mannitol salt agar (msa) only allows the growth of halophiles (salt-loving microbes); nonhalophiles will not grow. among the halophiles, mannitol fermenters will produce acid that turns the ph indicator yellow; mannitol nonfermenters leave the medium red. onto msa you inoculate a non-halophilic mannitol fermenter and a halophilic mannitol fermenter. in this case, the medium is acting as (a) medium(s). selective differential selective and differential enrichment