A Staphylococcus aureus from a 45-year-old man's skin abscess gives these results: oxacillin MIC 4 µg/mL (S ≤2, R ≥4); cefoxitin disk 26 mm (R ≤21, S ≥22); PBP2a latex test on growth taken next to a cefoxitin disk, negative; mecA and mecC PCR, not detected; nitrocefin test on induced growth, strongly positive; penicillin resistant. With clavulanate added, the oxacillin MIC falls to 0.5 µg/mL. Which mechanism best explains the raised oxacillin MIC?

Hyperproduction of penicillinase

A meropenem-resistant Klebsiella pneumoniae (MIC 16 µg/mL) from urine is tested by mCIM and eCIM. After incubation, the meropenem disks on the Escherichia coli ATCC 25922 lawn look as shown. The most likely carbapenemase class is:

Metallo-beta-lactamase, such as NDM

OXA-48-like serine carbapenemase

Serine carbapenemase, such as KPC

The D-zone test shown was set up on a Staphylococcus aureus isolate, with erythromycin (E) and clindamycin (CC) disks 18 mm apart edge to edge. The result is best explained by:

An msr(A) macrolide efflux pump

An inducible erm ribosomal methylase

An lnu(A) lincosamide-inactivating enzyme

A constitutive erm ribosomal methylase

Resistance: phenotypic detection (beta-lactamase, ESBL, D-test, carbapenemases)

19 ASCP MLS practice questions on Resistance: phenotypic detection (beta-lactamase, ESBL, D-test, carbapenemases), 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.

Question 1 · Hard
A Staphylococcus aureus from a 45-year-old man's skin abscess gives these results: oxacillin MIC 4 µg/mL (S ≤2, R ≥4); cefoxitin disk 26 mm (R ≤21, S ≥22); PBP2a latex test on growth taken next to a cefoxitin disk, negative; mecA and mecC PCR, not detected; nitrocefin test on induced growth, strongly positive; penicillin resistant. With clavulanate added, the oxacillin MIC falls to 0.5 µg/mL. Which mechanism best explains the raised oxacillin MIC?
1. AmecC-encoded PBP2a homologue
2. BHyperproduction of penicillinase
3. CModified native PBPs (MODSA)
4. DmecA-encoded PBP2a
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Answer: B · Hyperproduction of penicillinase
The isolate lacks mecA, mecC and PBP2a, and its cefoxitin zone is susceptible, so it is not MRSA. The strong beta-lactamase reaction and the fall in oxacillin MIC when clavulanate is added show that the enzyme itself is slowly breaking down oxacillin. This borderline phenotype (BORSA) is caused by hyperproduction of staphylococcal penicillinase and gives oxacillin MICs of about 1-8 µg/mL. Modified native PBPs (MODSA) can also raise the oxacillin MIC slightly, but a beta-lactamase inhibitor does not reverse that effect. Cefoxitin is a poor substrate for the penicillinase, which is why its zone stays susceptible.
- A: mecC was not detected, and mecC strains usually test cefoxitin resistant.
- C: Altered native PBPs are not reversed by clavulanate.
- D: mecA and PBP2a were both negative.
Question 2 · Hard
A 66-year-old man with a liver abscess has Enterobacter cloacae complex in his blood and is treated with ceftriaxone. The admission isolate is ceftriaxone S, ceftazidime S, cefepime S, meropenem S and cefoxitin R. On day 6 he is febrile again, and the abscess drain grows E. cloacae complex that is ceftriaxone R, ceftazidime R, piperacillin-tazobactam R, cefoxitin R, cefepime S and meropenem S. A cefepime disk with and without clavulanate shows no zone difference, and the mCIM is negative. The most likely mechanism is:
1. ADerepressed chromosomal AmpC
2. BOXA-48-like carbapenemase
3. CAcquired CTX-M-type ESBL
4. DKPC-type carbapenemase
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Answer: A · Derepressed chromosomal AmpC
E. cloacae complex carries a chromosomal ampC gene that is normally expressed only at a low, inducible level, which is why the first isolate was already cefoxitin resistant but ceftriaxone susceptible. During therapy with a third-generation cephalosporin, mutants that make AmpC constantly at a high level (derepressed) are selected, often within a few days. They resist ceftriaxone, ceftazidime and piperacillin-tazobactam, while cefepime, a poor AmpC substrate, and the carbapenems stay active. No clavulanate effect on the cefepime disk argues against an ESBL, and a negative mCIM rules out a carbapenemase. CLSI warns that Enterobacter, Klebsiella aerogenes and Citrobacter freundii can become resistant this way during therapy, so repeat isolates should be retested.
- B: An OXA-48-like enzyme would give a positive mCIM.
- C: An ESBL would usually be inhibited by clavulanate and would often affect cefepime.
- D: A KPC enzyme would make the isolate meropenem resistant and mCIM positive.
Question 3 · Medium
The Carba NP tubes shown, read after 2 hours at 35 °C, are from three Enterobacterales isolates. Which isolate should be reported as a carbapenemase producer?
Figure: MedExams
1. AIsolate 3 only
2. BIsolate 1 only
3. CIsolate 2 only
4. DIsolates 2 and 3
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Answer: C · Isolate 2 only
In the Carba NP test, a carbapenemase hydrolyses the imipenem in tube B, and the acid released turns the phenol red indicator from red to orange or yellow, while tube A, without imipenem, stays red as the control. Isolate 2 shows exactly this. Isolate 1 is negative, with both tubes red. Isolate 3 is invalid, not positive, because its control tube A also turned yellow, so the colour change does not depend on imipenem; repeat it or test by another method such as the mCIM.
- A: A yellow control tube makes isolate 3 uninterpretable, not positive.
- B: Isolate 1 kept both tubes red, which is a negative result.
- D: Isolate 3 is invalid because its control tube A also changed colour.
Question 4 · Hard
A meropenem-resistant Klebsiella pneumoniae (MIC 16 µg/mL) from urine is tested by mCIM and eCIM. After incubation, the meropenem disks on the Escherichia coli ATCC 25922 lawn look as shown. The most likely carbapenemase class is:
Figure: MedExams
1. AOXA-48-like serine carbapenemase
2. BSerine carbapenemase, such as KPC
3. CNone; AmpC with porin loss
4. DMetallo-beta-lactamase, such as NDM
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Answer: D · Metallo-beta-lactamase, such as NDM
In the mCIM, a meropenem disk is soaked in a suspension of the test organism and then placed on a lawn of susceptible E. coli ATCC 25922. The mCIM disk here has no zone (6 mm, the disk itself), so the isolate destroyed the meropenem: carbapenemase positive (6 to 15 mm is positive, 19 mm or more negative). In the eCIM, EDTA chelates the zinc that metallo-beta-lactamases need, and the eCIM zone of about 22 mm is at least 5 mm larger than the mCIM zone. That restored activity points to a metallo-beta-lactamase such as NDM, VIM or IMP; serine enzymes such as KPC and OXA-48 are not inhibited by EDTA.
- A: OXA-48-like enzymes are serine enzymes, and EDTA would not enlarge the zone.
- B: KPC is not inhibited by EDTA, so the eCIM zone would stay as small as the mCIM zone.
- C: AmpC with porin loss does not destroy the meropenem disk, so the mCIM zone would be 19 mm or more.
Question 5 · Medium
The D-zone test shown was set up on a Staphylococcus aureus isolate, with erythromycin (E) and clindamycin (CC) disks 18 mm apart edge to edge. The result is best explained by:
Figure: MedExams
1. AAn msr(A) macrolide efflux pump
2. BAn inducible erm ribosomal methylase
3. CAn lnu(A) lincosamide-inactivating enzyme
4. DA constitutive erm ribosomal methylase
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Answer: A · An msr(A) macrolide efflux pump
There is no zone around the erythromycin disk, so the isolate is erythromycin resistant. The clindamycin zone is round, with no flattening on the side facing erythromycin, so erythromycin did not induce clindamycin resistance. This fits an msr(A) efflux pump, which pumps out macrolides and streptogramin B but not lincosamides. Clindamycin can be reported susceptible (zone 21 mm or more); an inducible erm gene would have blunted the zone into a D.
- B: Inducible erm would flatten the clindamycin zone next to erythromycin into a D.
- C: lnu(A) inactivates clindamycin but leaves erythromycin active, the reverse of this plate.
- D: Constitutive erm also makes the isolate clindamycin resistant, with no zone around either disk.
Question 6 · Easy
Which two disks are placed close together in the D-zone test for inducible clindamycin resistance?
1. ACefoxitin and oxacillin
2. BVancomycin and linezolid
3. CErythromycin and clindamycin
4. DCeftazidime and clavulanate
Answer and explanation in LearnFree account
Question 7 · Hard
A meropenem-resistant Klebsiella pneumoniae gives an mCIM zone of 6 mm. In the paired eCIM, with EDTA added, the zone is 8 mm. What type of enzyme is most likely present?
1. AAmpC with porin loss, no carbapenemase
2. BESBL with efflux, no carbapenemase
3. CMetallo-beta-lactamase such as NDM
4. DSerine carbapenemase such as KPC
Answer and explanation in LearnFree account
Question 8 · Medium
An Enterococcus faecalis isolate from a patient with endocarditis grows on brain-heart infusion agar containing 500 µg/mL gentamicin. This means:
1. AThe isolate is resistant to vancomycin
2. BAmpicillin plus gentamicin will not be synergistic
3. CGentamicin alone will treat the infection
4. DSynergy with gentamicin is still expected
Answer and explanation in LearnFree account
Question 9 · Hard
A carbapenem-resistant Klebsiella pneumoniae is mCIM positive, ceftazidime-avibactam resistant and aztreonam susceptible. The most likely carbapenemase is:
1. AA class D oxacillinase (OXA-48 type)
2. BA class A serine enzyme (KPC type)
3. CAn AmpC enzyme with porin loss
4. DA metallo-beta-lactamase (NDM type)
Answer and explanation in LearnFree account
Question 10 · Medium
Which test names the carbapenemase family (KPC, NDM, VIM, IMP or OXA-48-like) from colonies in about 15 minutes, without nucleic acid amplification?
1. AModified carbapenem inactivation method
2. BCarba NP test
3. CModified Hodge test
4. DLateral flow immunoassay
Answer and explanation in LearnFree account
Question 11 · Easy
In a nitrocefin (chromogenic cephalosporin) test, a beta-lactamase producer causes:
1. ABlue fluorescence under UV light
2. BA yellow-to-red colour change
3. CBubbles when the reagent is added
4. DA red-to-yellow colour change
Answer and explanation in LearnFree account
Question 12 · Medium
Under the current CLSI cephalosporin breakpoints for Enterobacterales, ESBL confirmatory testing is:
1. ARequired before any cephalosporin result is reported
2. BNot needed for reporting but useful for infection control
3. CRecommended only for Pseudomonas aeruginosa
4. DRequired so that all cephalosporins can be edited to resistant
Answer and explanation in LearnFree account
Question 13 · Medium
In the Carba NP test, a carbapenemase producer is detected because:
1. AThe blaKPC gene is amplified
2. BIndicator E. coli grows near a disk
3. CImipenem hydrolysis lowers the pH
4. DAntibody-coated particles bind the enzyme
Answer and explanation in LearnFree account
Question 14 · Hard
ESBL confirmation on an Escherichia coli isolate gives these zones: ceftazidime 15 mm, ceftazidime-clavulanate 23 mm; cefotaxime 21 mm, cefotaxime-clavulanate 24 mm. What is the interpretation?
1. AESBL negative: both agents must increase by 5 mm
2. BIndeterminate: repeat with cefepime-clavulanate
3. CAmpC positive: cefotaxime rose by less than 5 mm
4. DESBL positive: one agent increased by 5 mm or more
Answer and explanation in LearnFree account
Question 15 · Hard
A patient with Staphylococcus aureus bacteremia has a penicillin MIC of 0.06 µg/mL (susceptible ≤0.12). The nitrocefin test is negative, but the penicillin zone-edge test shows a sharp, cliff-like edge. How should penicillin be reported?
1. AResistant: the sharp edge shows beta-lactamase
2. BSusceptible: the MIC and nitrocefin agree
3. CSusceptible: zone-edge results are not used for S. aureus
4. DResistant only if a repeat nitrocefin is positive
Answer and explanation in LearnFree account
Question 16 · Medium
A 68-year-old with a COPD exacerbation has Haemophilus influenzae in sputum, and the nitrocefin test is positive. This predicts resistance to:
1. AAmpicillin and amoxicillin
2. BCeftriaxone and cefotaxime
3. CAmoxicillin-clavulanate
4. DAzithromycin
Answer and explanation in LearnFree account
Question 17 · Medium
A Staphylococcus aureus isolate is erythromycin resistant and clindamycin susceptible by zone size. In a D-test, the clindamycin zone is flattened on the side facing the erythromycin disk. How is clindamycin reported?
1. AIntermediate
2. BSusceptible
3. CResistant
4. DSusceptible-dose dependent
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Question 18 · Hard
A 55-year-old with recurrent urinary infections has Klebsiella pneumoniae with these results: ceftazidime R, cefotaxime R, cefoxitin R, cefepime S, meropenem S; clavulanate does not enlarge the cephalosporin zones. The mechanism that best explains this is:
1. APlasmid-mediated AmpC beta-lactamase
2. BExtended-spectrum beta-lactamase
3. CKPC carbapenemase
4. DChromosomal SHV-1 penicillinase
Answer and explanation in LearnFree account
Question 19 · Hard
An E. coli isolate is resistant to ceftazidime and cefotaxime, and the zone of inhibition grows by 5 mm or more when clavulanic acid is added. What does this show?
1. AA plasmid-mediated AmpC beta-lactamase
2. BA carbapenemase such as KPC or NDM
3. CAn extended-spectrum beta-lactamase (ESBL)
4. DLoss of outer membrane porins only
Answer and explanation in LearnFree account

Resistance: phenotypic detection (beta-lactamase, ESBL, D-test, carbapenemases)

The Carba NP tubes shown, read after 2 hours at 35 °C, are from three Enterobacterales isolates. Which isolate should be reported as a carbapenemase producer?

A meropenem-resistant Klebsiella pneumoniae (MIC 16 µg/mL) from urine is tested by mCIM and eCIM. After incubation, the meropenem disks on the Escherichia coli ATCC 25922 lawn look as shown. The most likely carbapenemase class is:

The D-zone test shown was set up on a Staphylococcus aureus isolate, with erythromycin (E) and clindamycin (CC) disks 18 mm apart edge to edge. The result is best explained by:

Which two disks are placed close together in the D-zone test for inducible clindamycin resistance?

A meropenem-resistant Klebsiella pneumoniae gives an mCIM zone of 6 mm. In the paired eCIM, with EDTA added, the zone is 8 mm. What type of enzyme is most likely present?

An Enterococcus faecalis isolate from a patient with endocarditis grows on brain-heart infusion agar containing 500 µg/mL gentamicin. This means:

A carbapenem-resistant Klebsiella pneumoniae is mCIM positive, ceftazidime-avibactam resistant and aztreonam susceptible. The most likely carbapenemase is:

Which test names the carbapenemase family (KPC, NDM, VIM, IMP or OXA-48-like) from colonies in about 15 minutes, without nucleic acid amplification?

In a nitrocefin (chromogenic cephalosporin) test, a beta-lactamase producer causes:

Under the current CLSI cephalosporin breakpoints for Enterobacterales, ESBL confirmatory testing is:

In the Carba NP test, a carbapenemase producer is detected because:

ESBL confirmation on an Escherichia coli isolate gives these zones: ceftazidime 15 mm, ceftazidime-clavulanate 23 mm; cefotaxime 21 mm, cefotaxime-clavulanate 24 mm. What is the interpretation?

A patient with Staphylococcus aureus bacteremia has a penicillin MIC of 0.06 µg/mL (susceptible ≤0.12). The nitrocefin test is negative, but the penicillin zone-edge test shows a sharp, cliff-like edge. How should penicillin be reported?

A 68-year-old with a COPD exacerbation has Haemophilus influenzae in sputum, and the nitrocefin test is positive. This predicts resistance to:

A Staphylococcus aureus isolate is erythromycin resistant and clindamycin susceptible by zone size. In a D-test, the clindamycin zone is flattened on the side facing the erythromycin disk. How is clindamycin reported?

A 55-year-old with recurrent urinary infections has Klebsiella pneumoniae with these results: ceftazidime R, cefotaxime R, cefoxitin R, cefepime S, meropenem S; clavulanate does not enlarge the cephalosporin zones. The mechanism that best explains this is:

An E. coli isolate is resistant to ceftazidime and cefotaxime, and the zone of inhibition grows by 5 mm or more when clavulanic acid is added. What does this show?