Dissociation of Beta Hemolytic Streptococci Revealed by Size of Hemolytic Zone on Blood Agar Plates

Abstract

Summary Dissociation of a strain of group A beta hemolytic streptoccus was observed in which the first difference noted was the size of the hemolytic zone of subsurface colonies. The parent type (large hemolytic) produced a zone of 1.82 mm ± 0.05 mm and the variant type (small hemolytic) produced a zone of 1.19 mm ± 0.05 mm. An 18-hour blood broth culture from a single colony of the large hemolytic type which is smooth gives rise with mathematical precision to the small hemolytic dissociant which is matt. The mean rate of dissociation is 1 per cent ± 0.1 per cent. Of equal interest are the parallel differences exhibited by the two colonial types in two laboratory tests for pathogenicity; mouse virulence and ability to grow in human blood. When the large hemolytic type is injected into mice a majority of cultures of the heart's blood of those which die yield colonies predominantly of the small hemolytic type. On the other hand, cultures from the heart's blood of mice which succumb after inoculation with the small hemolytic type only show colonies of the small hemolytic type. Comparison of the lethality of the two types of colonies for mice made by the inoculation of serial tenfold dilutions indicates that the small hemolytic type is probably more virulent. When cultures of the large hemolytic type are inoculated into defibrinated human blood which is then incubated and mixed for three hours at 37°C, the large hemolytic type decreases to a fraction of the original inoculum whereas the small hemolytic dissociant which is about 1 per cent of the original inoculum increases to at least 200 per cent of the original inoculum. When the small hemolytic type is inoculated into human blood it multiplies profusely and it alone is observed in the test plates. The two types appear to be antigenically similar and give the same fermentation, toxin, and final pH reactions.