The tetracycline resistance gene tet(E) is frequently occurring and present on large horizontally transferable plasmids in Aeromonas spp. from fish farms

The aim of the study was to characterize 30 unrelated tetracycline resistant Aeromonas spp. from fish farms where none of the tetracycline resistance genes tet(A–E) were detected by multiplex PCR in a previous study. By cloning, sequencing and designing alternative PCR primers the tetracycline resistance gene tet(E) was found in (27/30) 90% of the previously negatively tested Aeromonas spp. The strains originated from farmed fish, water and sediment in Denmark (n = 27) and Canada (n = 3). The tet(E) gene was in several cases located on large horizontally transferable plasmids (approx. 150 kb) capable of interspecies transfer to Escherichia coli. This is the first report of horizontally transferable tet(E). Additionally, 15 motile Aeromonas strains with previously identified tetracycline resistance genes were tested, and this on one hand verified former findings of tet(E), but also identified more than one tet gene in some strains.The use of multiplex PCR for detection of tet(E) should be done with caution. tet(E) seems to occur frequently in Aeromonas spp. from fish farms and may spread horizontally among bacteria in the aquatic environment.