Which type of agar is used specifically for the selective growth of gram-positive cocci?

CNA agar is specifically formulated for the selective growth of gram-positive cocci, primarily due to the inclusion of colistin and nalidixic acid, which inhibit the growth of gram-negative bacteria. This makes it ideal for isolating organisms such as Staphylococcus and Streptococcus, allowing for their growth while suppressing other types of bacteria.

In contrast, MacConkey agar is designed to isolate gram-negative bacteria, particularly enterics, and it does not support the growth of gram-positive organisms. THIO agar is a general-purpose medium that supports a wide range of organisms but is not selective for gram-positive cocci specifically. TCBS agar is selective for vibrio species, particularly Vibrio cholerae, and is not used for growing gram-positive bacteria. Thus, CNA agar stands out as the correct choice for the selective growth of gram-positive cocci.