MacConkey II Agar is a slightly selective and differential medium for the detection of coliform organisms and enteric pathogens. MacConkey II Agar is a selective and differential medium. It is only slightly selective since the concentration of bile salts, which inhibit gram-positive microorganisms, is low in comparison with other enteric plating media. Crystal violet also is included in the medium to inhibit the growth of gram-positive bacteria, especially enterococci and staphylococci.