A simple plate assay for detection of group A streptococcus proteinase

A simple plate assay based upon the digestion of skim milk contained within Columbia agar base medium has been found to be a sensitive and reproducible method for detection of proteinase production by individual surface grown colonies of group A streptococci. Use of this medium enabled demonstration of proteinase activity by 24-h aerobic cultures, 79% of 72 group A streptococcus M prototype strains being proteinase positive. None of 40 ‘viridans’ streptococcus strains were positive on this medium and of 18 representative strains of different Lancefield serogroups, only the group P streptococcus had proteinase activity similar to that of the group A streptococci. This same medium can also be used for the detection of preformed proteinase present in liquid preparations by a well diffusion assay.