1. General Purpose Media: These media contain a diverse mixture of nutrients that can support the growth of a broad range of bacteria. Nonsynthetic (complex) media like nutrient agar, nutrient broth, and trypticase soy agar are good examples of general purpose media.
2. Enriched Media: Prepared with special complex nutrients or growth factors (i.e., specific amino acids, vitamins), these media promote the growth of certain species. Bacteria that require such substances are said to be fastidious. For example, adding sheep or horse blood to a culture medium provides several specific growth factors required by various species of streptococci.
3. Selective Media: These media are designed to help in the isolation and growth of certain species of bacteria from a mixed sample. This is accomplished by adding one or more agents to a medium that will promote (or select) the growth of the desired bacteria, while at the same time, inhibiting the growth of the unwanted organisms. Inhibitory agents include such substances as bile salts, NaCl, dyes, acids, and certain antimicrobial drugs. For instance, mannitol salt agar is formulated to a 7.5% NaCl concentration to promote the growth of Staphylococcus. This same salt concentration, however, establishes a hypertonic environment that is very inhibitory for most other organisms.
4. Differential Media: Differential media contain substances that are utilized or reacted to in different ways by microbes. These reactions produce specific visible changes in the appearance of the medium (e.g., color changes, formation of gases or precipitates) or the microbial colonies growing on it. These media are very useful in bacterial isolation and identification because they differentiate one species from another according to their particular biochemical traits.