A false positive occurs when the result of the differential test is positive whe

Question

A false positive occurs when the result of the differential test is positive when it should be negative. A false negative result occurs when the outcome of the differential test is negative when it should be positive. 1. When performing any differential test, which of the two results is more likely: a false positive or a false 2. Suppose you are performing a PR-glucose test on an grganism known to ferment glucose. After 48 hours of incubation, you look at the result and see that the color of the medium is red and that there is no gas in the Durham tube. Provide an explanation for this result 3. Suppose you inoculate a tube of PR-lactose with an organism that is a very slow-growing fermenter. After 48 hours, the medium is slightly cloudy, red, and there is no gas in the tube. You record the result as Neg/Neg. Is this a false positive false nerative or correct result? False Negotive 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 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

1. False positive and false negative are errors that occur during experiments and their probability depends on the type of the test and the process of doing it.

If we consider the type 1 error rates or the false positive rates, it depends on the specificity of the test. Increase in specificity of the test decreases the false positive results but increases the false negative rates. Complementarily, false negative rates depends on the sensitivity of the test.

2. Phenol red glucose (dextrose) broth  is used to determine whether the microbe can use the sugar glucose for carbon and energy. If glucose can be used, the microbe will accumulate acidic byproducts. In a positive test, the pH indicator in the medium changes color from its normal red to yellow, indicating acid production. If there no colour change and no gas production for a condition where it is to be present, it is false negative.

3. False negative result. If lactose can be used, the microbe will accumulate acidic byproducts. Since it is known that the organism is a slow fermentor and the results are negative, it is type II error.

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