Extracellular giant rickettsiae associated with bacteria in the gill of Crassostrea gigas (Mollusca, Bivalvia).

Extracellular giant rickettsiae closely associated with bacteria with a Gram-negative type wall are reported among the basal insertions of the cilia of the gill epithelium of the oyster Crassostrea gigas (Mollusca, Bivalvia) from the north of Spain. These rickettsiae are extremely pleomorphic cells with a maximum 9 microns length. The internal elements are ribosome-like particles and DNA strands are distributed in some light areas. Some rickettsial cells show one to two vacuole-like dense inclusions formed by curved concentric bands approximately 4 nm thick separated by an approximately 5-nm light space (myelin-like bodies). Numerous bacteria are closely associated with the rickettsiae in the surrounding zone. The cytopathic effects of this association seemed to give rise to the epithelial lysis and concomitant disappearance of the apical microvilli and cilia and the consequent destruction and disintegration of the gill epithelial cells, where these rickettsiae live in close proximity. This is believed to be the first description of extracellular giant pathogenic rickettsiae of these oyster species.