General microbiology: cell wall-deficient bacteria: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · General microbiology: cell wall-deficient bacteria

Sample item

A 19-year-old student has a persistent dry cough and low-grade fever for two weeks. The chest radiograph shows patchy infiltrates, and Gram stain of sputum shows no organisms. She fails to improve on ceftriaxone. The best explanation for the antibiotic failure is that the likely pathogen:

Ais an obligate intracellular virus insensitive to all antibioticsBproduces a metallo-beta-lactamase that hydrolyses cephalosporinsCforms spores that survive beta-lactam exposureDlacks a peptidoglycan cell wall, so beta-lactams have no target

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. lacks a peptidoglycan cell wall, so beta-lactams have no target

Mycoplasma pneumoniae causes atypical pneumonia with a dry cough. It has no cell wall, so it does not stain with Gram stain, and beta-lactams that inhibit peptidoglycan synthesis are ineffective. It has sterols in its membrane and grows slowly on enriched media with fried-egg colonies. Macrolides or doxycycline are effective.

Option by option

A. is an obligate intracellular virus insensitive to all antibiotics
Mycoplasma is a bacterium that responds to macrolides and tetracyclines, so antibiotics are not useless against it.
B. produces a metallo-beta-lactamase that hydrolyses cephalosporins
Metallo-beta-lactamase production is a carbapenem resistance mechanism of Enterobacterales, not of Mycoplasma.
C. forms spores that survive beta-lactam exposure
Mycoplasma does not form spores, and spore formation is not a beta-lactam resistance mechanism here.
D. lacks a peptidoglycan cell wall, so beta-lactams have no target (correct)
Absence of a cell wall removes the target of beta-lactams and explains the Gram stain result.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is D, lacks a peptidoglycan cell wall, so beta-lactams have no target. Mycoplasma pneumoniae causes atypical pneumonia with a dry cough.

Why is "is an obligate intracellular virus insensitive to all antibiotics" not the answer?

Mycoplasma is a bacterium that responds to macrolides and tetracyclines, so antibiotics are not useless against it.

Why is "produces a metallo-beta-lactamase that hydrolyses cephalosporins" not the answer?

Metallo-beta-lactamase production is a carbapenem resistance mechanism of Enterobacterales, not of Mycoplasma.

Why is "forms spores that survive beta-lactam exposure" not the answer?

Mycoplasma does not form spores, and spore formation is not a beta-lactam resistance mechanism here.

What concept does this question test?

It tests General microbiology: cell wall-deficient bacteria in Microbiology. It has no cell wall, so it does not stain with Gram stain, and beta-lactams that inhibit peptidoglycan synthesis are ineffective. It has sterols in its membrane and grows slowly on enriched media with fried-egg colonies. Macrolides or doxycycline are effective.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General microbiology: cell wall-deficient bacteria?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.