Question

suppose you have prepared a batch of emb agar and the starting ph is 0.5 higher than normal (ph 6.9-7.3). how would this difference affect the sensitivity and/or specificity of the agar?

Answer 1

A higher-than-normal pH in EMB agar reduces its specificity and sensitivity, compromising the inhibition of gram-positive bacteria and correcting bacterial identification due to color differentiation issues.

Step-by-step explanation:

If the starting pH of EMB agar is 0.5 higher than normal, it can have a profound effect on the sensitivity and specificity of the agar. Normally, EMB agar should have a pH range between 6.9 to 7.3 to effectively differentiate between lactose fermenters and non-fermenters.

higher starting pH can compromise the agar's ability to inhibit the growth of gram-positive bacteria and may not adequately differentiate between lactose fermenting organisms, which usually form metallic green or deep purple colonies, and non-lactose fermenters which form colorless colonies.

Therefore, the increased pH could potentially allow the growth of gram-positive bacteria and also interfere with the color differentiation between different types of bacteria, making it harder to correctly identify pathogenic bacteria. This is crucial because one of the main purposes of EMB agar is to distinguish pathogenic, lactose-fermenting bacteria such as E. coli, which often appear with a characteristic metallic sheen. Hence, maintaining the correct pH is essential for the proper functioning of EMB agar as a selective and differential medium.