as E. coli, and tube A (K/K) a non-fermenter. S. sonnei ferments lactose slowly, so a late reading can turn the slant yellow.

   • A: Tube C shows lactose or sucrose fermentation with gas.
   • B: Tube B shows H2S, which Shigella does not produce.
   • C: Tube A (K/K) shows no glucose fermentation, as in a non-fermenter.

6. Question 6 · Medium

   Colourless colonies on MacConkey agar from a leg ulcer swab are oxidase negative. Which species fits the tube reactions shown?

   

   EnlargeClose

   

   1. AProteus vulgaris
   2. BProteus mirabilis
   3. CSalmonella enterica
   4. DMorganella morganii

   Answer and explanation in LearnFree account

7. Question 7 · Hard

   Urine from a 70-year-old woman with a long-term catheter grows mucoid, lactose-fermenting, oxidase-negative rods. The isolate is non-motile, urease positive, lysine decarboxylase positive and ornithine decarboxylase negative; its IMViC tubes are shown. The most likely organism is:

   

   EnlargeClose

   

   1. ASerratia marcescens
   2. BKlebsiella pneumoniae
   3. CEnterobacter cloacae complex
   4. DKlebsiella oxytoca

   Answer and explanation in LearnFree account

8. Question 8 · Hard

   A nitrate broth inoculated with a suspected Enterobacterales isolate stays colorless after sulfanilic acid and the naphthylamine reagent are added; zinc dust is then added and the broth turns red. The interpretation is:

   1. AThe test is invalid and must be repeated
   2. BNitrate was not reduced, so the identification should be reviewed
   3. CNitrite was present but the reagents had failed
   4. DNitrate was reduced all the way to nitrogen gas

   Answer and explanation in LearnFree account

9. Question 9 · Hard

   Why should a spot indole test be performed on growth from sheep blood agar rather than from MacConkey agar?

   1. AMacConkey agar provides little tryptophan
   2. BBlood agar supplies the ferric ion the reaction needs
   3. CThe bile salts in the medium destroy indole
   4. DThe reagent cannot wet growth from a selective medium

   Answer and explanation in LearnFree account

10. Question 10 · Medium

    On eosin methylene blue (EMB) agar, colonies with a green metallic sheen indicate:

    1. AProduction of hydrogen sulfide
    2. BNo use of lactose by the colony
    3. CVigorous lactose fermentation
    4. DSlow, late fermentation of lactose

    Answer and explanation in LearnFree account

11. Question 11 · Medium

    A positive phenylalanine deaminase test on a lactose-negative gram-negative rod points to which group of organisms?

    1. AKlebsiella and Enterobacter
    2. BProteus, Morganella and Providencia
    3. CSerratia and Hafnia
    4. DEscherichia and Shigella

    Answer and explanation in LearnFree account

12. Question 12 · Medium

    A neonatal unit blood culture grows a yellow-pigmented Enterobacterales isolate from a baby fed reconstituted powdered formula. The most likely organism is:

    1. ACitrobacter freundii
    2. BKlebsiella aerogenes
    3. CCronobacter sakazakii
    4. DHafnia alvei

    Answer and explanation in LearnFree account

13. Question 13 · Hard

    Urine from a man with a long-term indwelling catheter grows a non-swarming, lactose-negative gram-negative rod: phenylalanine deaminase positive, indole positive, citrate positive, urease negative, hydrogen sulfide negative, ornithine decarboxylase negative. Which organism fits best?

    1. AProteus vulgaris
    2. BMorganella morganii
    3. CProvidencia stuartii
    4. DProteus mirabilis

    Answer and explanation in LearnFree account

14. Question 14 · Easy

    Which set of features is shared by members of the Enterobacterales?

    1. AOxidase negative, do not use glucose, reduce nitrate to nitrite
    2. BOxidase negative, oxidize glucose, do not reduce nitrate
    3. COxidase negative, ferment glucose, reduce nitrate to nitrite
    4. DOxidase positive, ferment glucose, reduce nitrate to nitrite

    Answer and explanation in LearnFree account

15. Question 15 · Medium

    A Christensen urea slant turns bright pink within 2 hours of incubation. Which organism is most likely?

    1. AKlebsiella pneumoniae
    2. BEnterobacter cloacae
    3. CEscherichia coli
    4. DProteus mirabilis

    Answer and explanation in LearnFree account

16. Question 16 · Easy

    On triple sugar iron agar, a yellow slant with a yellow butt and split agar (A/A with gas) shows that the isolate fermented:

    1. AGlucose plus lactose or sucrose
    2. BNone of the three sugars
    3. CSucrose only, without using glucose
    4. DGlucose only

    Answer and explanation in LearnFree account

17. Question 17 · Hard

    A lactose-fermenting sputum isolate is motile, lysine decarboxylase positive, ornithine decarboxylase positive, urease negative, Voges-Proskauer positive and citrate positive. Which identification fits best?

    1. ARaoultella ornithinolytica
    2. BKlebsiella pneumoniae
    3. CKlebsiella aerogenes
    4. DEnterobacter cloacae

    Answer and explanation in LearnFree account

18. Question 18 · Medium

    A Simmons citrate slant turns deep blue after overnight incubation. This shows that the organism:

    1. AFermented the citrate to acid products
    2. BProduced hydrogen sulfide from thiosulfate
    3. CHydrolyzed urea to ammonia and carbon dioxide
    4. DUsed citrate as its only carbon source

    Answer and explanation in LearnFree account

19. Question 19 · Medium

    A lysine iron agar tube shows a purple slant, a purple butt and a black precipitate. The correct interpretation is:

    1. AOnly glucose was fermented, with no lysine reaction
    2. BLysine was deaminated and hydrogen sulfide was produced
    3. CLysine was decarboxylated but no hydrogen sulfide was produced
    4. DLysine was decarboxylated and hydrogen sulfide was produced

    Answer and explanation in LearnFree account

20. Question 20 · Hard

    An identification panel reports Klebsiella pneumoniae for a mucoid sputum isolate, but the spot indole test on the same growth is positive. What is the best action?

    1. AResolve the discrepancy: an indole-positive Klebsiella is K. oxytoca
    2. BReport K. pneumoniae, because indole is unreliable in this genus
    3. CReport both organisms as a mixed culture
    4. DRepeat the panel after confirming the isolate is oxidase positive

    Answer and explanation in LearnFree account

21. Question 21 · Easy

    Which organism is a rapid lactose fermenter that forms mucoid pink colonies on MacConkey agar?

    1. ASalmonella Enteritidis
    2. BProteus mirabilis
    3. CShigella sonnei
    4. DKlebsiella pneumoniae

    Answer and explanation in LearnFree account

22. Question 22 · Easy

    On MacConkey agar, a lactose-fermenting colony appears:

    1. AGreen with a bright metallic sheen
    2. BBlack with a metallic-looking center
    3. CPink to red, with a bile salt precipitate
    4. DColorless and translucent on the plate

    Answer and explanation in LearnFree account

23. Question 23 · Medium

    A wound isolate is a lactose-fermenting, oxidase-negative gram-negative rod with these results: indole negative, Voges-Proskauer positive, citrate positive, motile, ornithine decarboxylase positive, lysine decarboxylase negative, urease negative. Which organism fits best?

    1. ASerratia marcescens
    2. BEscherichia coli
    3. CKlebsiella pneumoniae
    4. DEnterobacter cloacae complex

    Answer and explanation in LearnFree account

24. Question 24 · Medium

    Moeller decarboxylase broths are overlaid with mineral oil because:

    1. AThe oil protects the pH indicator from light
    2. BThe decarboxylation reaction needs anaerobic conditions
    3. CThe oil prevents evaporation during long incubation
    4. DThe oil sterilizes the surface of the broth

    Answer and explanation in LearnFree account

25. Question 25 · Hard

    A triple sugar iron tube inoculated with a lactose-negative urine isolate is incubated overnight with the cap screwed down tight, and the whole tube is yellow. The best action is:

    1. AAdd Kovac reagent to the slant and read again
    2. BRepeat the test with a loose cap
    3. CReport the result as K/A with gas production
    4. DReport the isolate as a lactose fermenter

    Answer and explanation in LearnFree account