Virulence of established vaccine strains and artificially passaged field strain of Mycoplasma mycoides subsp mycoides.

The virulence of Mycoplasma mycoides for mice, which appears to parallel virulence for cattle, was assessed in respect of four vaccine strains well known for their use in the control of contagious bovine pleuropneumonia. The T1 strain was slightly more virulent than V5, and both were much more virulent than F and KH3J, which could not be distinguished from each other. There was no significant difference between cultures of the T1 strain supplied by three different vaccine-producing laboratories, The mousevirulence test was also used to study the effect of serial passage in artificial systems on virulent field strains of M mycoides; two such strains underwent no appreciable change in virulence during many passages in chick embryos or in mice, although the same strains, and two others, were readily attenuated by repeated subculture in broth. With one strain of M mycoides there was evidence that the total number of subcultures in broth over a given period was more important than the duration of each subculture.