Bacteriology: Cutibacterium prosthetic joint infection: 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 · Bacteriology: Cutibacterium prosthetic joint infection

Sample item

Eight months after total shoulder arthroplasty, a 60-year-old man has persistent low-grade pain and prosthesis loosening without fever. Intra-operative cultures are negative at 3 days, but a Gram-positive pleomorphic anaerobic bacillus grows after 12 days of extended incubation. Which organism is it?

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

The answer and the reasoning

B. Cutibacterium acnes

Cutibacterium acnes (formerly Propionibacterium) is a skin commensal of the shoulder and sebaceous areas that causes indolent device-related infection. It grows slowly under anaerobic conditions, so cultures must be held for about 14 days.

Option by option

  • A. Bacteroides fragilis

    Bacteroides fragilis is a Gram-negative anaerobe of bowel flora.

  • B. Cutibacterium acnes (correct)

    Correct. A slow-growing pleomorphic anaerobic Gram-positive rod is typical of this skin commensal.

  • C. Clostridium perfringens

    Clostridium perfringens is a large boxcar-shaped spore-forming rod that grows quickly.

  • D. Staphylococcus aureus

    Staphylococcus aureus is a Gram-positive coccus that grows aerobically within a day or two.

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.

More Microbiology questions

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is B, Cutibacterium acnes. Cutibacterium acnes (formerly Propionibacterium) is a skin commensal of the shoulder and sebaceous areas that causes indolent device-related infection.

Why is "Bacteroides fragilis" not the answer?

Bacteroides fragilis is a Gram-negative anaerobe of bowel flora.

Why is "Clostridium perfringens" not the answer?

Clostridium perfringens is a large boxcar-shaped spore-forming rod that grows quickly.

Why is "Staphylococcus aureus" not the answer?

Staphylococcus aureus is a Gram-positive coccus that grows aerobically within a day or two.

What concept does this question test?

It tests Bacteriology: Cutibacterium prosthetic joint infection in Microbiology. It grows slowly under anaerobic conditions, so cultures must be held for about 14 days.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Bacteriology: Cutibacterium prosthetic joint infection?

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.