Furunculosis-inducing ability of microcosms seeded with Aeromonas salmonicida correlates with colony-forming ability but not with PCR and ELISA data

Two laboratory microcosms, constructed of sterilised sediment and water taken from different locations, were seeded with Aeromonas salmonicida. A non-quantitative PCR, a heterogeneous, noncompetitive, sandwich ELISA and colony formation were employed to generate survival data. In one microcosm, all three analytical methods generated positive results over the full 279 days of the incubation of the system at 18 degreesC. In the second microcosm, constructed with humic acid-rich material, colony formation was undetectable after 1 day but the PCR and ELISA methods generated positive results for up to 269 days.Direct injection into fish of material from the two microcosms both seeded with a passaged strain of A. salmonicida was used to determine the disease-producing potential of the bacteria in these systems. A direct correlation was observed between the presence of colony-forming units and the ability to produce furunculosis in test fish. None of the samples that generated positive PCR and ELISA signals but in which colony formation was not detected proved capable of inducing disease in fish. The significance of these data for the application of proxy methods in epizootiological studies is discussed. (C) 2002 Elsevier Science B.V. All rights reserved.