Colonisation and transmission of lux-marked and wild-type Aeromonas salmonicida strains in Atlantic salmon (Salmo salar L.)

Colonisation and transmission of Aeromonas salmonicida in Atlantic salmon was investigated using wild-type and lux-marked strains of A, salmonicida. An initial intra-peritoneal (i.p.) challenge showed that lux-marked cells were virulent only when injected at concentrations greater than or equal to 10(9) cfu ml(-1) and significantly less infective than wild-type MT463. The low virulence of A. salmonicida MT463 luxAB was probably due to loss of the proteinaceous A-layer, which is an important virulence factor involved in both intra- and inter-cellular A. salmonicida interactions. During the i.p. challenge, ail fish were held in one tank enabling assessment of transfer of lux-marked A. salmonicida between fish. Lux-marked cells shed from moribund and dead fish survived in the water column and cross-infected cohabitant fish. Cross-infection by A. salmonicida MT463 luxAB was investigated further by carrying out a cohabitation challenge. Lux-marked cells were recovered in low numbers from gill tissue and skin/mucus of cohabitant fish. Poor adhesion of cells may be due to loss of the A-layer protein. During a second cohabitation challenge using A-layer(+) and virulent wild-type strain MT432, between 10(2) and 10(7) cells g-l of fish gill tissue or skin/mucus were isolated. This result confirmed the preliminary observations obtained using lux-marked A. salmonicida MT463 and suggested that the gill and skin/mucus regions of fish were the main sites for attachment of A. salmonicida. None of the A. salmonicida strains was recovered from fish intestine samples during cohabitation challenges. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.