A tracheal aspirate from a ventilated patient who has received meropenem for 10 days grows a non-lactose-fermenting gram-negative rod that is resistant to meropenem and imipenem but susceptible to trimethoprim-sulfamethoxazole. The laboratory has no MALDI-TOF and must decide whether to raise a carbapenem-resistant organism alert for infection control. Results: TSI K/K (no change); oxidase negative; motile; DNase positive; lysine decarboxylase positive; good growth on MacConkey agar at 24 hours; no growth on CNA agar. What is the best interpretation?

Stenotrophomonas; the resistance is intrinsic

Burkholderia cepacia complex; refer it

Acinetobacter baumannii; raise a CRAB alert

P. aeruginosa; test for a carbapenemase

A blood culture from a 60-year-old ICU patient with a central line flags positive at 16 hours. The Gram stain is reported as gram-positive cocci in pairs, and the result is phoned to the ward. Next day, sheep blood and MacConkey agars both grow non-lactose-fermenting colonies, while colistin-nalidixic acid agar shows no growth. The isolate is oxidase negative, catalase positive and non-motile, and does not ferment glucose. A repeat Gram stain of the bottle broth, decolorized with fresh reagent, shows only plump gram-negative coccobacilli in pairs. The best explanation for the first report is:

Acinetobacter that resisted decolorization

A coccus in the broth that failed to subculture

Contamination of the MacConkey agar plate

Moraxella misread as gram-positive cocci

Carbapenem resistance in Acinetobacter baumannii is most often explained by:

Acquired OXA-type carbapenemases

An extended-spectrum beta-lactamase

Non-fermenting gram-negative bacilli

15 DOH Abu Dhabi (HAAD) lab licensing practice questions on Non-fermenting gram-negative bacilli, part of Microbiology. The first 15 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 tracheal aspirate from a ventilated patient who has received meropenem for 10 days grows a non-lactose-fermenting gram-negative rod that is resistant to meropenem and imipenem but susceptible to trimethoprim-sulfamethoxazole. The laboratory has no MALDI-TOF and must decide whether to raise a carbapenem-resistant organism alert for infection control. Results: TSI K/K (no change); oxidase negative; motile; DNase positive; lysine decarboxylase positive; good growth on MacConkey agar at 24 hours; no growth on CNA agar. What is the best interpretation?
1. ABurkholderia cepacia complex; refer it
2. BStenotrophomonas; the resistance is intrinsic
3. CAcinetobacter baumannii; raise a CRAB alert
4. DP. aeruginosa; test for a carbapenemase
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Answer: B · Stenotrophomonas; the resistance is intrinsic
An oxidase-negative, motile, DNase-positive and lysine-positive non-fermenter is Stenotrophomonas maltophilia. Acinetobacter is non-motile and DNase negative, Pseudomonas aeruginosa is oxidase positive, and the Burkholderia cepacia complex is DNase negative and usually weakly oxidase positive. S. maltophilia carries the chromosomal L1 metallo-beta-lactamase, so it is intrinsically resistant to carbapenems, and long carbapenem therapy selects it. Its carbapenem resistance is expected and does not call for a carbapenemase alert; trimethoprim-sulfamethoxazole is the usual treatment.
- A: The B. cepacia complex is DNase negative and usually weakly oxidase positive.
- C: Acinetobacter is non-motile and DNase negative.
- D: P. aeruginosa is oxidase positive.
Question 2 · Hard
A blood culture from a 60-year-old ICU patient with a central line flags positive at 16 hours. The Gram stain is reported as gram-positive cocci in pairs, and the result is phoned to the ward. Next day, sheep blood and MacConkey agars both grow non-lactose-fermenting colonies, while colistin-nalidixic acid agar shows no growth. The isolate is oxidase negative, catalase positive and non-motile, and does not ferment glucose. A repeat Gram stain of the bottle broth, decolorized with fresh reagent, shows only plump gram-negative coccobacilli in pairs. The best explanation for the first report is:
1. AA coccus in the broth that failed to subculture
2. BContamination of the MacConkey agar plate
3. CAcinetobacter that resisted decolorization
4. DMoraxella misread as gram-positive cocci
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Answer: C · Acinetobacter that resisted decolorization
Growth on MacConkey agar but not on CNA, an oxidase-negative, non-motile, non-fermenting pattern and coccobacilli in pairs identify Acinetobacter. Acinetobacter cells are plump and tend to hold crystal violet, so in a thick or under-decolorized smear they can look like gram-positive diplococci. The repeat smear of the same broth shows only gram-negative coccobacilli, so there is no second organism. Moraxella is oxidase positive and seldom grows on MacConkey agar, and contamination cannot explain the same growth on two plates. The laboratory must issue a corrected report and tell the ward, as empirical therapy may need to change.
- A: The repeat smear of the same broth shows only gram-negative coccobacilli, so no coccus is present.
- B: Contamination of one plate cannot explain the same organism on two media and in the broth.
- D: Moraxella is oxidase positive and seldom grows on MacConkey agar.
Question 3 · Easy
The blue-green pigment of Pseudomonas aeruginosa is:
1. AProdigiosin
2. BPyocyanin
3. CPyoverdin
4. DViolacein
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Answer: B · Pyocyanin
Pyocyanin is a water-soluble phenazine pigment made almost only by Pseudomonas aeruginosa, and with the yellow-green fluorescent pyoverdin it gives the familiar blue-green colony and agar. Pigment production is clearest on low-iron media such as Pseudomonas agar P. Some strains, particularly from cystic fibrosis sputum, lose pigment, so biochemical confirmation is still needed.
- A: Prodigiosin is the red pigment of some Serratia marcescens strains.
- C: Pyoverdin is the yellow-green fluorescent siderophore pigment.
- D: Violacein is the purple pigment of Chromobacterium violaceum.
Question 4 · Hard
Carbapenem resistance in Acinetobacter baumannii is most often explained by:
1. AA plasmid-borne mcr-1 gene
2. BAn extended-spectrum beta-lactamase
3. CAcquired OXA-type carbapenemases
4. DLoss of the OprD porin
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Answer: C · Acquired OXA-type carbapenemases
Most carbapenem-resistant Acinetobacter baumannii carries an acquired class D oxacillinase such as OXA-23, often with an upstream insertion sequence that drives strong expression; metallo-beta-lactamases such as NDM are also reported from the region. Because the enzyme is only one part of the picture, these strains usually test resistant to several classes at once. Carbapenem resistance in this species is therefore not a reliable screen for a particular enzyme, and molecular testing is needed to name it.
- A: mcr-1 confers colistin resistance, not carbapenem resistance.
- B: Extended-spectrum beta-lactamases do not hydrolyse carbapenems.
- D: OprD porin loss is a Pseudomonas aeruginosa mechanism.
Question 5 · Medium
A CSF Gram stain shows gram-negative coccobacilli in pairs, and Neisseria meningitidis is suspected. Which test separates Acinetobacter quickly?
1. AGlucose oxidation
2. BOxidase
3. CGrowth on chocolate agar
4. DCatalase
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Answer: B · Oxidase
Neisseria meningitidis is oxidase positive and Acinetobacter is oxidase negative, so the test can be done on a colony within a minute and prevents a serious misreport. Both organisms are catalase positive and both grow on chocolate agar, so neither of those helps. Acinetobacter also grows on MacConkey agar, which Neisseria does not.
- A: Glucose oxidation is variable in Acinetobacter and slow to read.
- C: Both grow on chocolate agar.
- D: Both organisms are catalase positive.
Question 6 · Hard
Sputum from a ventilated patient grows Stenotrophomonas maltophilia, and disk diffusion shows a wide zone around meropenem. The laboratory should report:
1. AMeropenem susceptible, as measured
2. BMeropenem intermediate, by inference
3. CMeropenem susceptible, with a comment on dosing and review
4. DNo carbapenem result; the L1 enzyme makes it unreliable
Answer and explanation in LearnFree account
Question 7 · Easy
On a Gram stain of a tracheal aspirate, Acinetobacter baumannii usually appears as:
1. AGram-negative coccobacilli in pairs
2. BCurved gram-negative rods
3. CLong thin gram-negative rods in chains
4. DGram-positive cocci in clusters
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Question 8 · Medium
Which pair of results separates Stenotrophomonas maltophilia from the Burkholderia cepacia complex?
1. AOxidase negative and DNase negative
2. BOxidase negative and DNase positive
3. COxidase positive and DNase negative
4. DOxidase positive and DNase positive
Answer and explanation in LearnFree account
Question 9 · Easy
In oxidative-fermentative (OF) medium, a non-fermenter such as Pseudomonas aeruginosa gives:
1. ANo acid in either tube
2. BAcid in the sealed tube only
3. CAcid in the open tube only
4. DAcid in the open and the sealed tube
Answer and explanation in LearnFree account
Question 10 · Hard
Mass spectrometry identifies a blood culture isolate as Acinetobacter calcoaceticus-baumannii complex. The best report is:
1. AAcinetobacter calcoaceticus-baumannii complex
2. BAcinetobacter lwoffii
3. CAcinetobacter calcoaceticus
4. DAn unidentified non-fermenting gram-negative rod
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Question 11 · Hard
A non-fermenter from a burn wound is oxidase positive and motile but makes no pigment. Which set of results confirms Pseudomonas aeruginosa?
1. ANo growth at 42 °C, arginine dihydrolase positive, ONPG positive
2. BNo growth at 42 °C, aesculin hydrolysed, urease positive
3. CGrowth at 42 °C, arginine dihydrolase positive, acetamide used
4. DGrowth at 42 °C, arginine dihydrolase negative, DNase positive
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Question 12 · Medium
Mucoid colonies of Pseudomonas aeruginosa in cystic fibrosis sputum are caused by:
1. AAccumulation of pyoverdin
2. BOverproduction of alginate
3. CA thickened peptidoglycan layer
4. DA K1 polysaccharide capsule
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Question 13 · Medium
Pseudomonas aeruginosa is separated from Pseudomonas fluorescens and Pseudomonas putida by:
1. AGrowth at 42 °C
2. BPolar flagellar motility
3. CGrowth on MacConkey agar
4. DA positive oxidase test
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Question 14 · Hard
Sputum from a 19-year-old with cystic fibrosis grows a non-fermenter on Burkholderia cepacia selective agar; it is oxidase positive and resistant to colistin and gentamicin. Before reporting, the laboratory should:
1. AConfirm the identification by a reference method
2. BReport colistin as the agent of choice
3. CReport it as mucoid Pseudomonas aeruginosa
4. DDiscard it as oropharyngeal flora
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Question 15 · Medium
An oxidase-negative, non-fermenting, non-motile gram-negative coccobacillus is isolated from an ICU patient and is resistant to many antibiotics. What is the most likely organism?
1. AMoraxella catarrhalis
2. BAcinetobacter baumannii
3. CKlebsiella pneumoniae
4. DPseudomonas aeruginosa
Answer and explanation in LearnFree account

Non-fermenting gram-negative bacilli

The blue-green pigment of Pseudomonas aeruginosa is:

Carbapenem resistance in Acinetobacter baumannii is most often explained by:

A CSF Gram stain shows gram-negative coccobacilli in pairs, and Neisseria meningitidis is suspected. Which test separates Acinetobacter quickly?

Sputum from a ventilated patient grows Stenotrophomonas maltophilia, and disk diffusion shows a wide zone around meropenem. The laboratory should report:

On a Gram stain of a tracheal aspirate, Acinetobacter baumannii usually appears as:

Which pair of results separates Stenotrophomonas maltophilia from the Burkholderia cepacia complex?

In oxidative-fermentative (OF) medium, a non-fermenter such as Pseudomonas aeruginosa gives:

Mass spectrometry identifies a blood culture isolate as Acinetobacter calcoaceticus-baumannii complex. The best report is:

A non-fermenter from a burn wound is oxidase positive and motile but makes no pigment. Which set of results confirms Pseudomonas aeruginosa?

Mucoid colonies of Pseudomonas aeruginosa in cystic fibrosis sputum are caused by:

Pseudomonas aeruginosa is separated from Pseudomonas fluorescens and Pseudomonas putida by:

Sputum from a 19-year-old with cystic fibrosis grows a non-fermenter on Burkholderia cepacia selective agar; it is oxidase positive and resistant to colistin and gentamicin. Before reporting, the laboratory should:

An oxidase-negative, non-fermenting, non-motile gram-negative coccobacillus is isolated from an ICU patient and is resistant to many antibiotics. What is the most likely organism?