Which is the medium of choice for the selective recovery of gram-negative anaerobes?

Kanamycin-vancomycin (KV) agar is specifically designed to cultivate gram-negative anaerobes. This medium contains kanamycin and vancomycin, which inhibit the growth of most gram-positive bacteria, thereby providing a selective environment for gram-negative anaerobes to thrive. The combination of these antibiotics effectively suppresses the contaminants from other bacterial types, allowing for the isolation and recovery of the desired anaerobic gram-negative species.

In contrast, the other media listed do not provide the same level of selectivity necessary for isolating gram-negative anaerobes. Phenylethyl alcohol (PEA) agar, while it does inhibit some gram-negative organisms, is not as effective for strictly anaerobic environments and primarily supports the growth of gram-positive bacteria. Cycloserin-cefoxitin-fructose agar (CCFA) is typically used for the isolation of Clostridium difficile, which is specifically a gram-positive anaerobe rather than a gram-negative. Finally, THIO broth is a general-purpose, enriched broth that supports the growth of a wide variety of organisms but lacks the specific inhibiting factors required for selective recovery of gram-negative anaerobes. Therefore, KV agar is the most suitable choice for this purpose.