Question

# Microbiology 

My bacteria is staphylococcus epidermidis and these are the tests I need some help with:

PR-mannitol

PR-raffinose

I need to describe what results I would expect to get if I was completing this test in the lab.

Answer 1

Staphylococcus epidermidis is a Gram-positive, facultative anaerobic bacteria. In both PR-mannitol & PR-raffinose test, it will give negative result.

Mannitol salt agar is both selective & differential media. It has a high concentration (~7.5%-10%) of salt (NaCl) & is selective for gram-positive bacteria Staphylococci. It can also differentiate mannitol fermenters. When an organism is able to ferment mannitol, it produces an acidic byproduct which changes the color of phenol red to yellow in the agar. Since, Staphylococcus epidermidis give negative result for PR-mannitol test, it is unable to ferment mannitol & pH indicator phenol red remains red in the media.

PR-raffinose is done to see if an organism can ferment raffinose as carbon source. When an organism is able to ferment raffinose, it produces an acidic byproduct which changes the color of phenol red to yellow in the agar. As Staphylococcus epidermidis give negative result for PR-mannitol test, it is unable to ferment raffinose & will not turn phenol red to yellow, i.e., no color change occurs to the medium.