Gene cassettes encoding resistance to quaternary ammonium compounds: a role in the origin of clinical class 1 integrons?

DNA sequencing, phylogenetic and mapping studies suggest that the class 1 integron found in pathogens arose when one member of the diverse family of environmental class 1 integrons became embedded into a Tn402 transposon. However, the timing of this event and the selective forces that first fixed the newly formed element in a bacterial lineage are still unknown. Biocides have a longer use in clinical practice than antibiotics, and a qac (quaternary ammonium compound) resistance gene, or remnant thereof, is a normal feature of class 1 integrons recovered from clinical isolates. Consequently, it is possible that the initial selective advantage was conferred by resistance to biocides, mediated by qac. Here, we show that diverse qac gene cassettes are a dominant feature of cassette arrays from environmental class 1 integrons, and that they occur in the absence of any antibiotic resistance gene cassettes. They are present in arrays that are dynamic, acquiring and rearranging gene cassettes within the arrays. The abundance of qac gene cassettes makes them a likely candidate for participation in the original insertion into Tn402, and as a source of a readily selectable phenotype. More broadly, the increasing use of qac and other biocides at the present time seems likely to promote the fixation of further novel genetic elements, with unpredictable and potentially adverse consequences for human health and agriculture.