In the decomposition of substrates, the ability of the bacteria to hydrolyze or degrade a specific agar (medium) helps to differentiate members of the Nocardia and Streptomyces genera. The most often used solid media include casein, xanthine, tyrosine, and hypoxanthine.
Media is formulated to promote the growth of aerobic actinomycetes. Each specific solid media formulation assists in the identification of the bacteria. Detection of a hydrolytic enzyme that degrades adenine, casein, xanthine, tyrosine, and/or hypoxanthine is recorded as the clearing of the medium surrounding and beneath areas of bacterial growth (producing a halo effect).
Figure 14. CDC/Berd. (1972). Image #11999. This photograph depicted a QUAD Plate containing a xanthine growth media, upon which Gram-positive, Nocardia, and Streptomyces bacterial colonies were grown. The results here, were two positive, i.e., the ability for the organisms to decompose xanthine, and two negative growth patterns. Positive results included a clear halo around the colonies, while a negative result manifests as an absence of this clear halo, and instead, one will note the deposition of a melanin-like pigment. These colonies were grown in an aerobic environment, which means these organisms thrive in the presence of oxygen. PHIL public domain. https://phil.cdc.gov/Details.aspx?pid=11999Figure 15. CDC/Georg. (1964). Image #18859. These three Petri dish culture plates contained a casein-rich growth medium that had been inoculated from left to right, with Nocardia asteroides [now referred to as complex to include N. abscessus and others], N. brasiliensis, and a Streptomyces sp. organism. Note that the N. asteroides [now known as individual species, and no longer referred to as asteroides] produced no hydrolysis reaction halo, while N. brasiliensis produced a minor reaction, and the Streptomyces sp. bacterium produced a strong hydrolytic reaction. Within the cleared halo region, casein had been hydrolyzed. PHIL public domain. https://phil.cdc.gov/Details.aspx?pid=18859
