In vivo transfer of plasmid pRAS1 between Aeromonas salmonicida and Aeromonas hydrophila in artificially infected Cyprinus carpio L.

This study investigated the possible in vivo transfer of plasmid pRAS1 between Aeromonas salmonicida and A. hydrophila inhabiting two different organs of Cyprinus carpio L. To distinguish transconjugants from naturally occurring antibiotic resistant bacteria, twelve luminescent transposon-tagged A. hydrophila strains using mini Tn5luxCDABEKm2 transposon were generated. In conjugal transfer experiments, fish were conditioned with the donor bacteria and subsequently immersed in water containing the recipient strain. Bacteria were recovered from gills and intestines and isolated by growth on selective plates. Transconjugants were identified by their resistance to the pRAS1 encoded antimicrobials and by light emission. In vivo transfer frequencies ranged between 10(-3) and 10(-6) and were somewhat lower in intestines, compared to gills. Transfer frequencies were also smaller relative to those obtained in vitro. The minimal amount of donor and recipient bacteria needed to yield detectable transconjugants in vivo was 1 x 10(4) CFU mL(-1). Implications of this plasmid transfer in natural settings and its possible consequences to human health are discussed.