Enteric pathogens

19 DOH Abu Dhabi (HAAD) lab licensing practice questions on Enteric pathogens, part of Microbiology. The first 19 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 26-year-old man returned to Abu Dhabi from a family visit in South Asia 10 days ago and has had a rising fever for a week, with headache and a relatively slow pulse. Two blood culture sets grow a motile, oxidase-negative gram-negative rod that does not ferment lactose. Results: TSI K/A with gas and no H2S; urease negative; indole negative; citrate negative; lysine decarboxylase negative; ornithine decarboxylase positive. The isolate agglutinates in Salmonella polyvalent O antiserum. Which organism is most likely?

   1. ASalmonella Paratyphi A
   2. BShigella flexneri
   3. CSalmonella Enteritidis
   4. DSalmonella Typhi

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   Answer: A · Salmonella Paratyphi A

   Fever after travel to South Asia, with a blood isolate that reacts in Salmonella antiserum, points to enteric fever. Salmonella Paratyphi A differs from most salmonellae in being H2S negative or weak, citrate negative and lysine decarboxylase negative, while it makes gas and is ornithine positive. S. Typhi makes no gas, gives a small amount of H2S, and is lysine positive and ornithine negative. Non-typhoidal serotypes such as Enteritidis are H2S, citrate and lysine positive, and Shigella is non-motile and does not react in Salmonella antisera. Serotyping (O group A) at a reference laboratory confirms the identification.

   • B: Shigella is non-motile and does not agglutinate in Salmonella antisera.
   • C: Enteritidis and other non-typhoidal serotypes are H2S, citrate and lysine positive.
   • D: S. Typhi gives no gas and a little H2S, and is lysine positive and ornithine negative.

2. Question 2 · Hard

   An outbreak of bloody diarrhea with fever affects 30 workers who eat from one kitchen in a labor camp, and two of them develop hemolytic uremic syndrome. Stools grow red colonies without black centers on XLD agar. The isolates are oxidase-negative gram-negative rods with these results: TSI K/A with no gas and no H2S; non-motile; urease negative; lysine decarboxylase negative; ornithine decarboxylase negative; mannitol not fermented; no reaction in Salmonella polyvalent O antiserum. The isolates most likely belong to:

   1. AShigella serogroup A (S. dysenteriae)
   2. BShigella serogroup B (S. flexneri)
   3. CShigella serogroup C (S. boydii)
   4. DShigella serogroup D (S. sonnei)

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   Answer: A · Shigella serogroup A (S. dysenteriae)

   Red XLD colonies without H2S, a K/A TSI without gas, non-motility and a negative lysine decarboxylase identify Shigella. Among the serogroups, S. dysenteriae (group A) is the one that does not ferment mannitol, and S. sonnei (group D) is the one that is ornithine positive. A mannitol-negative, ornithine-negative isolate therefore belongs to group A, which is confirmed with group A antiserum. S. dysenteriae type 1 makes Shiga toxin, which explains the cases of hemolytic uremic syndrome. The isolates should be referred to the public health laboratory for serotyping.

   • B: S. flexneri usually ferments mannitol.
   • C: S. boydii ferments mannitol.
   • D: S. sonnei ferments mannitol and is ornithine decarboxylase positive.

3. Question 3 · Easy

   Selenite F broth is included in a stool culture set-up in order to:

   1. AEnrich Campylobacter during overnight transport
   2. BEnrich Vibrio species before plating on TCBS agar
   3. CEnrich small numbers of Salmonella while holding back coliforms
   4. DNeutralise the acid in stool so that Shigella survives transport

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   Answer: C · Enrich small numbers of Salmonella while holding back coliforms

   Selenite holds back Escherichia coli and other normal flora for the first 12 to 18 hours while Salmonella multiply, so a light load in a carrier or a convalescent patient is still recovered. The broth is subcultured to xylose-lysine-deoxycholate or Hektoen enteric agar after that time; left longer, the flora overgrow it. Shigella is recovered poorly, so a direct plate is always set up as well.

   • A: Campylobacter enrichment uses Bolton or Preston broth in a microaerobic atmosphere.
   • B: Vibrio enrichment uses alkaline peptone water, not selenite.
   • D: Cary-Blair is the transport medium that protects acid-sensitive Shigella.

4. Question 4 · Hard

   A traveller returning from South Asia has watery diarrhoea. A stool isolate gives yellow colonies on TCBS agar, is oxidase positive and string test positive, grows in broth without added sodium chloride, and does not agglutinate in O1 or O139 antiserum. How should it be reported?

   1. AVibrio cholerae serogroup O1, El Tor biotype
   2. BVibrio parahaemolyticus
   3. CVibrio cholerae, non-O1 and non-O139
   4. DVibrio alginolyticus

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   Answer: C · Vibrio cholerae, non-O1 and non-O139

   Growth without added salt and a positive string test fit Vibrio cholerae, and sucrose fermentation gives the yellow colony on TCBS agar. Strains outside serogroups O1 and O139 usually lack cholera toxin; they cause sporadic gastroenteritis and wound infection but not epidemic cholera, so they are reported as non-O1, non-O139. Only O1 and O139 isolates carry the public health urgency of epidemic cholera.

   • A: An O1 isolate would agglutinate in O1 antiserum.
   • B: V. parahaemolyticus is halophilic and gives green colonies on TCBS agar.
   • D: V. alginolyticus is halophilic and needs added salt to grow.

5. Question 5 · Medium

   Campylobacter is easier to see in a direct stool smear when the Gram stain is counterstained with:

   1. ACarbol fuchsin
   2. BMalachite green
   3. CSafranin
   4. DCrystal violet

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   Answer: A · Carbol fuchsin

   The cells are thin, curved and take up safranin weakly, so they are easily missed; carbol fuchsin applied for 2 to 3 minutes stains them well and the gull-wing and S-shaped forms become obvious. A positive smear supports an early report, but sensitivity is only moderate, so culture or a molecular panel is still needed. Dark-field or phase microscopy of fresh stool shows the darting motility.

   • B: Malachite green is a spore stain counterstain, not a Gram counterstain.
   • C: Safranin stains these thin cells too faintly to be reliable.
   • D: Crystal violet is the primary stain of the Gram method.

6. Question 6 · Easy

   Primary isolation of Campylobacter jejuni from stool needs incubation at:

   1. A35 °C in ambient air
   2. B42 °C in a microaerobic atmosphere
   3. C35 °C in a 5% carbon dioxide incubator
   4. D42 °C in an anaerobic jar

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7. Question 7 · Medium

   Why does Escherichia coli O157:H7 form colourless colonies on sorbitol-MacConkey agar?

   1. AIt does not ferment the lactose in the medium
   2. BThe bile salts in the medium hold back its growth
   3. CIt does not ferment the sorbitol in the medium
   4. DIts Shiga toxin bleaches the pH indicator

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8. Question 8 · Hard

   A 4-year-old with bloody diarrhoea becomes pale and passes little urine. Which set of results best fits haemolytic uraemic syndrome?

   1. ATarget cells on the film, platelets 150 x 10^9/L, creatinine normal
   2. BSchistocytes on the film, platelets 420 x 10^9/L, creatinine normal
   3. CSchistocytes on the film, platelets 45 x 10^9/L, creatinine rising
   4. DSpherocytes on the film, platelets 250 x 10^9/L, direct antiglobulin test positive

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9. Question 9 · Hard

   Blood cultures from a 70-year-old man with cirrhosis grow a curved gram-negative rod. It grows at 25 °C and 37 °C but not at 42 °C, is catalase positive and hippurate hydrolysis negative. Which organism fits best?

   1. ACampylobacter coli
   2. BCampylobacter jejuni
   3. CCampylobacter fetus
   4. DHelicobacter pylori

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10. Question 10 · Hard

    Which limitation of Salmonella-Shigella (SS) agar makes XLD or Hektoen enteric agar the better second plate for stool?

    1. AIt does not differentiate lactose fermenters
    2. BSalmonella ferments its sucrose and looks like E. coli
    3. CIt cannot show hydrogen sulfide production
    4. DIt inhibits some Shigella strains, above all S. dysenteriae

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11. Question 11 · Hard

    A Shiga toxin immunoassay on a diarrhoeal stool is positive, but every colony on sorbitol-MacConkey agar ferments sorbitol. The best explanation is:

    1. AThe sorbitol plate was read before 24 hours
    2. BA non-O157 Shiga toxin-producing E. coli is present
    3. CAn enterotoxigenic E. coli is present
    4. DThe immunoassay cross-reacted with Shigella sonnei

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12. Question 12 · Easy

    Stool may be enriched before plating on TCBS agar for Vibrio species in:

    1. AThioglycollate broth
    2. BAlkaline peptone water
    3. CTetrathionate broth
    4. DSelenite F enrichment broth

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13. Question 13 · Hard

    A stool isolate does not ferment lactose. Results: TSI K/A with abundant H2S and gas; urease negative; lysine decarboxylase positive; citrate positive; motile. The most likely identification is:

    1. AProteus mirabilis
    2. BNon-typhoidal Salmonella
    3. CCitrobacter freundii complex
    4. DSalmonella Typhi

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14. Question 14 · Medium

    A stool isolate is a non-lactose-fermenting gram-negative rod that is non-motile, lysine decarboxylase negative and produces no gas from glucose. Which genus fits best?

    1. ASalmonella
    2. BShigella
    3. CProteus
    4. DCitrobacter

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15. Question 15 · Medium

    On xylose-lysine-deoxycholate (XLD) agar, Salmonella forms red colonies with black centres because the organism:

    1. AFerments the xylose only, and makes no H2S
    2. BUses the xylose, then decarboxylates lysine, and makes H2S
    3. CFerments the lactose and sucrose in the medium, and makes H2S
    4. DUses none of the three sugars, and makes H2S

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16. Question 16 · Medium

    In suspected enteric fever, which specimen is most likely to be positive during the first week of illness?

    1. AUrine culture
    2. BBlood culture
    3. CStool culture
    4. DSerum for antibody

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17. Question 17 · Hard

    A 5-year-old has bloody diarrhoea without fever. Stool on sorbitol-MacConkey agar grows colourless colonies that agglutinate in O157 antiserum. What should the laboratory do next?

    1. ASet up disk diffusion and report susceptibilities
    2. BRepeat the culture on Hektoen enteric agar
    3. CReport the isolate as an enteropathogenic E. coli strain
    4. DConfirm Shiga toxin or stx genes and keep the isolate

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18. Question 18 · Medium

    Curved, gull-wing shaped gram-negative rods grow on selective agar at 42 °C under microaerophilic conditions. What is the organism?

    1. AVibrio cholerae
    2. BHelicobacter pylori
    3. CShigella sonnei
    4. DCampylobacter jejuni

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19. Question 19 · Medium

    Yellow colonies grow on TCBS agar from a patient with profuse rice-water stools. What is the most likely organism?

    1. ACampylobacter jejuni
    2. BVibrio parahaemolyticus
    3. CVibrio cholerae
    4. DSalmonella

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