Discontinuous Variation in the Virulence of Bact. aertrycke Mutton

A given strain of Bact. aertrycke Mutton has been tested repeatedly for its virulence to mice, and on some of these occasions the virulence of 10 single colony cultures taken from this strain has likewise been tested. Between these single colony cultures such marked differences in virulence have been found as to constitute definite discontinuous variations. Side by side in the same culture there have been found virulent and avirulent organisms. Daily subculture in broth under certain atmospheric conditions resulted in the fall in virulence of the whole culture; this was accompanied by a replacement of the virulent organisms by organisms that were either completely avirulent or were only weakly virulent. The evidence suggests that the fall in virulence of the whole culture is not due to a simultaneous fall in the virulence of each of its constituent organisms, but to a replacement of the highly virulent organisms by organisms of a lower degree of virulence. During the process of replacement two or three different variants, showing discontinuous variations in virulence, may be demonstrated together in the same culture. The conclusions to be drawn from these findings, and their bearing on the interpretation of the results of experimental epidemiology, are discussed.