Coexistence of the stippled Rickettsiales-like prokaryote with Candidatus Xenohaliotis californiensis and the bacteriophage pCXc in farmed red abalone Haliotis rufescens from Mexico

Candidatus Xenohaliotis californiensis (CXc) is a Rickettsiales-like organism (RLO) that is considered the causal agent of withering syndrome (WS), a chronic and potentially fatal disease for several abalone species. CXc can be infected by a bacteriophage (pCXc), and both phage-infected and non-phage-infected CXc can coexist with a presumable different species of RLO tentatively named stippled Rickettsiales-like organism (ST-RLO). Currently, the taxonomic affiliation of the ST-RLO is unknown, and no studies have been conducted to determine its presence in infected abalones in the Peninsula of Baja California, Mexico. To determine whether the ST-RLO is a histological variant of CXc or a new species of RLO, we performed histology and species-specific in situ hybridization (ISH) for CXc and pCXc in red abalone, Haliotis rufescens, infected with CXc and hyperparasitized by pCXc, which also showed the presence of the ST-RLO. Additionally, a histological analysis of the presence of all of these RLOs from 2008 to 2019 from an aquaculture facility was performed. The results showed that all STRLOs were negative by ISH for CXc and pCXc, suggesting that the ST-RLO is a different species of bacterium and that it is not susceptible to infection by pCXc. Histologically similar alterations to those produced by CXc in the post esophagus were observed. The three RLOs regularly coexist in all the infected abalones, and there is a relatively constant proportion of each kind of RLO over time, suggesting that equilibrium is reached during coexistence; the largest percentage corresponded to CXc non phage infected (81%), followed by CXc infected by pCXc (16%) and the ST-RLO (3%). In all cases, the ST-RLO infection remained at a subclinical level making it difficult to determine if it acts as pathogen or as a commensal. Therefore, the genetic characterization of the STRLO and pathogenicity studies are needed to further elucidate the role of this new RLO in the development of of WS in abalone species of North America.

Automated summary

Candidatus Xenohaliotis californiensis (CXc) is considered the causal agent of withering syndrome in abalones and can be infected by a bacteriophage, while another presumed species of RLO called stippled Rickettsiales-like organism (ST-RLO) coexists. The ST-RLO is shown to be a different bacterium species and not susceptible to phage infection. Histological analysis revealed a relatively constant proportion of each RLO coexisting in infected abalones over time.