Purification and biological features of a rickettsia-like prokaryote from the scallop Argopecten irradians in China

Three methods were tried to purify rickettsia-like prokaryotes (RLPs) from infected Argopecten irradians tissues in China and Renografin density gradient centrifugation was found to be the optimal method. The purified organisms were of two morphological cell types: a large cell type (LC) and a small cell type (SC). In Renografin density gradient centrifugation, LCs and SCs in the band at the 20-25% interface were pure and intact. The prokaryotes exhibit two trilaminar membranes, of which the outer membrane is thicker, rippled or even. The prokaryotes possess a nucleoid structure in the center and a high electron-dense ribosorne zone in the periphery. The intact RLP purified by Renografin density gradient centrifugation has a thick and obvious slime layer associated with the outer membrane in the negatively stained sections under electron microscopy. LCs averaging 960.6 +/- 275.5 x 823.0 +/- 222.7 nm (n = 38) are polymorphic, usually coccoid, ellipsoidal or rod-shaped. SCs averaging 251.6 +/- 101.9 x 204.3 +/- 74.5 nm(n = 24) are usually coccoid and ellipsoidal. Two propagation modes, transverse binary fission and budding, was observed. (C) 2004 Elsevier B.V. All rights reserved.