The S. aureus organism remain viable when there are still nutrients in the medium.As soon as nutrients are exhausted, the organism cells enter in dead phase and number of dead cells is higher than number of live cells. For S aureus the viability differs among strains . Some still alive one month . Others lasted to 3 months. Glycerol protect the organism alive and continue live in deep freez  . Add 15% glycerol to MS anar broth inoculated with few turbidity of S, aureus

Salt mannitol agar is similar to any nutrient media. However it is more of selective media for S aureus rather than maintainance media. S aureus predominate on salt mannitol agar in presence of other organisms.Similar to MacCocnkey agar for E coli. And colonies of S aureus will remain viable  for  time  till they reach a death phase similar to any nutrient media as mentioned by Gamal.

For maiantaince purpose and storage of pure culture, common nutrient media like nutrient agar or soyabean caesin digest agar  in slants are preferred over selective media.

In addition to the above comments, if you have a fridge 2-8oC, place the culture plates. They will be preserved for longer periods if you're not interested in inoculating slopes or nutrient broth/glycerol.

Thank you very much, Gamal Enan, Trupti Ashar and Deborah Nanjebe for your valuable guidance and expert opinions.

Dear Mr. Muhammed,This media was defined as differential media for S.aureus, and it is poor in nutrients content, specially the carbon source (mannitol). For this reason, I think this media was not optimal for viable growth of this species more than 20 days.