Which media types are selective for gram-positive bacteria?

The selection of media types that are selective for gram-positive bacteria is correctly identified. Both phenylethyl alcohol agar (PEA) and colistin-nalidixic acid agar (CNA) are formulated specifically to inhibit the growth of gram-negative bacteria while allowing gram-positive bacteria to grow.

PEA works by disrupting the outer membrane of gram-negative bacteria, contributing to the selective conditions for gram-positive bacteria, which typically lack this outer membrane. CNA employs the antibiotics colistin and nalidixic acid, which specifically target and inhibit the growth of certain gram-negative bacteria, thus fostering a favorable environment for the growth of gram-positive organisms.

Other media options listed are not selective for gram-positive bacteria. For instance, MacConkey Agar (MAC) and Eosin Methylene Blue Agar (EMB) are primarily selective for gram-negative bacteria. Mannitol Salt Agar (MSA) can support the growth of staphylococcus species, particularly the salt-tolerant varieties, but it is not exclusively selective for gram-positive bacteria as it can also support some gram-negative organisms. Blood Agar Plate (BAP) is an enriched medium that supports the growth of many bacteria, both gram-positive and gram-negative, without selectivity. Tryptic Soy