Question

Assuming, as in Question 1. you tested an organism using PR Glucose and PR Base broths, which of the combi nations of results in the following table would be reliable. Interpret each of the combinations and explain how the results might be obtained. PR PR Base Reliable Y/N Glucose A/G AIG Interpretation fermentation with acid and gas end products in PR base Hermentation with acid end products In PR glucoses no ferm. in Pr buse fermentation w/ acid s gas end products in all plates y N AG no termentation A/G Suppose you inoculate a PR broth with a slow-growing fermenter. After 48 hours, you see slight turbidity but score it as 6-). Is this result a false positive or a false negative? Is this a result of poor specificity or poor sensitivity of the test system?

Answer 1

Answer:

2):

KEY: A = growth with acid (yellow color) K = growth with alkaline reaction (red color) G=gas formation A/G-K Growth with both acid and gas as end products in PR glucose Growth with alkaline reaction in PR base broth KK Growth with alkaline reaction in PR glucose Growth with alkaline reaction in PR base broth A/- & K Growth with only acid as end product in PR glucose Growth with alkaline reaction in PR base broth A-& AG Growth with only acid as end product in PR glucose Growth with both acid and gas as end products in PR base broth

3):

• Phenol red (PR) is a differential test medium used to distiguish between different types of gram negative bacteria.
• In it carbohydrates are added for bacterial fermantation and phenolic red as pH indicator.
• All the fermenting bacteria produce acid and some produce gas also. At pH below 6.8, yellow colour is produced when bacteria ferments sugar.
• At pH above 7.4,pink or red colour may be seen. The PR broth should not be incubated for too long.
• In that case, the bacteria may utilize acid or the peptone amino acids and may give rise to red colour to the media from yellow colour because of neutralization of acid or increased alkalinity.
• Degradation of peptone amino acids raises the pH and produces pink colour.
• In the given problem, the PR broth was tested after 48 hours. Though this is not a very short gap to detect positive change ( yellow colour), still, the result seemed to be negative.
• So the result is a false negative. Because if all the ingredients are present in the medium, necessary for fermentation and growth of a fermenter supposed to give positive result, then absence of the desired result should be regarded as false negative.

$\rightarrow$The result is false because of poor sensitivity of the test system. The fermenter was slow growing.

So the amount of acid produced in 48 hours was low and because of lack of sensitivity of the medium, the change could not be detected with this small amount of acid.

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