For prenatal group B streptococcus (GBS) screening, which specimen is collected?

A swab of the lower vagina and then the rectum

A clean-catch midstream urine sample

A cervical swab taken during a speculum examination

A high vaginal swab from the posterior fornix

Twelve wedding guests develop vomiting and cramps 2 to 4 hours after eating the same rice dish, and all recover within a day. The most useful laboratory investigation is:

Culture and toxin testing of leftover food

Routine stool culture for Salmonella and Shigella

Blood cultures from the most unwell guests

Ova and parasite examination of stool

For culture of a deep wound infection, the most useful specimen is:

Tissue or aspirated pus from the debrided wound base

A dry swab rolled over the wound surface

Dressing material removed from the wound

A swab of the skin around the wound edge

A Gram stain of a swab from a diabetic patient's leg ulcer shows many squamous epithelial cells, no neutrophils and mixed bacteria. What does this most likely indicate?

Surface contamination; a better specimen is needed

An anaerobic infection needing anaerobic culture

A severe infection in a neutropenic patient

A deep abscess containing many different organisms

Wound, genital and stool cultures

14 DOH Abu Dhabi (HAAD) lab licensing practice questions on Wound, genital and stool cultures, part of Microbiology. The first 14 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 Gram-stained vaginal smear from a 30-year-old woman with discharge is scored by Nugent criteria, averaging per oil-immersion field. Lactobacillus morphotypes: none = 4, under 1 = 3, 1-4 = 2, 5-30 = 1, over 30 = 0. Gardnerella/Bacteroides morphotypes: none = 0, under 1 = 1, 1-4 = 2, 5-30 = 3, over 30 = 4. Curved gram-variable rods: none = 0, under 1 to 4 = 1, 5 or more = 2. Totals: 0-3 normal, 4-6 intermediate, 7-10 bacterial vaginosis. The smear shows under 1 lactobacillus, 5-30 small gram-variable rods and 1-4 curved gram-variable rods per field, with clue cells. What is the result?
1. AScore 8: bacterial vaginosis
2. BScore 7: bacterial vaginosis
3. CScore 5: intermediate flora
4. DScore 6: intermediate flora
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Answer: B · Score 7: bacterial vaginosis
Nugent scoring adds three parts. Lactobacilli score inversely, so under 1 per field gives 3 points; small gram-variable rods of the Gardnerella/Bacteroides type at 5-30 per field give 3; and curved gram-variable rods of the Mobiluncus type at 1-4 per field give 1. The total of 7 falls in the bacterial vaginosis range, and the clue cells agree. Culture for Gardnerella vaginalis is not useful, because many healthy women carry it; the scored Gram stain is the laboratory standard. An intermediate score of 4-6 is reported as such and interpreted with the clinical findings.
- A: Curved rods at 1-4 per field score 1, not 2.
- C: Lactobacilli score inversely: under 1 per field scores 3, not 1.
- D: This total leaves out the 1 point for curved rods at 1-4 per field.
Question 2 · Easy
For prenatal group B streptococcus (GBS) screening, which specimen is collected?
1. AA clean-catch midstream urine sample
2. BA cervical swab taken during a speculum examination
3. CA swab of the lower vagina and then the rectum
4. DA high vaginal swab from the posterior fornix
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Answer: C · A swab of the lower vagina and then the rectum
GBS colonizes the gut and spreads to the lower vagina, so sampling both sites, with one swab or two swabs placed in the same broth, gives the highest yield. Cervical samples and a speculum are not needed. The swab goes into a selective enrichment broth, such as Lim broth, before subculture or NAAT. GBS bacteriuria found in pregnancy is reported, but urine does not replace the vaginal-rectal screen.
- A: Urine detects GBS bacteriuria but is not the screening specimen.
- B: The cervix is not sampled and a speculum is not needed for GBS screening.
- D: A high vaginal swab misses rectal carriage and has a lower yield.
Question 3 · Hard
Twelve wedding guests develop vomiting and cramps 2 to 4 hours after eating the same rice dish, and all recover within a day. The most useful laboratory investigation is:
1. ACulture and toxin testing of leftover food
2. BRoutine stool culture for Salmonella and Shigella
3. CBlood cultures from the most unwell guests
4. DOva and parasite examination of stool
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Answer: A · Culture and toxin testing of leftover food
Vomiting within a few hours of a meal points to a preformed toxin, such as Bacillus cereus emetic toxin (classically from rice) or Staphylococcus aureus enterotoxin. Routine stool culture looks for invasive pathogens with incubation periods of a day or more and would not explain this outbreak. Large numbers of the organism, or its toxin, in the implicated food confirm the source. Public health authorities usually coordinate food and stool sampling.
- B: Salmonella and Shigella have incubation periods of a day or more and cause fever and diarrhea.
- C: This is a short toxin-mediated illness without bacteremia.
- D: Parasites do not cause vomiting within hours of a meal.
Question 4 · Easy
For culture of a deep wound infection, the most useful specimen is:
1. ATissue or aspirated pus from the debrided wound base
2. BA dry swab rolled over the wound surface
3. CDressing material removed from the wound
4. DA swab of the skin around the wound edge
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Answer: A · Tissue or aspirated pus from the debrided wound base
Tissue or pus aspirated from the base or advancing edge of a cleaned, debrided wound contains the organisms causing the infection. Surface swabs pick up colonizing skin and environmental flora, which can mislead treatment. Aspirates and tissue also allow anaerobic culture. If a swab must be used, it is taken after cleaning, from viable tissue, and sent in transport medium.
- B: Surface swabs mainly collect colonizing flora, not the infecting organisms.
- C: Dressings carry contaminants and are not acceptable culture specimens.
- D: The skin around the wound carries normal flora rather than the pathogen.
Question 5 · Medium
A Gram stain of a swab from a diabetic patient's leg ulcer shows many squamous epithelial cells, no neutrophils and mixed bacteria. What does this most likely indicate?
1. AAn anaerobic infection needing anaerobic culture
2. BA severe infection in a neutropenic patient
3. CSurface contamination; a better specimen is needed
4. DA deep abscess containing many different organisms
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Answer: C · Surface contamination; a better specimen is needed
Squamous epithelial cells come from the skin surface, and the lack of neutrophils argues against inflammation at the sampled site. The swab has most likely collected colonizing flora rather than infected tissue. Many laboratories grade wound smears, report poor quality and ask for tissue or an aspirate. Full workup of such a specimen can lead to treatment of colonizers.
- A: Mixed bacteria without neutrophils suggest contamination, not proven anaerobic infection.
- B: Neutropenia is not the likely explanation when skin cells dominate the smear.
- D: Pus from an abscess would contain many neutrophils.
Question 6 · Easy
Many laboratories do not perform routine stool culture for Salmonella, Shigella and Campylobacter on patients who have been in hospital for more than:
1. A12 hours
2. B3 days
3. C14 days
4. D7 days
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Question 7 · Medium
A swab from a patient's infected surgical wound grows a moderate amount of Staphylococcus aureus with a few colonies of coagulase-negative staphylococci and diphtheroids. The best report is:
1. AS. aureus with susceptibilities, plus mixed skin flora
2. BAll three organisms identified, each with susceptibilities
3. CCoagulase-negative staphylococci with susceptibilities only
4. DMixed skin flora only, with no identification
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Question 8 · Hard
A multiplex gastrointestinal PCR panel on a child's stool detects Salmonella. A portion of the stool was also sent in Cary-Blair medium. What should the laboratory do next?
1. AReport the PCR result and discard the Cary-Blair stool sample
2. BRequest blood cultures instead of culturing the stool
3. CCulture the stool to recover an isolate for AST and typing
4. DRepeat the PCR on a second stool before reporting
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Question 9 · Medium
A saline wet mount of vaginal discharge is examined 3 hours after collection and no trichomonads are seen, but a Trichomonas vaginalis NAAT from the same visit is positive. What is the most likely explanation?
1. AThe NAAT cross-reacts with Gardnerella vaginalis
2. BThe wet mount should have been refrigerated first
3. CTrichomonads are seen only in Gram-stained smears
4. DTrichomonads lost their motility during the delay
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Question 10 · Hard
An aquarium worker has a slowly enlarging nodule on the back of her hand, and a biopsy is sent for mycobacterial culture. At which temperature must the culture be incubated to recover the most likely agent?
1. A4 °C for cold enrichment
2. B42-45 °C, as for M. xenopi
3. C28-30 °C as well as 35-37 °C
4. D35-37 °C only, as for M. tuberculosis
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Question 11 · Hard
A clinician sends a vaginal swab requesting 'culture for Gardnerella vaginalis' from a woman with a thin, fishy-smelling discharge. What is the most appropriate laboratory response?
1. ACulture on chocolate agar and report the colony count
2. BCulture on human blood bilayer Tween agar and report any growth
3. CPerform anaerobic culture to identify the main organisms
4. DAdvise a Gram stain scored by Nugent criteria instead
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Question 12 · Hard
A 0.2 g burn wound biopsy is homogenized in saline to a final volume of 2 mL. The homogenate is diluted 1:100, and 0.1 mL of this dilution grows 60 colonies. Using a cut-off of 10^5 CFU/g for invasive infection, the result is:
1. A6 × 10^4 CFU/g, below the cut-off
2. B6 × 10^3 CFU/g, below the cut-off
3. C1.2 × 10^5 CFU/g, above the cut-off
4. D6 × 10^5 CFU/g, above the cut-off
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Question 13 · Medium
When stool from an asymptomatic food handler is screened for Salmonella carriage, why is an enrichment broth such as selenite F also inoculated?
1. AThe broth allows the Salmonella serotype to be read directly
2. BSelenite F favors growth of the normal coliforms
3. CEnrichment replaces the need for selective agar
4. DCarriers often shed only small numbers of Salmonella
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Question 14 · Medium
A urethral swab for Neisseria gonorrhoeae culture should be:
1. AFrozen at -20 °C in glycerol broth for batch testing
2. BRefrigerated at 2-8 °C until the morning run
3. CPlated at once onto warmed selective agar in CO2
4. DPlaced in a dry tube and sent at room temperature
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Wound, genital and stool cultures

For prenatal group B streptococcus (GBS) screening, which specimen is collected?

Twelve wedding guests develop vomiting and cramps 2 to 4 hours after eating the same rice dish, and all recover within a day. The most useful laboratory investigation is:

For culture of a deep wound infection, the most useful specimen is:

A Gram stain of a swab from a diabetic patient's leg ulcer shows many squamous epithelial cells, no neutrophils and mixed bacteria. What does this most likely indicate?

Many laboratories do not perform routine stool culture for Salmonella, Shigella and Campylobacter on patients who have been in hospital for more than:

A swab from a patient's infected surgical wound grows a moderate amount of Staphylococcus aureus with a few colonies of coagulase-negative staphylococci and diphtheroids. The best report is:

A multiplex gastrointestinal PCR panel on a child's stool detects Salmonella. A portion of the stool was also sent in Cary-Blair medium. What should the laboratory do next?

A saline wet mount of vaginal discharge is examined 3 hours after collection and no trichomonads are seen, but a Trichomonas vaginalis NAAT from the same visit is positive. What is the most likely explanation?

An aquarium worker has a slowly enlarging nodule on the back of her hand, and a biopsy is sent for mycobacterial culture. At which temperature must the culture be incubated to recover the most likely agent?

A clinician sends a vaginal swab requesting 'culture for Gardnerella vaginalis' from a woman with a thin, fishy-smelling discharge. What is the most appropriate laboratory response?

A 0.2 g burn wound biopsy is homogenized in saline to a final volume of 2 mL. The homogenate is diluted 1:100, and 0.1 mL of this dilution grows 60 colonies. Using a cut-off of 10^5 CFU/g for invasive infection, the result is:

When stool from an asymptomatic food handler is screened for Salmonella carriage, why is an enrichment broth such as selenite F also inoculated?

A urethral swab for Neisseria gonorrhoeae culture should be: