Gentamicin as a selective agent for the isolation of beta haemolytic streptococci

The isolation of beta haemolytic streptococci was compared using Columbia sheep blood agar with and without gentamicin in a final concentration of 5.5 micrograms per ml. The inclusion of gentamicin gave greatly improved results whenever the isolation of this organism in pure culture was complicated by overgrowth with Gram-negative bacilli or staphylococci.