General microbiology and sterilisation: anaerobic jar indicator: 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 and sterilisation: anaerobic jar indicator

Sample item

A GasPak anaerobic jar for culturing Clostridium species contains a methylene blue indicator strip. After 48 hours the strip is still blue. What is the correct interpretation?

AThe catalyst has become overheated and inactiveBExcess carbon dioxide has built up inside the jarCOxygen is still present, so anaerobiosis has not been achievedDAnaerobic conditions are perfect and the strip is working

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

The answer and the reasoning

C. Oxygen is still present, so anaerobiosis has not been achieved

Methylene blue is colourless in reduced, anaerobic conditions and stays blue when oxygen is present. A persistently blue strip means the jar has failed, usually from a leak or an exhausted catalyst or gas generator. Cultures from such a jar should not be relied on for obligate anaerobes.

Option by option

A. The catalyst has become overheated and inactive
Catalyst damage could cause the problem, but the strip itself only reports oxygen status, not catalyst temperature.
B. Excess carbon dioxide has built up inside the jar
Carbon dioxide does not change the colour of methylene blue.
C. Oxygen is still present, so anaerobiosis has not been achieved (correct)
Correct. A blue strip means oxidised dye, hence oxygen remains in the jar.
D. Anaerobic conditions are perfect and the strip is working
In a proper anaerobic atmosphere the strip is decolourised, so blue colour signals failure.

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 C, Oxygen is still present, so anaerobiosis has not been achieved. Methylene blue is colourless in reduced, anaerobic conditions and stays blue when oxygen is present.

Why is "The catalyst has become overheated and inactive" not the answer?

Catalyst damage could cause the problem, but the strip itself only reports oxygen status, not catalyst temperature.

Why is "Excess carbon dioxide has built up inside the jar" not the answer?

Carbon dioxide does not change the colour of methylene blue.

Why is "Anaerobic conditions are perfect and the strip is working" not the answer?

In a proper anaerobic atmosphere the strip is decolourised, so blue colour signals failure.

What concept does this question test?

It tests General microbiology and sterilisation: anaerobic jar indicator in Microbiology. A persistently blue strip means the jar has failed, usually from a leak or an exhausted catalyst or gas generator. Cultures from such a jar should not be relied on for obligate anaerobes.

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 and sterilisation: anaerobic jar indicator?

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.