Cytotoxic effect of Mycoplasma fermentans on mouse thymocytes

The in vitro response of mouse thymocytes to various mycoplasmas was evaluated. Cultures of thymus cells from BALB mice were prepared in Earle minimal essential medium with 10 per cent fetal calf serum. After an initial drop in viability, cell populations stabilized at approximately 10-5 viable cells/ml for 3 to 5 days. Concentration of 10-6 to 10-8 colony-forming units of toxic isolates of Mycoplasma fermentans per ml killed over 50 per cent of these cells in a dose-dependent fashion. Four other mycoplasmas (M. pneumoniae, M. hominis, M. arthritidis and a nontoxic strain of M. fermentans) did not induce cytotoxicity of mouse thymocytes. Toxic isolates of M. fermentans multiplied in the presence of thymus cells as they were being inactivated. However, nonviable membrane preparations of these mycoplasma were also toxic, indicating that growth of the organisms is not a prerequisite for the toxic effect. The relevance of these findings for the isolation and identification of the membrane-associated toxic factor is discussed.