Question

Suppose you inoculate a PR broth with a slow-growing fermenter. After 48 hours, you see slight turbidity but score it as (-/-). Is this result a false positive or afalse negative? Is this a result of poor specificity or poor sensitivity of the test system?

Answer 1

The PR (phenol red) broth is deferential media, which is commonly used to distinguish gram negative enteric bacteria, namely E.coli, E. faecalis, etc. The organisms which use carbohydrate turn the media into yellow and produce gas bubbles; whreas organism’s which use peptone turn the media into fascia color.

For example, E. faecalis produces only acid, but no gas hence, it is represented as A/- and E.coli produce both gas and acid, so it is represented with A/G. The Micrococcus luteus provides no reaction and it is represented as -/-.

Here, the Micrococcus luteus grow in the media, but do not produce compounds, which change the color of phenol red-media. This type of microbes have poor sensitivity, because they do not produce either alkaline or acidic compounds.   Hence, they exhibit a false negative result.