Integrons from Aeromonas bacteria collected from streams and beaches

Antibiotic resistance is a growing concern for public health. Aquatic environments in particular are considered large reservoirs for antibiotic resistance genes. Gram-negative bacteria from the genus Aeromonas are commonly associated with aquatic environments and include pathogens known to cause systemic infections as well as wound infections. Integrons are genetic units able to capture and incorporate single genes known as gene cassettes through site-specific recombination. They are often associated with other mobile genetic elements and can therefore rapidly disperse between different bacterial species and spread the genes they contain. Integrons in certain classes also share a relatively small gene cassette pool comprised almost entirely of antibiotic resistance genes and as such are important contributors to the spread of antibiotic resistance across bacterial species. The main goal of this study was to characterize and describe the integrons found in environmental bacterial isolates belonging to the genus Aeromonas. Water samples were taken from streams and several beaches in Southern California. The bacteria were isolated using filtering and growth on selective media. The bacterial DNA was isolated and PCR amplification was conducted with primers obtained from the literature and designed by our lab to screen the isolates for the presence of integrons. PCR products were run through a 1% agarose gel and positive samples were sequenced. The sequences were assembled using Lasergene software. The program Mega5 and NCBI tools were used for sequence analyses. 8 out of 19 (42%) total Aeromonas isolates from the environment contained an integron, which ranged from 1,833 to 3,574 base pairs in length. The Aeromonas species that contained integrons were A. culicicola and A. caviae. All integrons we discovered belong to class 1. The most common gene cassettes contained dfrA12 and aadA2 genes, which code for trimethoprim and aminoglycoside resistance, respectively. Other gene cassettes included dfrA17, aadB, and dfrA15. The gene dfrA15 has never been previously reported in Aeromonas, and most of the gene cassette combinations discovered in our integrons have also been previously unreported in Aeromonas. Aeromonas isolates are therefore acquiring new resistance genes in the environment, perhaps due to physical proximity of diverse bacteria in natural habitats. The spread of these genes through integrons suggests that the environment is a formidable source for the spread of antibiotic resistance genes through horizontal gene transfer.