Penetration of host cells by Rickettsia rickettsii appears to be mediated by a phospholipase of rickettsial origin.

Internalization of obligate intracellular bacteria belonging to the genus Rickettsia by eukaryotic cells requires participation of both the parasitized host and the microorganism. The term "induced phagocytosis" has been used specifically to describe the entry of Rickettsia prowazekii, although a similar mechanism is likely for R. rickettsii. A role for a phospholipase in the internalization process has been proposed for both of these organisms, with the strongest supporting evidence provided for R. prowazekii. Despite general acceptance of the notion that phospholipase activity is involved in the internalization process of these bacteria, the origin of the enzyme is not known. The results of the study presented here, which used R. rickettsii and Vero cells, suggest that a rickettsial phospholipase, rather than a host cell phospholipase, mediates internalization of the organism. This conclusion is based upon results which show that pretreatment of R. rickettsii, but not of host cells, with a specific chemical inhibitor of phospholipase, and also antiserum to this enzyme, significantly reduces uptake of the organism and its ability to cause plaque formation.