In an attempt to develop a reproducible experimental model of bacterial infection inCrassostrea gigas, oysters taken from very localised sub-populations suffering natural mortality outbreaks were used in cohabitation trials under laboratory conditions. From these trials, a collection of Vibrio strains was isolated from moribund and healthy oysters. In a second step, strains were experimentally tested for virulence by means of injection into healthy oysters. This screening revealed a span of virulence among isolated strains from none to medium. When pooling injected strains, results suggest increased virulence. Vibrio strains may have additive/synergistic action leading to higher C. gigas mortality rates in experimental challenges. Although the study initially aimed to develop a simple experimental model, a complex of interactions emerged between several bacterial strains during the pathogenic process in their molluscan host. Selected strains provide a suitable model of experimental disease for further studies and better understanding of bacterial interaction and pathogenesis in C. gigas.

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Microcell-type parasites of oysters are associated with a complex of diseases in different oyster species around the world. The etiological agents are protists of very small size that are very difficult to characterize taxonomically. Associated lesions may vary according to the host species, and their occurrence may be related to variations in tissue structure. Lesion morphology cannot be used to distinguish the different agents involved. Ultrastructural observations on Mikrocytos roughleyi revealed similarities with Bonamia spp., particularly in regard to the presence of electron-dense haplosporosomes and mitochondria, whose absence from M. mackini also indicate that M. roughleyi and M. mackini are not congeneric. A partial small subunit (ssu) rRNA gene sequence of M. roughleyi was determined. This partial sequence, 951 nucleotides in length, has 95.2 and 98.4% sequence similarities with B. ostreae and B. exitiosus ssu rDNA sequences, respectively. Polymorphisms among the ssu rDNA sequences of B. ostreae, B. exitiosus and M. roughleyi allowed identification of restriction enzyme digestion patterns diagnostic for each species. Phylogenetic analysis based on the ssu rDNA data suggested that M. roughleyi belongs in the phylum Haplosporidia and that it is closely related to Bonamia spp. On the basis of ultrastructural and molecular considerations, M. roughleyi should be considered a putative member of the genus Bonamia.KEY WORDS: Mikrocytos roughleyi · Saccostrea glomerata · Bonamiosis · Microcell · Taxonomy · Small subunit rDNA Resale or republication not permitted without written consent of the publisherDis Aquat Org 54: [209][210][211][212][213][214][215][216][217] 2003 Farley et al. 1988), and M. roughleyi causes winter mortality in the Sydney rock oyster Saccostrea glomerata in SE Australia (Roughley 1926, Farley et al. 1988). Experimentally, Bonamia spp. can infect Ostrea spp. (Grizel et al. 1983, Le Borgne & Le Pennec 1983, Bougrier et al. 1986, Pascual et al. 1991 and C. rivularis (Cochennec et al. 1998). M. mackini has been shown to infect C. gigas, C. virginica, O. edulis and O. concaphila (Bower et al. 1997) (present Table 1). Therefore, Bonamia spp. and Mikrocytos spp. cannot be distinguished solely by the hosts which they infect. However, to date the geographic distributions of the 2 Mikrocytos species appear to be distinct, with M. mackini having been reported only in western Canada and M. roughleyi only in eastern Australia.Several studies have included observations of the ultrastructure of Bonamia ostreae (Comps et al. 1980, Pichot et al. 1980, Bréhelin et al. 1982, Grizel et al. 1983, Grizel 1985, Chagot et al. 1992, Montes et al. 1994 and B. exitiosus (Dinamani et al. 1987, Hine 1991, Hine & Wesney 1994a,b, Hine et al. 2001b), but there are none on Bonamia sp. in Ostrea angasi in Australia or Tiostrea chilensis in Chile. A recent study on the ssu rDNA gene sequences of B. ostreae (Carnegie et al. 2000, Cochennec et al. 2000 and Bonamia sp. from New Zealand showed that they are co...

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