The ultrastructure of three mycoplasma species, Mycoplasma pneumoniae, Mycoplasma gallisepticum, and Mycoplasma neurolyticum, has been studied in isolated form as well as in HeLa cell cultures and following incubation with human peripheral blood leukocytes. During log growth phase, the organisms could be distinguished from each other on the basis of their fine structure. In mammalian cell cultures, PPLO's appeared to proliferate on the plasma membranes which had markedly increased their surface area by means of long cytoplasmic processes which extended toward and surrounded them. Some of the microorganisms affected in this way may well have lain, not in vacuoles, but at the bottom of crypts.

It is suggested that the cytopathogenic effect exerted by PPLO's on some tissue cultures may be attributable to membrane damage. Mycoplasmas adhered to leukocyte plasma membranes in a similar manner. They were avidly phagocytosed by neutrophils and eosinophils with accompanying degranulation of the white cells. It is thus likely that the local inflammatory reaction induced by PPLO's does not differ in essence from that caused by bacteria.

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