4. The protocol for the VP test requires you to incubate the medium for 48 hours

Question

4. The protocol for the VP test requires you to incubate the medium for 48 hours, then add the two VP reagents and record the results every 10 minutes. A. If you were to incubate inoculated VP medium for 24 hours, would this be more likely to produce a false positive or a false negative? B. If you added the VP reagents and then recorded the results immediately, is this more likely to produce a false positive or a false negative? C. What would be the effect of forgetting to shake the tube after adding the VP reagents? D. Suppose you inoculated VP medium with a known VP negative organism. After adding the VP reagents, you noticed the medium turned red after about 10 minutes. Provide one possible explanation for this result. 5. When performing the nitrate test on a microaerophile, microbiologists sometimes use nitrate medium that contains a small amount of agar, similar to fluid thioglycolate broth. Why is this done?

Explanation / Answer

4. Some bacteria produces neutral-reacting end products when grown in specific media. Particular eneteric bacteria that ferment glucose, further metabolize pyruvic acid to form acetoin. This end product in the presence of atmospheric O2 and potassium hydroxide, is converted to diacetyl. This diacetyl is catalyzed by alpha-napthol and creatine to form red complex (VP positive).

A. If incubate inoculated VP media for 24 hours, this would be more likely to produce false negative.

B. If record the result immediately after addition, it is more likely to produce false negative. Because the result must be recorder after 15 mins and within 1 hr. If immediately no reactions won't occur by the VP reagents.

C. Shaking is required to supply adequate O2 to all the bacterias in a container or simply you can say exposing the media to atmospheric O2. If forget to shake, may be the result will not correct.

D. One possible explanation may be medium is contaminated with VP positive organisms.

5. Agar is added to make the medium semi-solid which helps in demonstrating the motility of the organism along the stab line of inoculation (stab culture).

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