Phenylethyl Alcohol Agar (PEA) is used primarily for which purpose?

Phenylethyl Alcohol Agar (PEA) is designed specifically to selectively promote the growth of Gram-positive bacteria while inhibiting the growth of Gram-negative bacteria. The phenylethyl alcohol in the medium disrupts the outer membrane of Gram-negative bacteria, which makes it less favorable for their growth. This characteristic makes PEA an effective selective medium for isolating Gram-positive microorganisms, such as staphylococci and streptococci, from mixed cultures.

The choice of this answer reflects an understanding of the selective properties of PEA, which are vital in microbiological techniques for isolating and identifying specific groups of bacteria in complex samples. The use of PEA is particularly valuable in clinical settings where isolating pathogenic Gram-positive bacteria can be critical for diagnosis and treatment.