A 6-year-old girl has had cramping and bloody diarrhea for 3 days without fever. Stool cultures for Salmonella, Shigella and Campylobacter are negative. On sorbitol-MacConkey agar all colonies are pink, and five pink colonies tested with E. coli O157 latex reagent are negative. A Shiga toxin immunoassay on an overnight gram-negative broth culture of the stool is positive. Her complete blood count and creatinine are still normal. The physician asks how to interpret the discordant results. What should the laboratory do next?

Send the broth to a public health lab

Report E. coli O157:H7 from the culture

Report the toxin result as a false positive

Test the pink colonies with H7 antiserum

A 7-month-old boy who attends daycare has had vomiting and watery, nonbloody diarrhea for 2 days and is mildly dehydrated. He has not received antibiotics, and several children in his daycare group have the same illness. A multiplex gastrointestinal PCR panel on his stool detects norovirus GI/GII and Clostridioides difficile toxin A/B genes; all other bacterial, viral and parasitic targets are not detected. The laboratory follows IDSA/SHEA guidance on C. difficile testing in children. Which interpretation is most appropriate?

Norovirus illness with probable C. difficile carriage

C. difficile infection that needs treatment

False-positive norovirus; repeat the panel

Dual infection, both agents causing illness

Gastrointestinal

23 ASCP MLS practice questions on Gastrointestinal, part of Microbiology. The first 23 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 54-year-old man who cleans his freshwater aquarium by hand has had watery diarrhea and cramps for 4 days; he has not traveled. His stool grows blue-green colonies with black centers on Hektoen enteric agar. The isolate is an oxidase-negative, lactose-nonfermenting gram-negative rod with these reactions: TSI K/A with H2S and gas; indole positive; urease negative; lysine decarboxylase positive; citrate negative; motile. It does not agglutinate in Salmonella polyvalent O antiserum. Which organism is most likely?
1. AProteus vulgaris
2. BEdwardsiella tarda
3. CSalmonella enterica
4. DCitrobacter freundii
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Answer: B · Edwardsiella tarda
Black-centered colonies on Hektoen agar and an H2S-positive K/A TSI suggest Salmonella, but the indole reaction separates the look-alikes. Edwardsiella tarda is H2S positive, indole positive, lysine decarboxylase positive and citrate negative, and it is linked to fish, fresh water and reptiles. Salmonella is indole negative and usually citrate positive, and it would normally agglutinate in polyvalent O antiserum. Citrobacter freundii is indole negative, citrate positive and lysine negative, and Proteus vulgaris is urease positive. Checking the full biochemical profile before reporting stops a Salmonella mimic from being reported as Salmonella.
- A: P. vulgaris is urease positive and lysine decarboxylase negative.
- C: Salmonella is indole negative, usually citrate positive, and agglutinates in polyvalent O antiserum.
- D: C. freundii is indole negative, lysine decarboxylase negative and citrate positive.
Question 2 · Hard
A 6-year-old girl has had cramping and bloody diarrhea for 3 days without fever. Stool cultures for Salmonella, Shigella and Campylobacter are negative. On sorbitol-MacConkey agar all colonies are pink, and five pink colonies tested with E. coli O157 latex reagent are negative. A Shiga toxin immunoassay on an overnight gram-negative broth culture of the stool is positive. Her complete blood count and creatinine are still normal. The physician asks how to interpret the discordant results. What should the laboratory do next?
1. AReport E. coli O157:H7 from the culture
2. BReport the toxin result as a false positive
3. CTest the pink colonies with H7 antiserum
4. DSend the broth to a public health lab
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Answer: D · Send the broth to a public health lab
Sorbitol-MacConkey agar screens only for E. coli O157, which does not ferment sorbitol; most non-O157 Shiga toxin-producing E. coli, such as O26, O103 and O111, ferment sorbitol and look like ordinary E. coli. A positive Shiga toxin test with no O157 isolate therefore points to a non-O157 STEC, which can also cause bloody diarrhea and hemolytic uremic syndrome. CDC recommends that toxin-positive broths or stools without an O157 isolate be sent promptly to a public health laboratory for isolation and serotyping. The physician is told that Shiga toxin was detected, so the child can be watched for HUS and antibiotics avoided.
- A: All colonies were O157 negative, so O157 has not been isolated.
- B: Non-O157 STEC ferment sorbitol, so a negative O157 screen does not make the toxin result false.
- C: H7 typing follows an O157 isolate; these colonies are not O157.
Question 3 · Hard
A 7-month-old boy who attends daycare has had vomiting and watery, nonbloody diarrhea for 2 days and is mildly dehydrated. He has not received antibiotics, and several children in his daycare group have the same illness. A multiplex gastrointestinal PCR panel on his stool detects norovirus GI/GII and Clostridioides difficile toxin A/B genes; all other bacterial, viral and parasitic targets are not detected. The laboratory follows IDSA/SHEA guidance on C. difficile testing in children. Which interpretation is most appropriate?
1. AC. difficile infection that needs treatment
2. BFalse-positive norovirus; repeat the panel
3. CDual infection, both agents causing illness
4. DNorovirus illness with probable C. difficile carriage
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Answer: D · Norovirus illness with probable C. difficile carriage
A large share of healthy infants carry toxigenic C. difficile without disease, probably because their gut lacks mature toxin receptors. IDSA/SHEA guidance therefore advises against routine C. difficile testing in infants 12 months of age or younger, and a positive result at this age is not taken as proof of infection. Norovirus explains the acute vomiting, the watery diarrhea and the daycare cluster. Many laboratories suppress or add a comment to the C. difficile target on panels from young children for this reason. Treatment for C. difficile is not indicated.
- A: C. difficile carriage is common in infants, and testing at this age is not advised.
- B: Norovirus fits the vomiting, watery stools and daycare cluster; nothing suggests a false positive.
- C: A C. difficile target in an infant does not show disease, and norovirus explains the illness.
Question 4 · Easy
Which specimen is appropriate for Clostridioides difficile testing?
1. AFormed stool in a sterile cup
2. BFresh unformed stool
3. CStool in 10% formalin
4. DStool in PVA fixative
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Answer: B · Fresh unformed stool
C. difficile testing should be done only on unformed stool, which takes the shape of its container, from a patient with diarrhea. Many people, especially in hospital, carry C. difficile without disease, so a positive result on formed stool may only mean colonization. The toxins break down at room temperature, so the stool is tested promptly or refrigerated. Fixatives such as formalin and PVA make the specimen unsuitable for toxin and molecular tests.
- A: Formed stool suggests no diarrhea, and a positive result may only mean colonization.
- C: Formalin preserves parasites for ova and parasite testing and is unsuitable for C. difficile tests.
- D: PVA is a fixative for parasite smears, not for C. difficile testing.
Question 5 · Easy
Which noninvasive test detects current Helicobacter pylori infection and is suitable for confirming eradication after treatment?
1. ASerum H. pylori IgG
2. BUrea breath test
3. CBiopsy rapid urease test
4. DGastric biopsy culture
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Answer: B · Urea breath test
In the urea breath test the patient swallows urea labelled with carbon-13 or carbon-14. If H. pylori is present, its urease splits the urea and labelled CO2 appears in the breath. Because the test detects living organisms, it turns negative after successful treatment; the stool antigen test works the same way. IgG antibodies can stay positive for months to years after cure, so serology is not used to confirm eradication.
- A: Antibodies persist after cure, so serology cannot show eradication.
- C: The rapid urease test needs a biopsy taken at endoscopy, so it is invasive.
- D: Culture needs an endoscopic biopsy and is used mainly for susceptibility testing.
Question 6 · Hard
Six weeks after finishing H. pylori eradication therapy, a patient who still takes omeprazole daily has a negative urea breath test. The most appropriate interpretation is that the result:
1. AShould be confirmed with H. pylori IgG serology
2. BIs invalid because it was done too soon after therapy
3. CConfirms eradication; no further testing is needed
4. DMay be falsely negative; retest 2 weeks off the PPI
Answer and explanation in LearnFree account
Question 7 · Medium
A nonmotile, lysine decarboxylase-negative stool isolate forms green colonies without black centers on Hektoen enteric agar. What is the most likely organism?
1. AShigella flexneri
2. BSalmonella Typhimurium
3. CProteus mirabilis
4. DEscherichia coli
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Question 8 · Hard
Serotyping of a lactose-negative, H2S-positive stool isolate gives: Salmonella polyvalent O antiserum negative; Vi antiserum positive; after heating the suspension at 100 °C for 15 minutes, polyvalent O and group D O antisera positive. These findings show that:
1. AThe polyvalent O antiserum had lost activity
2. BThe Vi capsule was masking the O antigens
3. CHeating created new O antigens on the cells
4. DThe isolate has lost its flagellar H antigens
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Question 9 · Easy
Which enteric pathogen is CIN (cefsulodin-irgasan-novobiocin) agar designed to recover from stool?
1. AShigella sonnei
2. BYersinia enterocolitica
3. CVibrio cholerae
4. DCampylobacter jejuni
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Question 10 · Medium
Which test separates Campylobacter jejuni from Campylobacter coli?
1. AGrowth at 42 °C
2. BHippurate hydrolysis
3. COxidase
4. DCatalase
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Question 11 · Medium
A stool tested by a two-step Clostridioides difficile algorithm is glutamate dehydrogenase (GDH) antigen positive and toxin A/B immunoassay negative. The next step is to:
1. AReport toxigenic C. difficile infection
2. BRequest a new stool in 7 days
3. CReport the stool as negative
4. DTest the same stool by a toxin gene NAAT
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Question 12 · Hard
A 4-year-old had bloody diarrhea after a barbecue. A week later the platelet count is 45 × 10^3/µL (45 × 10^9/L), the blood film shows schistocytes and creatinine is 2.1 mg/dL (186 µmol/L). Which test on the original stool is most likely to be positive?
1. AInvasion plasmid gene (ipaH)
2. BC. difficile toxin B gene (tcdB)
3. CHeat-stable toxin gene (st)
4. DShiga toxin 2 gene (stx2)
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Question 13 · Hard
A 14-year-old has fever and right lower quadrant pain, but the appendix is normal at surgery. A stool isolate is lactose negative, oxidase negative, urease positive and H2S negative, and is motile at 25 °C but not at 37 °C. The most likely organism is:
1. ASalmonella Enteritidis
2. BAeromonas hydrophila
3. CYersinia enterocolitica
4. DProteus mirabilis
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Question 14 · Hard
A stool from a food handler screened as a possible typhoid carrier yields a lactose-nonfermenting isolate: TSI K/A with a small amount of H2S and no gas; lysine decarboxylase positive; ornithine decarboxylase negative; citrate negative; motile; urease negative. What is the most likely identification?
1. ASalmonella Paratyphi A
2. BSalmonella Typhi
3. CShigella sonnei
4. DCitrobacter freundii
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Question 15 · Easy
Which Vibrio cholerae serogroups produce cholera toxin and cause epidemic cholera?
1. AO1 and O139
2. BO1 and O157
3. CO157 and O111
4. DNon-O1, non-O139 strains
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Question 16 · Hard
A lactose-nonfermenting, H2S-positive stool isolate agglutinates weakly in Salmonella polyvalent O antiserum. Further results: lysine decarboxylase negative, growth in KCN, ONPG positive, urease negative. The most likely identification is:
1. ASalmonella Typhimurium
2. BProteus mirabilis
3. CEdwardsiella tarda
4. DCitrobacter freundii
Answer and explanation in LearnFree account
Question 17 · Medium
Why do current CDC recommendations call for Shiga toxin (or stx gene) testing of diarrheal stools in addition to sorbitol-MacConkey culture?
1. ASorbitol-MacConkey agar does not support growth of O157
2. BShiga toxin tests separate ETEC from EPEC strains
3. CCulture cannot detect E. coli O157 once diarrhea is bloody
4. DMost non-O157 STEC ferment sorbitol like normal E. coli
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Question 18 · Medium
Helicobacter pylori and Campylobacter jejuni are both curved, oxidase-positive gram-negative rods. H. pylori differs in being:
1. ACatalase positive
2. BMicroaerophilic in growth
3. CStrongly and rapidly urease positive
4. DHippurate hydrolysis positive
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Question 19 · Hard
A man has watery diarrhea and cramps a day after eating raw oysters. Stool results: green colonies on TCBS agar; oxidase positive; growth in 6% NaCl but not in 0% NaCl; ONPG negative. Which organism is most likely?
1. AVibrio cholerae
2. BVibrio parahaemolyticus
3. CVibrio alginolyticus
4. DVibrio vulnificus
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Question 20 · Medium
A student develops watery, non-bloody diarrhea on the third day of a trip to Mexico. The most common bacterial cause of this traveler's diarrhea is:
1. AEnterotoxigenic E. coli (ETEC)
2. BEnterohemorrhagic E. coli (EHEC)
3. CEnteropathogenic E. coli (EPEC)
4. DEnteroinvasive E. coli (EIEC)
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Question 21 · Hard
A lactose-nonfermenting stool isolate agglutinates in Shigella group D (S. sonnei) antiserum. Which additional result shows that it is not Shigella?
1. APositive ornithine decarboxylase
2. BNo H2S on TSI agar
3. CPositive oxidase test
4. DNegative motility test
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Question 22 · Hard
An inpatient's liquid stool arrives for Clostridioides difficile NAAT. The order notes three loose stools today and two doses of lactulose in the past 24 hours. The laboratory follows IDSA/SHEA guidance. The most appropriate action is to:
1. APerform the NAAT and report the result
2. BAsk for two more stools before testing
3. CContact the provider, as laxatives argue against testing
4. DTest by toxin immunoassay instead of NAAT
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Question 23 · Medium
A stool isolate gives an alkaline slant, acid butt with H2S (K/A, black butt) on TSI, and is urease negative and motile. What is the most likely organism?
1. ASalmonella
2. BShigella
3. CProteus mirabilis
4. DEscherichia coli
Answer and explanation in LearnFree account

Gastrointestinal

Which specimen is appropriate for Clostridioides difficile testing?

Which noninvasive test detects current Helicobacter pylori infection and is suitable for confirming eradication after treatment?

Six weeks after finishing H. pylori eradication therapy, a patient who still takes omeprazole daily has a negative urea breath test. The most appropriate interpretation is that the result:

A nonmotile, lysine decarboxylase-negative stool isolate forms green colonies without black centers on Hektoen enteric agar. What is the most likely organism?

Serotyping of a lactose-negative, H2S-positive stool isolate gives: Salmonella polyvalent O antiserum negative; Vi antiserum positive; after heating the suspension at 100 °C for 15 minutes, polyvalent O and group D O antisera positive. These findings show that:

Which enteric pathogen is CIN (cefsulodin-irgasan-novobiocin) agar designed to recover from stool?

Which test separates Campylobacter jejuni from Campylobacter coli?

A stool tested by a two-step Clostridioides difficile algorithm is glutamate dehydrogenase (GDH) antigen positive and toxin A/B immunoassay negative. The next step is to:

A 4-year-old had bloody diarrhea after a barbecue. A week later the platelet count is 45 × 10^3/µL (45 × 10^9/L), the blood film shows schistocytes and creatinine is 2.1 mg/dL (186 µmol/L). Which test on the original stool is most likely to be positive?

A 14-year-old has fever and right lower quadrant pain, but the appendix is normal at surgery. A stool isolate is lactose negative, oxidase negative, urease positive and H2S negative, and is motile at 25 °C but not at 37 °C. The most likely organism is:

Which Vibrio cholerae serogroups produce cholera toxin and cause epidemic cholera?

A lactose-nonfermenting, H2S-positive stool isolate agglutinates weakly in Salmonella polyvalent O antiserum. Further results: lysine decarboxylase negative, growth in KCN, ONPG positive, urease negative. The most likely identification is:

Why do current CDC recommendations call for Shiga toxin (or stx gene) testing of diarrheal stools in addition to sorbitol-MacConkey culture?

Helicobacter pylori and Campylobacter jejuni are both curved, oxidase-positive gram-negative rods. H. pylori differs in being:

A man has watery diarrhea and cramps a day after eating raw oysters. Stool results: green colonies on TCBS agar; oxidase positive; growth in 6% NaCl but not in 0% NaCl; ONPG negative. Which organism is most likely?

A student develops watery, non-bloody diarrhea on the third day of a trip to Mexico. The most common bacterial cause of this traveler's diarrhea is:

A lactose-nonfermenting stool isolate agglutinates in Shigella group D (S. sonnei) antiserum. Which additional result shows that it is not Shigella?

An inpatient's liquid stool arrives for Clostridioides difficile NAAT. The order notes three loose stools today and two doses of lactulose in the past 24 hours. The laboratory follows IDSA/SHEA guidance. The most appropriate action is to:

A stool isolate gives an alkaline slant, acid butt with H2S (K/A, black butt) on TSI, and is urease negative and motile. What is the most likely organism?