Sterilisation: Tyndallisation: 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: Tyndallisation

Sample item

A laboratory must sterilise a sugar-containing fermentation medium that caramelises and decomposes at 121 degrees C. Which method is most appropriate?

ABoiling at 100 degrees C for 10 minutes onceBSteaming at 100 degrees C for 20 to 30 minutes on three successive daysCHolding at 63 degrees C for 30 minutesDHot air oven at 160 degrees C for 2 hours

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

The answer and the reasoning

B. Steaming at 100 degrees C for 20 to 30 minutes on three successive days

Tyndallisation (fractional sterilisation) exposes the medium to flowing steam at 100 degrees C on three successive days. Vegetative cells die each day, and spores that survive germinate overnight and are killed by the next exposure. It suits heat-labile media such as those with sugars, gelatin or serum.

Option by option

A. Boiling at 100 degrees C for 10 minutes once
A single boil kills vegetative cells but spores survive, so the medium is not sterile.
B. Steaming at 100 degrees C for 20 to 30 minutes on three successive days (correct)
Correct. Three intermittent exposures kill vegetative cells and spores that germinate between exposures.
C. Holding at 63 degrees C for 30 minutes
This is the Holder pasteurisation temperature and is not a sterilising process.
D. Hot air oven at 160 degrees C for 2 hours
Dry heat would burn the sugar and cannot be used for liquid media in glass tubes.

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 B, Steaming at 100 degrees C for 20 to 30 minutes on three successive days. Tyndallisation (fractional sterilisation) exposes the medium to flowing steam at 100 degrees C on three successive days.

Why is "Boiling at 100 degrees C for 10 minutes once" not the answer?

A single boil kills vegetative cells but spores survive, so the medium is not sterile.

Why is "Holding at 63 degrees C for 30 minutes" not the answer?

This is the Holder pasteurisation temperature and is not a sterilising process.

Why is "Hot air oven at 160 degrees C for 2 hours" not the answer?

Dry heat would burn the sugar and cannot be used for liquid media in glass tubes.

What concept does this question test?

It tests Sterilisation: Tyndallisation in Microbiology. Vegetative cells die each day, and spores that survive germinate overnight and are killed by the next exposure. It suits heat-labile media such as those with sugars, gelatin or serum.

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: Tyndallisation?

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.