Rickettsiales‐like organisms in bivalves and marine gastropods: a review

Abstract

Rickettsiales‐like organisms (RLOs) are intracellular bacteria that have been reported to be symbionts of over 60 species of bivalves and marine gastropod molluscs of aquaculture importance around the world. Morphologically, some of these prokaryotes are similar to bacteria from the order Rickettsiales, and in most cases, they form intracytoplasmic inclusions similar to those observed in bacteria of the family Anaplasmataceae. Genetically, only Candidatus Xenohaliotis californiensis (CXc), a parasite of several abalone species, has been classified as a member of the order Rickettsiales. The other two characterized RLOs are distantly related; one belongs to the same class (α‐proteobacteria), and the other belongs to the γ‐proteobacteria. Hyperparasitism of RLOs by bacteriophages has been reported in several species of mollusc and appears to have a negative effect on RLOs. The intracellular lifestyle of RLOs has been associated with cell alterations including hypertrophy and compression of the cytoplasmic organelles, and in most cases, no host reaction is observed by histology. In some cases, a high infection intensity of RLOs has been related to dysfunction of the affected organs that can result in disease, and in other cases, high burdens of RLOs have been associated with mortality, but most of these cases have yet to be confirmed. To date, only CXc has been confirmed as the aetiological agent of a chronic and potentially lethal disease of several abalone species known as withering syndrome. This disease is listed by the World Organization of Animal Health. With the ever‐increasing interest in preserving fisheries and expanding the aquaculture production of mollusc species, more efforts should be focused on the characterization and study of RLOs to elucidate the type of symbiosis they maintain with their host and determine when they are acting as true pathogens that should be subjected to control in aquaculture. This review provides an overview of the literature on RLOs that infect bivalves and gastropods of aquaculture importance, and we discuss the most important aspects of these unique prokaryotes.

High-resolution spatial and genomic characterization of coral-associated microbial aggregates in the coral Stylophora pistillata

“…Endozoicomonas strains have been isolated from corals, and their metabolic potential was inferred from derived genomic studies on the isolates and metagenomes assembled from culture-independent studies, suggesting integrated metabolic links facilitating nutrient acquisition and provision and therefore potentially important roles in host health (14,(21)(22)(23)(24). Some Endozoicomonas have been reported to locate within aggregates of the coral tissues (14) and other marine invertebrates (10,11,13), although other bacterial taxa including Rickettsiales (Alphaproteobacteria) and Chlamydia (Chlamydiae) have also been reported to form aggregations in corals (23,25) and mollusks (26,27); hence, these coral-associated microbial aggregates (CAMAs) could be constructed with polymorphous types of bacteria (28,29). However, the spatial localization of these symbionts is still largely overlooked, particularly with respect to individual phylotypes and populations across bacterial lineages.…”

16S rRNA–Based Analysis Reveals Differences in the Bacterial Community Present in Tissues of Choromytilus chorus (Mytilidae, Bivalvia) Grown in an Estuary and a Bay in Southern Chile

“…Studies have suggested that the microbiota in bivalves can be influenced by local factors [8,9] and can contribute beneficial functions to the host, such as the improvement of digestive tract development, the increase in an immune response against pathogen attack, and the provision of essential nutrients (e.g., vitamins, enzymes, and fatty acids) [10,11]. In contrast, studies have also revealed the occurrence of pathogenic bacteria in the microbiota, including both zoonotic pathogens (e.g., Vibrio splendidus, Vibrio aestuariuanus, and Nocardia crassostreae) and human pathogens (e.g., Vibrio parahaemolyticus and Vibrio vulnificus) [12][13][14][15]. Similarly, studies have also linked shifts in microbiota composition to bivalve mass mortality events [16][17][18].…”

Cosmopolitan Distribution of Endozoicomonas-Like Organisms and Other Intracellular Microcolonies of Bacteria Causing Infection in Marine Mollusks

“…In the case of rickettsia infections in abalone Haliotis spp. it has been shown that temperature, hostsusceptibility and pathogen-specificity can affect the growth of the IMCs and subsequently cause disease and mortality (Cruz-Flores and Cáceres-Martínez, 2020).…”

“…However, the vast majority of IMC infecting mollusks remains unclassified, despite some of them being associated with population declines. This information is essential to establish the type of symbiosis they maintain with their host (mutualisms, commensalism or parasitism) and whether they can act as true pathogens and from there to determine the necessity of implementing control measures in aquaculture (Cruz-Flores and Cáceres-Martínez, 2020). In the present study, the microbial diversity associated to IMCs infecting economically important marine molluskan species was interrogated by a 16S rRNA gene amplicon sequencing and phylogenetic analysis approach, through international collaboration, to progress in the molecular identification of IMCs infecting mollusks.…”