There are many commercial rapid tests designed to quickly identify one or two bacteria/yeast species. At 7.5% - 10% salt concentration, Mannitol Salt Agar is one of them.
There are three possible results:
1. Mannitol-positive (ie, Staphyloccocus aureus) - bacterial growth which turns agar to yellow color.
2. Mannitol-negative (ie, Staphyloccocus epidermis) - bacterial growth which does not turn agar to yellow color.
3. No bacterial growth - most other species