In numerous histopathological studies carried out on several species of bivalve mollusks of commercial aquaculture importance around the world , the presence of characteristic tissue inclusions has been related to Rickettsiales-like organism (RLOs). In some records , these RLOs were associated with mass mortalities and tissue damage but not in others . Thus , the type of symbiotic association between the oyster and the bacterium have not been clearly defined. Typically, the studies on RLOs are limited to ultrastructural and histological descriptions which are inadequate for a robust taxonomic classification. These RLOs have been observed on the west coast of Mexico in routine health surveys of the pleasure oyster Crassostrea corteziensis (Figure 1). In addition to RLOs being the causative agents of the inclusion bodies observed in bivalves, the genus Endozoicomonas has been recently found to cause very similar inclusion bodies in a wide range of bivalve hosts. The genus Endozoicomonas has been reported on the west coast of Mexico suggesting the presence of these bacteria in C. corteziensis.
Employing histological methodologies in combination with transmission electron microscopy (TEM), in situ hybridization and Next generation sequencing (NGS) from DNA extracted from formalin-fixed paraffin-embedded (FFPE) tissues we attempt to identify and characterize the bacteria observed and causing the inclusion bodies in C. corteziensis. The NGS was performed from 10 years old FFPE tissues. The sequences were compared using BLASTn and mapped to the partial sequence of 16S rRNA for Endozoicomonas sp. In addition, microbiome analysis of the bacterial community associate with C. corteziensis (table 1) showing the tissue inclusions of the RLOs are underway. Histologically, the intracytoplasmic vacuoles (Ivs) (figure 1) presented sizes of 15.70-15.24 µm in length and 15.42-14.95 µm in width. The prevalence of RLOs in the study period was 6% while the infection intensity of the RLOs was grade one in 89% of the infected oysters.