PH AND CALCIUM-DEPENDENCE OF HEMOLYSIS DUE TO RICKETTSIA-PROWAZEKII -COMPARISON WITH PHOSPHOLIPASE-ACTIVITY

Rickettsia prowazekii invades nucleated cells through phagocytosis and subsequently proliferates in the cytoplasm of the host cell. Hemolysi s and a phospholipase A(2) (PLA(2)) activity at neutral pHs have previ ously been reported, even though the phagosomal environment is most li kely acidic. We here show that R. prowazekii and R. typhi also lyse er ythrocytes at mildly acidic pHs, compatible with an early phagosomal c ompartment. For R. prowazekii, hemolysis at an acidic pH but not a neu tral pH is enhanced by Ca2+, raising the possibility that more than on e membranolytic factor may be produced by the rickettsiae. The rickett siae alone display PLA(2) activity, implying that the enzyme is of bac terial rather than erythrocyte or host cell origin. Moreover, the PLA( 2) activity requires divalent cations (Ca2+ or Mg2+), and, as with man y extracellular PLA(2)s from other species, it has a preference for ac idic over neutral phospholipids. The pH dependence of PLA(2) is simila r to that of the hemolysis without Ca2+, but in the presence of the he molysis buffers (which contain Mg2+), there is no calcium-induced enha ncement at acidic pHs. Thus, these rickettsiae are endowed with a memb ranolytic activity that could contribute to the escape of the bacteria from early phagosomal compartments, and it is likely that multiple to xins may be used for membrane lysis.