Molecular typing of Aeromonas salmonicida- the cause of serous disease in farmed marine fish

Most if not all marine-fish currently held in captivity in Norway are susceptible to infection with Aeromonas salmonicida. Current expansion in farming of new marine species will almost certainly lead to increased isolations of A. salmonicida. Losses can be high and the bacterium is often difficult to eradicate from infected stocks. Repeated antibiotic treatments are often necessary. Vaccination in marine species may be only partially successful. Typing of A. salmonicida isolated from marine fish will al low greater understanding of the routes and risks for spread of infection, provide support for eradication measures, allow mapping of infection within susceptible fish populations, help identify common and variable antigens and suitable strains for auto-v accine production. Both subsp. salmonicida and “atypical” strains pose relevant problems in terms of strain characterisation and typing. The homogenous subsp. salmonicida strains have so far thwarted almost all efforts at sub-typing, while atypical strai n taxonomy remains vague and satisfactory typing systems have not yet been fully established. It is proposed that the recently sequenced (Institute for Marine Biosciences, Canada) A. salmonicida genome shall form the basis for a Multi-Locus sequence Typi ng Scheme (MLST) based on heterogeneity within “housekeeping” genes. It is considered that this typing system may be particularly useful for discrimination of subsp. salmonicida strains which although very homogenous, must be subject to micro-evolutionary influences. A Multi-Locus Variable number of tandem repeat Analysis (MLVA) typing system, based on repetitive elements identified from the genome sequence is also proposed. This assay will if successful, result in a reproducible, portable, PCR-based as say with high discriminatory power, most probably of use in differentiation of the heterogenic atypical isolates. A. salmonicida has a pan-global range. Such typing systems have world-wide relevance.