Susceptibility of pathogenic and nonpathogenic Naegleria spp. to complement-mediated lysis

The susceptibility of four species of Naegleria amoebae to complement-mediated lysis was determined. The amoebicidal activity of normal human serum (NHS) and normal guinea pig serum (NGPS) for Naegleria amoebae was measured by an in vitro cytotoxicity assay. Release of radioactivity from amoebae labeled with [3H]uridine and visual observation with a compound microscope were used as indices of lysis. Highly pathogenic mouse-passaged N. fowleri was less susceptible to the lytic effects of NHS and NGPS than the weakly pathogenic, axenically grown N. fowleri or N. australiensis and the nonpathogenic amoebae N. gruberi and N. lovaniensis. However, both pathogenic and nonpathogenic Naegleria spp. depleted complement as assessed by total hemolytic activity. Treatment of serum with EDTA, heat (56 degrees C, 30 min), cobra venom factor, or antibody to C3 or C9 complement components decreased the amoebicidal activity of NHS. The presence of specific agglutinating antibody to N. fowleri enhanced the amoebicidal activity of NGPS for N. fowleri.