An RT PCR-DGGE survey of gill-associated bacteria in Norwegian seawater-reared Atlantic salmon suffering proliferative gill inflammation

Proliferative gill inflammation, a respiratory disease of apparently multifactorial aetiology, affects seawater-farmed Atlantic salmon in Norway and may be associated with significant losses. In the present study, the gill-associated bacterial community in fish suffering PGI as diagnosed by histology was compared with that of clinically healthy fish by Reverse Transcriptase Polymerase Chain Reaction-Denaturing Gradient Gel Electrophoresis. 'Candidatus Piscichlamydia salmonis', which forms histologically detectable intracellular inclusions commonly called epitheliocysts, was identified in 16 of 21 fish displaying PGI, normally as the dominant phylotype. Although identified by RT-DGGE in many clinically healthy fish from PGI affected farms, this bacterium was only detected in 7 of 48 fish from PGI-free farms, of which all positives interestingly originated from a single farm. Taken together, these results reveal a positive association between 'Candidatus Piscichlamydia salmonis' and PGI in the investigated material from 10 geographically distinct farms. Furthermore, a wide distribution of gamma-proteobacteria was detected, but the bacterial communities individual fish gills were generally quite simple and often dominated by one or two phylotypes, most commonly Burkholderia-like bacteria (previously Pseudomonads) and Psychrobacter spp. Representatives spoilage or fish pathogenic genera e.g. Photobacterium, Flavobacterium, Allivibrio, Shewanella and Tenacibaculum, were less frequently identified and apparently in low numbers. Sequences representing at least one undescribed species closely related to fish pathogenic Francisella spp. were also identified. (C) 2009 Elsevier B.V. All rights reserved.