The fish pathogen Aeromonas salmonicida was chromosomally marked with genes encoding bacterial luciferase, luxAB, isolated from Vibrio fischeri, resulting in constitutive luciferase production. During exponential growth in liquid batch culture, luminescence was directly proportional to biomass concentration, and luminometry provided a lower detection limit of approximately 10 3 cells ml ؊1 , 1 order of magnitude more sensitive than enzyme-linked immunosorbent assay detection. In sterile seawater at 4؇C, lux-marked A. salmonicida entered a dormant, nonculturable state and population activity decreased rapidly. The activity per viable cell, however, increased by day 4, indicating that a proportion of the population remained active and culturable. Putative dormant cells were not resuscitated after the addition of a range of substrates.

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 ≥109 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, all 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 102 and 107 cells g−1 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.

Colonisation and transmission of Aeromonas salmonicida in Atlantic salmon was investigated using wild-type and luxmarked strains of A. salmonicida. An initial intra-peritoneal (i.p.) challenge showed that lux-marked cells were virulent only when injected at concentrations v10 W cfu ml 3I 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, all 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 P and 10 U cells g 3I 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. z 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.