Increased selectivity of Mitis-Salivarius agar containing polymyxin.

Incorporation of the antibiotic polymixin B sulfate in plates of Mitis-Salivarius medium at a level of 100 U per ml effectively suppressed the growth of gram-negative spreading organisms which occasionally overgrow Mitis-Salivarius plates of oral samples and make them unsuitable for enumeration and differentiation of streptococci. At the indicated level the antibiotic did not affect the growth or typical colonial characteristics of the oral streptococci.