Integron diversity in bacterial communities of freshwater sediments at different contamination levels

Integrons, genetic elements known to be involved in the adaptation of pathogenic bacteria, were first discovered in the clinical setting. However, they are ancient structures found in various environments. When clinical integrons have a low diversity, with three integrases and gene cassettes essentially encoding antibiotic resistance, in natural environments, integrons show a greater diversity, of both gene cassettes and integrases. Although a large number of gene cassettes from environmental samples have been identified, integrase diversity remains poorly documented, and has not yet been investigated in freshwater environments. The work presented here explores environmental integrons in sediments from a freshwater environment, with emphasis on integrases. Integron diversity in bacterial communities was analyzed at sampling stations with different contamination levels and contaminant types. A total of 684 integrase sequences were obtained and grouped into 322 previously undescribed integron classes, revealing a diversity wider than that previously expected in non-clinical environments. The bacterial community structures did not fully explain the integron diversity suggesting that integrase diversity could be influenced by contamination level, and that contaminant type could influence gene cassette diversity. These results provide further arguments for the involvement of integrons in the adaptation of bacterial communities in response to contaminants in natural environments.