Variation in the susceptibility of bovine mycoplasmas to killing by the alternative complement pathway in bovine serum.

All but one of nine Acholeplasma strains were susceptible to killing by gnotobiotic-calf serum while all but one of fifteen Mycoplasma strains were resistant. The mechanism of killing was examined using inhibitors of complement, viz. EDTA. MgEGTA. epsilon-amino caproic acid, and also by desalting serum and adding MgCl2 and CaCl2. The results indicated that, when strains were killed by serum from gnotobiotic calves it was by the alternative complement pathway. However, experiments with mycoplasmas and Escherichia coli indicated that the effect of MgEGTA was not identical for bovine and human sera. Sensitivity to killing by the alternative complement pathway in bovine serum appeared to be a property of avirulent strains and resistance to killing may be regarded as a virulence determinant of mycoplasmas.