Fourteen people fall ill 1 to 5 hours after a staff lunch. All ate fried rice made from rice boiled the day before, left at room temperature overnight and briefly reheated. Most have nausea and vomiting, few have diarrhea, and none has fever. Leftover rice is cultured: mannitol egg yolk polymyxin (MYP) agar grows 10^7 CFU/g of pink colonies surrounded by a white precipitate. The isolate is a large, beta-hemolytic, motile gram-positive bacillus with central to subterminal spores that do not swell the cell. The illness is best explained by:

Bacillus cereus emetic toxin (cereulide)

Clostridium perfringens enterotoxin

B. cereus heat-labile diarrheal toxin

Staphylococcus aureus enterotoxin A

Which set of results identifies Listeria monocytogenes?

Catalase positive, bile esculin positive, tumbling motility at 25 °C

Catalase positive, bile esculin negative, motile at 37 °C only

Catalase negative, bile esculin positive, non-motile

Catalase negative, bile esculin negative, tumbling motility at 25 °C

A blood culture from a woman 32 weeks pregnant grows short gram-positive rods that are catalase positive, show tumbling motility at 25 °C and give narrow beta-hemolysis. Which comment belongs on the report?

Cephalosporins are not active; use ampicillin

Vancomycin is the only agent with reliable activity

A third-generation cephalosporin is the drug of choice

Therapy can wait until full susceptibility results are available

Aerobic gram-positive bacilli

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Aerobic gram-positive bacilli, part of Microbiology. The first 10 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
Fourteen people fall ill 1 to 5 hours after a staff lunch. All ate fried rice made from rice boiled the day before, left at room temperature overnight and briefly reheated. Most have nausea and vomiting, few have diarrhea, and none has fever. Leftover rice is cultured: mannitol egg yolk polymyxin (MYP) agar grows 10^7 CFU/g of pink colonies surrounded by a white precipitate. The isolate is a large, beta-hemolytic, motile gram-positive bacillus with central to subterminal spores that do not swell the cell. The illness is best explained by:
1. ABacillus cereus emetic toxin (cereulide)
2. BClostridium perfringens enterotoxin
3. CB. cereus heat-labile diarrheal toxin
4. DStaphylococcus aureus enterotoxin A
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Answer: A · Bacillus cereus emetic toxin (cereulide)
Pink (mannitol-negative), lecithinase-positive colonies on MYP agar, with large, motile, beta-hemolytic spore-forming rods, identify Bacillus cereus. An incubation of 1 to 6 hours with mainly vomiting points to the emetic syndrome, caused by cereulide, a small heat-stable toxin preformed in starchy food such as cooked rice left warm. Reheating does not destroy it. The diarrheal syndrome is caused by heat-labile enterotoxins made in the gut and starts after 8 to 16 hours. Staphylococcal food poisoning also has a short incubation, but the rice grew B. cereus, not staphylococci.
- B: C. perfringens causes diarrhea and cramps 8 to 16 hours after eating, usually from meat dishes.
- C: The diarrheal toxins act after 8 to 16 hours and cause mainly diarrhea, not vomiting.
- D: The rice grew Bacillus cereus at 10^7 CFU/g, not staphylococci.
Question 2 · Medium
Which set of results identifies Listeria monocytogenes?
1. ACatalase positive, bile esculin negative, motile at 37 °C only
2. BCatalase negative, bile esculin positive, non-motile
3. CCatalase positive, bile esculin positive, tumbling motility at 25 °C
4. DCatalase negative, bile esculin negative, tumbling motility at 25 °C
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Answer: C · Catalase positive, bile esculin positive, tumbling motility at 25 °C
Listeria is a catalase-positive, bile esculin-positive short gram-positive rod with a narrow zone of beta-hemolysis. Its flagella work best at 20 to 25 °C, which gives tumbling motility in a wet mount and an umbrella pattern in semisolid medium incubated at room temperature. It also multiplies at 4 °C, the basis of cold enrichment from food and tissue. A positive CAMP test with Staphylococcus aureus confirms the identification.
- A: Listeria is motile at 25 °C and sluggish or non-motile at 37 °C, the opposite pattern.
- B: A catalase-negative, esculin-positive, non-motile organism fits the enterococci and group D streptococci.
- D: A catalase-negative result rules out Listeria.
Question 3 · Hard
A blood culture from a woman 32 weeks pregnant grows short gram-positive rods that are catalase positive, show tumbling motility at 25 °C and give narrow beta-hemolysis. Which comment belongs on the report?
1. AVancomycin is the only agent with reliable activity
2. BA third-generation cephalosporin is the drug of choice
3. CTherapy can wait until full susceptibility results are available
4. DCephalosporins are not active; use ampicillin
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Answer: D · Cephalosporins are not active; use ampicillin
The findings identify Listeria monocytogenes, which is intrinsically resistant to all cephalosporins, including third-generation agents. Ampicillin or penicillin, often with gentamicin for synergy, is the accepted treatment, so flagging the organism changes therapy at once. Listeriosis in pregnancy threatens the fetus even when the mother is only mildly ill. CLSI provides breakpoints for a small number of agents, so a full panel is neither available nor needed.
- A: Vancomycin has activity but is not the preferred agent; ampicillin is.
- B: Cephalosporins have no useful activity against Listeria.
- C: Delay is unsafe because the fetus is at risk and the organism is predictable.
Question 4 · Medium
Sputum from a kidney transplant recipient grows dry, chalky white colonies with aerial filaments after 4 days of aerobic incubation. The organisms are beaded, branching gram-positive rods that are positive by the modified acid-fast stain. Which genus fits best?
1. AActinomyces
2. BNocardia
3. CRhodococcus
4. DStreptomyces
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Answer: B · Nocardia
Nocardia is a strictly aerobic actinomycete that grows slowly on routine media, forms dry adherent colonies with aerial hyphae, and keeps carbolfuchsin when a weak (1%) acid decolorizer is used. Partial acid-fastness plus aerobic growth separates it from the anaerobic actinomycetes. Pulmonary and disseminated disease is typical of transplant recipients and others on long-term steroids. Plates must be held beyond the routine 48 hours or the organism is missed.
- A: Actinomyces is anaerobic to microaerophilic and is not acid-fast.
- C: Rhodococcus is coccoid to short rod-shaped, often salmon-pink, and does not form branching filaments.
- D: Streptomyces forms aerial hyphae but is not acid-fast.
Question 5 · Easy
Metachromatic granules in Corynebacterium diphtheriae are best demonstrated by which stain?
1. AZiehl-Neelsen stain
2. BAlbert stain
3. CGiemsa stain
4. DIndia ink preparation
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Answer: B · Albert stain
Albert, Neisser and Loeffler methylene blue stains show the polymetaphosphate granules that give C. diphtheriae its beaded appearance, together with club-shaped cells in Chinese-letter arrangements. The granules are a morphological clue only: identification still needs culture on a tellurite-containing medium and biochemical tests. Toxigenicity must then be shown before diphtheria is confirmed. Other corynebacteria of the skin flora can look identical on a smear.
- A: Ziehl-Neelsen stains acid-fast organisms, not metachromatic granules.
- C: Giemsa is used for blood parasites and intracellular organisms.
- D: India ink outlines yeast capsules such as those of Cryptococcus.
Question 6 · Medium
A throat isolate forms gray-black colonies with brown halos on Tinsdale medium. Which single result excludes Corynebacterium diphtheriae?
1. AA positive catalase test
2. BGrowth on sheep blood agar
3. CA positive urease test
4. DA positive nitrate reduction test
Answer and explanation in LearnFree account
Question 7 · Hard
An identification system reports "Corynebacterium species" for a catalase-positive gram-positive rod from the blood culture of a 70-year-old man taking corticosteroids. The isolate is beta-hemolytic and motile at 25 °C. The best next step is:
1. ARepeat the Gram stain and report the morphology only
2. BWork the isolate up as Listeria monocytogenes
3. CReport it as Corynebacterium species, as identified
4. DReport it as a skin contaminant of no significance
Answer and explanation in LearnFree account
Question 8 · Medium
On mannitol egg yolk polymyxin (MYP) agar, Bacillus cereus forms pink colonies surrounded by a white precipitate. These two reactions show that the isolate:
1. ADoes not ferment mannitol and produces lecithinase
2. BDoes not ferment mannitol and hydrolyzes starch
3. CFerments mannitol and produces no extracellular enzyme
4. DFerments mannitol and produces lecithinase
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Question 9 · Hard
Why should a Nocardia isolate from a lung abscess be identified to species level and tested by broth microdilution?
1. ASusceptibility differs widely between species
2. BDisk diffusion results for Nocardia are reliable
3. CNocardia species are uniformly susceptible to trimethoprim-sulfamethoxazole
4. DA species name removes the need for susceptibility testing
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Question 10 · Hard
A large, non-hemolytic, non-motile aerobic gram-positive bacillus from a skin lesion is subcultured onto agar containing penicillin and forms chains of swollen round cells. This "string of pearls" reaction supports:
1. ABacillus thuringiensis
2. BBacillus anthracis
3. CBacillus subtilis
4. DBacillus cereus
Answer and explanation in LearnFree account

Aerobic gram-positive bacilli

Which set of results identifies Listeria monocytogenes?

A blood culture from a woman 32 weeks pregnant grows short gram-positive rods that are catalase positive, show tumbling motility at 25 °C and give narrow beta-hemolysis. Which comment belongs on the report?

Sputum from a kidney transplant recipient grows dry, chalky white colonies with aerial filaments after 4 days of aerobic incubation. The organisms are beaded, branching gram-positive rods that are positive by the modified acid-fast stain. Which genus fits best?

Metachromatic granules in Corynebacterium diphtheriae are best demonstrated by which stain?

A throat isolate forms gray-black colonies with brown halos on Tinsdale medium. Which single result excludes Corynebacterium diphtheriae?

An identification system reports "Corynebacterium species" for a catalase-positive gram-positive rod from the blood culture of a 70-year-old man taking corticosteroids. The isolate is beta-hemolytic and motile at 25 °C. The best next step is:

On mannitol egg yolk polymyxin (MYP) agar, Bacillus cereus forms pink colonies surrounded by a white precipitate. These two reactions show that the isolate:

Why should a Nocardia isolate from a lung abscess be identified to species level and tested by broth microdilution?

A large, non-hemolytic, non-motile aerobic gram-positive bacillus from a skin lesion is subcultured onto agar containing penicillin and forms chains of swollen round cells. This "string of pearls" reaction supports: