Selective Culture Media

Selective culture media are used to recover pathogens from samples containing mixture of microorganisms such as feces. For example, Salmonella-Shigella (SS) agar contains about five times the concentration of bile salts compared with MacConkey agar and is more inhibitory to E.coli and more selective for the recovery of Salmonella species from stool cultures.

Various compounds may serve to inhibit the growth of certain undesired microorganisms, thus making the medium selective. Relative concentrations of these substances are important in determining the selectivity of the medium.

1. Aniline dyes (e.g., brilliant green and eosin)
2. Heavy metals (e.g., bismuth)
3. Chemicals (e.g., azide, citrate, deoxycholate, selenite, and phenyl ethyl alcohol), and
4. Antimicrobial agents (e.g., neomycin, colistin, vancomycin, and chloramphenicol)