Low Cassette Variability in Class 2 and Class 1 Integrons of Aeromonas spp. Isolated from Environmental Samples

Integrons are prokaryotic genetic elements known to carry and exchange antibiotic resistance gene cassettes through a site-specific recombinase called integrase. In this work, 107 Aeromonas isolates from environmental origin, including fish, water, and sediments, were investigated for the presence of integrons. Using specific primers for Class 1, 2 and 3 integrases, only Class 1 and Class 2 integrons were detected. Detection of Class 2 integrases and their associated variable regions required two rounds of polymerase chain reaction (PCR). Sequencing of the intI2 amplicons confirmed them as integrase-derived products. Class 1 integrons were detected in 26 out of 107 isolates. PCR amplification of the variable regions associated to these integrons revealed an outstanding homogeneity, 25 of them having variable regions with an identical dfrA12-orfF-aadA2 cassette array and one integron carrying only the dfrA16 cassette. To assess clone diversity, chromosomal DNA from isolates was subjected to enterobacterial repetitive intergenic consensus-PCR (ERIC-PCR), which discarded clonality in all instances. Class 2 integrons were surprisingly more prevalent than Class1 integrons, being detected in 60 out of 107 isolates. Forty-six of them showed a unique ERIC profile, while the remaining 14 strains displayed profiles that could be grouped in five different patterns. Cassette arrangements of all Class 2 variable regions were those described as the most prevalent (dfrA1-sat2-aadA1). A rather startling result of this work is the sensitivity to trimethoprim, streptomycin, and streptothricin of most strains, despite the presence of the cognate resistance genes. To know the integron distribution in environmental Aeromonas species, a phylogenetic reconstruction was done using rpoD/gyrB or rpoD/gyrA gene sequences. Isolates bearing these elements corresponded to Aeromonas hydrophila, Aeromonas veronii, Aeromonas salmonicida, Aeromonas dhakensis, Aeromonas sanarellii, Aeromonas taiwanensis, Aeromonas media, Aeromonas caviae, Aeromonas jandaei, and Aeromonas sp. This work revealed an unusual high incidence of Class 2 integrons and a low variability of cassette arrangements in environmental Aeromonas species.