Development of a PCR protocol for the detection of Aeromonas salmonicida in fish by amplification of the fstA (ferric siderophore receptor) gene.

The aims of the study were to evaluate a new PCR protocol designed to detect Aeromonas salmonicida in fish tissues and to develop a non-destructive method for the diagnosis of furunculosis. A set of primers (Fer3, Fer4), flanking a fragment of the fstA gene (coding for the ferric-siderophore receptor) was designed, showing to be sensitive and specific. When compared to PCR methods previously reported, the new protocol recognized all the 69 A. salmonicida strains evaluated, with no cross-reactions with the other bacterial species analysed. Sensitivity assays were performed in fish tissues seeded with serial dilutions of pure cultures of A. salmonicida and mixed cultures of this bacterium with Vibrio anguillarum and Aeromonas hydrophila. Detection limits obtained were of 60 and 450 bacterial cells 100 mg(-1) of tissue, respectively. Mucus and blood were evaluated in order to develop a non-destructive tool to detect the pathogen. The detection limits in seeded mucus and blood samples were 2.5 x 10(2) and 1 x 10(5) bacterial cells mL(-1), respectively. When the method was used to detect A. salmonicida in asymptomatic wild salmon, four samples of mucus and six of blood were positive, corresponding to 6 out of the 31 fish examined, whereas only one of the samples resulted positive by culture methods. It is concluded that the PCR protocol evaluated is fast, specific and sensitive to detect A. salmonicida in infected and asymptomatic fish, and will be helpful for the control of the disease through the prompt detection of carriers within fish populations.