Many Class I Integrons Comprise Distinct Stable Structures Occurring in Different Species of Enterobacteriaceae Isolated from Widespread Geographic Regions in Europe

Three sizes of inserted regions of DNA (800, 1,000, and 1,500 bp) were shown to be common among class I integrons in unrelated clinical isolates ofEnterobacteriaceae from different European hospitals. Sequencing showed that 800-bp inserted regions comprised identical sequences includingaacA4 , that 1,000-bp inserted regions includedaadA , and that 1,500-bp inserted regions includeddfrI andaadA1 , irrespective of host species and geographic origin. In addition promoter sequences were mostly identical for each size class. These data suggest that inserted gene cassettes and promoter regions of integrons are conserved and stable, with resistance genes transferred more often as part of the entire integron structure than as individual gene cassettes.