Variation in the virulence of strains of Mycoplasma pulmonis related to susceptibility to killing by macrophages in vivo.

The virulence of five strains of Mycoplasma pulmonis, as judged by their ability to survive in the respiratory tract and induce pneumonia in CBA mice, was related to the ability of viable organisms to persist in the peritoneal cavity. This appeared to be the result of differences in the ability of the strains to resist killing by peritoneal macrophages in vivo. It is suggested that resistance to phagocytosis by macrophages is an important determinant of virulence for M. pulmonis.