ORIGINAL RESEARCH published: 30 October 2020 doi: 10.3389/fmicb.2020.577481 Edited by: Sébastien Duperron, Muséum National d’Histoire Naturelle, France Reviewed by: Clara F. Rodrigues, University of Aveiro, Portugal Sigmund Jensen, University of Bergen, Norway Suzanne Dufour, Memorial University of Newfoundland, Canada *Correspondence: Irene Cano irene.canocejas@cefas.co.uk Specialty section: This article was submitted to Microbial Symbioses, a section of the journal Frontiers in Microbiology Received: 30 June 2020 Accepted: 14 October 2020 Published: 30 October 2020 Citation: Cano I, Ryder D, Webb SC, Jones BJ, Brosnahan CL, Carrasco N, Bodinier B, Furones D, Pretto T, Carella F, Chollet B, Arzul I, Cheslett D, Collins E, Lohrmann KB, Valdivia AL, Ward G, Carballal MJ, Villalba A, Marigómez I, Mortensen S, Christison K, Kevin WC, Bustos E, Christie L, Green M and Feist SW (2020) Cosmopolitan Distribution of Endozoicomonas-Like Organisms and Other Intracellular Microcolonies of Bacteria Causing Infection in Marine Mollusks. Front. Microbiol. 11:577481. doi: 10.3389/fmicb.2020.577481 Cosmopolitan Distribution of Endozoicomonas-Like Organisms and Other Intracellular Microcolonies of Bacteria Causing Infection in Marine Mollusks Irene Cano 1 * , David Ryder 1 , Steve C. Webb 2 , Brian J. Jones 3 , Cara L. Brosnahan 3 , Noelia Carrasco 4 , Barbara Bodinier 4 , Dolors Furones 4 , Tobia Pretto 5 , Francesca Carella 6 , Bruno Chollet 7 , Isabelle Arzul 7 , Deborah Cheslett 8 , Evelyn Collins 8 , Karin B. Lohrmann 9 , Ana L. Valdivia 9 , Georgia Ward 10 , María J. Carballal 11 , Antonio Villalba 11,12,13 , Ionan Marigómez 13 , Stein Mortensen 14 , Kevin Christison 15 , Wakeman C. Kevin 16 , Eduardo Bustos 17 , Lyndsay Christie 1 , Matthew Green 1 and Stephen W. Feist 1 1 International Centre of Excellence for Aquatic Animal Health, Cefas Weymouth Laboratory, Weymouth, United Kingdom, 2 Cawthron Institute, Nelson, New Zealand, 3 Animal Health Laboratory, Ministry for Primary Industries, Upper Hutt, New Zealand, 4 Institut de Recerca i Tecnologia Agroalimentaries (IRTA), Sant Carles de la Ràpita, Tarragona, Spain, 5 Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy, 6 Department of Biology, University of Naples Federico II, Naples, Italy, 7 SG2M-LGPMM, Laboratoire De Génétique Et Pathologie Des Mollusques Marins, Ifremer, La Tremblade, France, 8 Marine Institute, Galway, Ireland, 9 Departamento de Biología Marina, Facultad de Ciencias del Mar, Universidad Católica del Norte, Centro Innovación Acuícola Aquapacífico, Coquimbo, Chile, 10 Life Sciences Department, Natural History Museum, London, United Kingdom, 11 Centro de Investigacións Mariñas, Consellería do Mar da Xunta de Galicia, Vilanova de Arousa, Spain, 12 Departamento de Ciencias de la Vida, Universidad de Alcalá, Alcalá de Henares, Spain, 13 Research Centre for Experimental Marine Biology and Biotechnology (PIE), University of the Basque Country (UPV/EHU), Plentzia, Spain, 14 Institute of Marine Research, Bergen, Norway, 15 Department of Environment, Forestry and Fisheries, Cape Town, South Africa, 16 Institute for International Collaboration, Hokkaido University, Sapporo, Japan, 17 Centro Acuícola Pesquero de Investigación Aplicada (CAPIA), Universidad Santo Tomás, Sede Puerto Montt, Chile Intracellular microcolonies of bacteria (IMC), in some cases developing large extracellular cysts (bacterial aggregates), infecting primarily gill and digestive gland, have been historically reported in a wide diversity of economically important mollusk species worldwide, sometimes associated with severe lesions and mass mortality events. As an effort to characterize those organisms, traditionally named as Rickettsia or Chlamydia-like organisms, 1950 specimens comprising 22 mollusk species were collected over 10 countries and after histology examination, a selection of 99 samples involving 20 species were subjected to 16S rRNA gene amplicon sequencing. Phylogenetic analysis showed Endozoicomonadaceae sequences in all the mollusk species analyzed. Geographical differences in the distribution of Operational Taxonomic Units (OTUs) and a particular OTU associated with pathology in king scallop (OTU_2) were observed. The presence of Endozoicomonadaceae sequences in the IMC was visually confirmed by in situ hybridization (ISH) in eight selected samples. Sequencing data also indicated other symbiotic bacteria. Subsequent phylogenetic analysis of those OTUs revealed a novel microbial diversity associated with molluskan Frontiers in Microbiology | www.frontiersin.org 1 October 2020 | Volume 11 | Article 577481