Question

1. (2 points) A patient presents in the ER with a 2nd degree burn that has become significantly infected. To identify the causative agent responsible for the infection, a sample of the pus is sent to the Clinical Microbiology lab. Describe one test that could be used to distinguish pathogenic and frequently drug resistant aureus from non-pathogenic S. epidermidis in this clinical sample. Explain specifically how the test would distinguish between these organisms. 2. (2 points) A patient presents in the ER with severe diarrhea consistent with food poisoning. To identity the causative agent, a stool sample is sent to the Clinical Microbiology lab. Describe one test that could be use to distinguish enterotoxic coli from Salmonella in this clinical sample. Explain specifically how the test would distinguish between these organisms.

Answer 1

Answer 1: To distinguish between S.aureus and S.epidermidis, the sample will be cultured on Mannitol salt agar. Mannitol salt agar is a selective and differential medium.

Mannitol salt agar contains high concentration of salt(approximately 7.5% or more) hence it is selective for Gram positive organism like Staphylococci and inhibitory for other organisms. It has phenol red as the indicator which turns yellow under acidic conditions.

Most non pathogenic Staphylococci will not ferment mannitol. S.aureus ferments mannitol and hence it will produce yellow colonies with yellow zones while Staphylococcus epidermidis will produce small pink or red colonies with no colour change as they are unable to ferment mannitol.

Answer 2: To distinguish between Escherichia and Salmonella the sample can be cultured on MacConkey’s agar

MacConkey agar is a selectve and differential medium to differentiate Gram negative organisms. The presence of bile salts and crystal violet inhibits the growth of Gram positive bacteria making the medium selective for Gram negative organisms. The medium is designed for growth of enteric bacteria. The medium is able to differentiate between lactose fermenting and lactose non fermenting organisms.. Neutral red is the P^H indicator which will turn red below P^H of 6.8.

After the inoculation, the lactose fermenting bacteria like Escherichia will produce lactic acid and will turn pink because of acid production and consequently lowering the P^H below 6.8. the colonies of these bacteria will be surrounded by a hazy zone because of precipitation of bile salts due to the presence of acid.

The lactose non fermenting bacteria like Salmonella. will not change the color of the medium and their colonies will appear colorless or transparant.