Salmon lice are one of the most important threats in Norwegian salmonid aquaculture and cleaner-fish of several species are used for biological control of salmon lice. The health situation in cleaner fish species is both alarming and unsustainable, with almost total loss of cleaner-fish during the salmon culture cycle being relatively common. Infection with the bacterium Aeromonas salmonicida represents one of the most important causes of death among cleaner-fish in salmon farms. This bacterium is known to cause disease in many different fresh- and saltwater fish species worldwide. Historically, focus has however been upon strains belonging to the sub-species A. salmonicida subsp. salmonicida (also known as ‘typical’ A. salmonicida), which primarily infects salmonid fish. Other fish species, such as cleaner fish, are more prone to infection with other A. salmonicida sub-species/-types (collectively referred to as ‘atypical’ A. salmonicida), and this highly diverse group of bacteria remains very poorly described and systematized. This has e.g. hampered vaccine development and made reproducible studies on ‘atypical’ A. salmonicida difficult to conduct.
A previous study carried out by our group found that A. salmonicida (‘typical’ and ‘atypical’) can be divided into different sub-types (A-layer types) based on sequencing of the gene encoding a protein layer (A-layer) exposed on the surface of the bacterial cell. Furthermore, each of the identified A-layer types seem to cause disease only in a few specific fish species, and primarily two such A. salmonicida A-layer types are found in cleaner fish, both in Norway and the British Isles. By expanding the collection of A-layer typed A. salmonicida isolates further with regard to origin (host, geography time), we can make a globally representative sub-selection of isolates for whole genome sequencing (WGS). Subsequent bioinformatics analysis will illuminate the overall population structure of this bacterium, while possibly also facilitating a better understanding of factors underlying the observed host specificity. Knowledge arising from this project could be potentially valuable for optimization of mitigating measures against A. salmonicida in farmed fish, including cleaner fish species.
