A problem encountered using staphylococcus/streptococcus supplement.

A problem was encountered while using a commercial staphylococcus/streptococcus selective supplement in blood agar culture medium. A number of strains of Staphylococcus aureus failed to grow, although they grew well on non-selective media. The phenomenon was shown to be a result of a pH change caused by incubation in a carbon dioxide-enriched atmosphere, potentiating the activity of the nalidixic acid component. It is recommended that media containing this supplement are not incubated in CO2 enriched atmospheres.