A non-fermenting gram-negative rod from the sputum of a ventilated patient is resistant to meropenem but susceptible to trimethoprim-sulfamethoxazole and levofloxacin. Which organism does this intrinsic pattern fit?

Acinetobacter baumannii complex

An Enterococcus faecalis isolate from a patient with endocarditis shows high-level gentamicin resistance. What does this mean for treatment?

Ampicillin and gentamicin will no longer act synergistically

Gentamicin can now be used alone at standard doses

Vancomycin resistance can be assumed as well

The isolate must also be resistant to ampicillin

Fluoroquinolone resistance in Enterobacterales most often arises from:

Mutations in DNA gyrase and topoisomerase IV

An aminoglycoside-modifying acetyltransferase

Acquisition of the binding protein PBP2a

Methylation of the 23S ribosomal RNA target

Resistance: antibiotic mechanisms and intrinsic resistance

10 ASCP MLS practice questions on Resistance: antibiotic mechanisms and intrinsic resistance, 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
Blood cultures from a patient receiving vancomycin grow catalase-negative, non-sporing gram-positive rods that are vancomycin resistant. The most likely organism is:
1. ACorynebacterium striatum
2. BListeria monocytogenes
3. CCutibacterium acnes
4. DLactobacillus species
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Answer: D · Lactobacillus species
Lactobacillus, like Leuconostoc and Pediococcus, is intrinsically resistant to vancomycin, which is why it can cause breakthrough bacteremia in a patient already on the drug. Lactobacilli are catalase negative, which separates them from Listeria, Corynebacterium and Cutibacterium. Penicillin or ampicillin, often with gentamicin, is used for serious infection. This intrinsic glycopeptide resistance does not involve the van genes of enterococci.
- A: Corynebacterium striatum is catalase positive and remains vancomycin susceptible.
- B: Listeria is catalase positive and vancomycin susceptible.
- C: Cutibacterium acnes is catalase positive and vancomycin susceptible.
Question 2 · Medium
A non-fermenting gram-negative rod from the sputum of a ventilated patient is resistant to meropenem but susceptible to trimethoprim-sulfamethoxazole and levofloxacin. Which organism does this intrinsic pattern fit?
1. ABurkholderia cepacia
2. BStenotrophomonas maltophilia
3. CAcinetobacter baumannii complex
4. DPseudomonas aeruginosa
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Answer: B · Stenotrophomonas maltophilia
S. maltophilia carries two chromosomal beta-lactamases, including the L1 metallo-enzyme, so it is intrinsically resistant to all carbapenems. Trimethoprim-sulfamethoxazole is the drug of choice, with levofloxacin and minocycline as alternatives. Carbapenems are left out of its CLSI table because reporting them would mislead the clinician. The isolate is oxidase negative and DNase positive, which separates it from Pseudomonas.
- A: B. cepacia is usually susceptible to meropenem.
- C: Carbapenem resistance in the Acinetobacter baumannii complex is acquired, not intrinsic.
- D: P. aeruginosa is usually carbapenem susceptible and is intrinsically resistant to trimethoprim-sulfamethoxazole.
Question 3 · Hard
An Enterococcus faecalis isolate from a patient with endocarditis shows high-level gentamicin resistance. What does this mean for treatment?
1. AGentamicin can now be used alone at standard doses
2. BAmpicillin and gentamicin will no longer act synergistically
3. CVancomycin resistance can be assumed as well
4. DThe isolate must also be resistant to ampicillin
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Answer: B · Ampicillin and gentamicin will no longer act synergistically
Enterococci have low-level intrinsic aminoglycoside resistance, so an aminoglycoside alone is never adequate; it is added to a cell wall agent for synergy. High-level resistance, from a modifying enzyme or a ribosomal change, abolishes that synergy, so the result is reported to let the clinician choose another regimen such as ampicillin with ceftriaxone. Screening uses gentamicin at 500 micrograms/mL in broth or a 120 microgram disk. The result says nothing about ampicillin or vancomycin.
- A: Aminoglycosides are never used alone against enterococci because of intrinsic low-level resistance.
- C: Vancomycin resistance is tested separately and is not implied.
- D: Ampicillin is tested separately; the two results are independent.
Question 4 · Easy
Which antibiotic is Klebsiella pneumoniae intrinsically resistant to?
1. ACefepime
2. BMeropenem
3. CGentamicin
4. DAmpicillin
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Answer: D · Ampicillin
K. pneumoniae carries a chromosomal SHV-1 penicillinase that hydrolyses ampicillin, so wild-type strains are ampicillin resistant. The enzyme is inhibited by clavulanic acid, which is why ampicillin-sulbactam may still have activity. Resistance to cefepime, gentamicin or meropenem is acquired rather than intrinsic. An ampicillin-susceptible Klebsiella result should prompt a check of the identification.
- A: Cefepime resistance in Klebsiella is acquired, usually through an ESBL or AmpC enzyme.
- B: Meropenem resistance is acquired, usually with a carbapenemase.
- C: Gentamicin resistance is acquired through modifying enzymes.
Question 5 · Medium
Fluoroquinolone resistance in Enterobacterales most often arises from:
1. AAn aminoglycoside-modifying acetyltransferase
2. BMutations in DNA gyrase and topoisomerase IV
3. CAcquisition of the binding protein PBP2a
4. DMethylation of the 23S ribosomal RNA target
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Answer: B · Mutations in DNA gyrase and topoisomerase IV
Mutations in gyrA and parC alter the target enzymes and raise the MIC in steps, and efflux pumps and plasmid-borne qnr genes add to the effect. Because resistance is stepwise, an isolate may be reported susceptible to one quinolone and resistant to another, so each agent is tested. Ribosomal methylation, modifying enzymes and PBP2a are mechanisms for other drug classes. Quinolone MICs must be read against current CLSI breakpoints, which were lowered.
- A: Modifying enzymes are the main route to aminoglycoside resistance.
- C: PBP2a gives methicillin resistance in staphylococci.
- D: Ribosomal methylation by erm genes causes macrolide and clindamycin resistance.
Question 6 · Easy
Which organism is intrinsically resistant to beta-lactams because it has no cell wall?
1. AMycoplasma pneumoniae
2. BLegionella pneumophila
3. CBordetella pertussis
4. DMoraxella catarrhalis
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Question 7 · Medium
Why does vancomycin have no useful activity against gram-negative bacilli?
1. AThe molecule is too large to cross the outer membrane
2. BGram-negative cells hydrolyse it with a beta-lactamase
3. CThe drug binds only gram-positive ribosomes
4. DGram-negative cells have no D-alanyl-D-alanine precursors
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Question 8 · Medium
Which susceptibility result for a urine isolate of Proteus mirabilis should be checked before it is reported?
1. ASusceptible to gentamicin
2. BSusceptible to ampicillin
3. CSusceptible to nitrofurantoin
4. DResistant to colistin and tetracycline
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Question 9 · Easy
Obligate anaerobes are intrinsically resistant to aminoglycosides because:
1. AThey hydrolyse the drug with a beta-lactamase
2. BTheir cell wall contains no peptidoglycan
3. CThey have no ribosomal target for the drug
4. DDrug uptake needs oxygen-dependent transport
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Question 10 · Easy
Enterococcus faecalis is intrinsically resistant to all cephalosporins because:
1. AIts outer membrane keeps cephalosporins out
2. BIts penicillin-binding proteins bind cephalosporins poorly
3. CIt has no peptidoglycan in the cell wall
4. DIt produces an extended-spectrum beta-lactamase
Answer and explanation in LearnFree account

Resistance: antibiotic mechanisms and intrinsic resistance

Blood cultures from a patient receiving vancomycin grow catalase-negative, non-sporing gram-positive rods that are vancomycin resistant. The most likely organism is:

A non-fermenting gram-negative rod from the sputum of a ventilated patient is resistant to meropenem but susceptible to trimethoprim-sulfamethoxazole and levofloxacin. Which organism does this intrinsic pattern fit?

An Enterococcus faecalis isolate from a patient with endocarditis shows high-level gentamicin resistance. What does this mean for treatment?

Which antibiotic is Klebsiella pneumoniae intrinsically resistant to?

Fluoroquinolone resistance in Enterobacterales most often arises from:

Which organism is intrinsically resistant to beta-lactams because it has no cell wall?

Why does vancomycin have no useful activity against gram-negative bacilli?

Which susceptibility result for a urine isolate of Proteus mirabilis should be checked before it is reported?

Obligate anaerobes are intrinsically resistant to aminoglycosides because:

Enterococcus faecalis is intrinsically resistant to all cephalosporins because: