Question

A nasal swab was inoculated to a mannitol salt agar. The examination shows colonies on yellow agar. The organism is:

Answer 1

The organism likely to be identified by a yellow color change on mannitol salt agar is Staphylococcus aureus, caused by its fermentation of mannitol resulting in acid production.

Step-by-step explanation:

The organism that turns a mannitol salt agar yellow is most likely Staphylococcus aureus. Mannitol salt agar (MSA) is a selective and differential medium that encourages the growth of halotolerant bacteria such as staphylococci. It contains a high concentration of salt which inhibits most other bacteria.

The medium also has mannitol, a fermentable sugar, and phenol red, a pH indicator that changes color in the presence of acid. When S. aureus ferments mannitol, it produces acid and as a result, the phenol red in the agar turns yellow.

This ability to ferment mannitol and the subsequent acidification of the medium is characteristic of S. aureus. Other staphylococcal species, like S. epidermidis, do not ferment mannitol and therefore do not cause the color change from red to yellow.

Answer 2

The organism that changes mannitol salt agar to yellow is likely Staphylococcus aureus, indicating its ability to ferment mannitol and tolerate high salt concentrations.

Step-by-step explanation:

The organism that grows on mannitol salt agar and changes the color of the agar to yellow is most likely Staphylococcus aureus. This bacterium is capable of fermenting mannitol which leads to the production of acids, resulting in the phenol red pH indicator within the medium turning yellow. Since S. aureus is a halotolerant bacterium, it can grow in the high salt concentration of mannitol salt agar, which is inhibitory to many other bacteria. The presence of yellow colonies on the mannitol salt agar is a key indication that S. aureus is present, as opposed to other non-mannitol fermenting staphylococci that would leave the medium red.

The organism responsible for the yellow colonies on the mannitol salt agar is Staphylococcus aureus. This bacterium ferments mannitol, producing acids that lower the pH of the agar, causing the pH indicator phenol red to turn yellow. Other tests, such as gram staining and catalase activity, can also be used to confirm the presence of S. aureus.