Sterilisation: filtration: 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 · Sterilisation: filtration

Sample item

Heat-labile foetal calf serum for cell culture is sterilised by passage through a 0.22 micrometre membrane filter. Which contaminant could still be present in the filtrate?

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

The answer and the reasoning

B. Viruses

A 0.22 micrometre membrane retains bacteria, yeasts and spores, which are larger than the pores. Viruses are much smaller and pass through, as may some mycoplasmas. Filtration is therefore not a virucidal step.

Option by option

  • A. Bacillus spores

    Spores are around 1 micrometre and are retained.

  • B. Viruses (correct)

    Correct. Viruses are smaller than the pore size and are not retained.

  • C. Pseudomonas aeruginosa

    Pseudomonas is about 0.5 to 1 micrometre wide and is retained.

  • D. Candida albicans yeast cells

    Yeasts are several micrometres across and are retained by the membrane.

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, Viruses. A 0.

Why is "Bacillus spores" not the answer?

Spores are around 1 micrometre and are retained.

Why is "Pseudomonas aeruginosa" not the answer?

Pseudomonas is about 0.5 to 1 micrometre wide and is retained.

Why is "Candida albicans yeast cells" not the answer?

Yeasts are several micrometres across and are retained by the membrane.

What concept does this question test?

It tests Sterilisation: filtration in Microbiology. 22 micrometre membrane retains bacteria, yeasts and spores, which are larger than the pores. Viruses are much smaller and pass through, as may some mycoplasmas. Filtration is therefore not a virucidal step.

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 Sterilisation: filtration?

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.