Double-zone beta hemolytic streptococci

four cows in a herd where mastitis was prevalent, four strains of streptococci with the hemolytic properties to be described. In the primary blood agar plate the deep colony of one strain was surrounded by "a clear inner circle (of hemolysis) three milli-meters in diameter and a partly cleared outer zone, up to ten millimeters in total diameter. " Later study and repeated re-plating in blood agar has shown that these four strains are alike in all respects. One of them, designated Streptococcus A-18 (Cow) B, has been described and photographed by Brown (1919). Apparently similar strains were isolated from horses by Jones (1919) who also published a photograph of one of them. Since then, many strains producing similar appearances in blood agar have been isolated from bovine and human sources. A brief description has been published (Brown, 1934). The method for demonstrating the double-zones is as follows. To a tube of infusion agar, melted and cooled to 45 to 50'C., is added about 1 cc. of defibrinated blood and a loop of suitably diluted culture. The dilution of the culture should be such that there may be fewer than 100 colonies in the plate. After thorough mixing the inoculated medium is poured into a Petri dish and allowed to harden. The plates are incubated at 370C. and then refrigerated. During the first 24 hours of incubation deep colo-nies with small zones of hemolysis appear and become larger as incubation is continued. Usually the zones of hemolysis have a rather poorly defined periphery but they will be found to b