ACANTHAMOEBA SP FROM THE PHILIPPINES - ELECTRON-MICROSCOPY STUDIES ONNATURALLY-OCCURRING BACTERIAL SYMBIONTS

The isolation of two plasmind-like circular DNAs, measuring 52 and 42 kbp, from an Acanthamoeba sp. from the Philippines has led to the demo nstration of a bacterial endosymbiont occurring in this free-living am oeba. The 52-kbp band hybridized with a short sequence of cytochrome b gene and was identified as the mitochondrial DNA, whereas the 42-kbp band was identified as plasmid DNA of the bacterial symbionts on the b asis of electron microscopy. The endosymbionts are gram-negative, rod- shaped bacteria measuring approximately 1.3x0.43 mu m and numbering ab out eight to ten cells per section. They are randomly distributed in b oth cysts and trophozoites and are surrounded neither by a phagolysoso mal membrane nor by a clear or electron-translucent region. The endosy mbiont membrane appears to have a close association with ribosomes, wh ich are seen to be more concentrated within the vicinity of the symbio nts than elsewhere within the cytoplasm. Attempts to grow the symbiont s and the amoebae separately have failed.