Bacterial diversity in a marine hatchery: Balance between pathogenic and potentially probiotic bacterial strains

Aquatic hatcheries contain diverse microbial communities that include pathogenic, innocuous and beneficial bacteria, and the ability to maintain a proper balance of this microflora may be the key to a successful culture environment. Herein, we undertook to identify the bacterial diversity present in a marine hatchery in British Columbia that cultures both fish and shellfish species. Bacterial strains were cultured from numerous microalgae and shellfish species as well as a single marine fish (sablefish) species grown in the facility. In addition, several bacterial isolates were taken from wild bivalve shellfish used as broodstock (geoducks and oysters), surrounding marine waters and sediments, and from macroalgae collected in the field. Characterizations were limited to culturable bacteria, as the ultimate aim of the project was to identify strains that were beneficial and could be used as probionts in the future. Of the 598 bacterial isolates cultured, 172 unique phylotypes were identified through 16S ribosomal DGGE genotyping. Sixty percent of the unique phylotypes were sequenced, yielding 112 different strains/species of bacteria. Twenty-two percent of the bacterial strains were found to be ubiquitous in the hatchery and marine environment, having been identified in three or more species. Potential pathogens were identified by their strain ID, ability to lyse red blood cells, or prevalence in moribund individuals. Sixteen of the strains identified were known fish or shellfish pathogens, and six strains were reported human pathogens. Fifty-seven percent of the bacterial isolates tested were haemolytic positive and 23% were identified from moribund or dead larvae. In addition, Vibrio logei, a luminous bacterial species that is symbiotic in many shellfish species, was identified as a potential pathogen of sablefish larvae.