title: The evolution of class 1 integrons and the rise of antibiotic resistance.
abstract: Class 1 integrons are central players in the worldwide problem of antibiotic resistance, because they can capture and express diverse resistance genes. In addition, they are often embedded in promiscuous plasmids and transposons, facilitating their lateral transfer into a wide range of pathogens. Understanding the origin of these elements is important for the practical control of antibiotic resistance and for exploring how lateral gene transfer can seriously impact on, and be impacted by, human activities. We now show that class 1 integrons can be found on the chromosomes of nonpathogenic soil and freshwater Betaproteobacteria. Here they exhibit structural and sequence diversity, an absence of antibiotic resistance genes, and a phylogenetic signature of lateral transfer. Some examples are almost identical to the core of the class 1 integrons now found in pathogens, leading us to conclude that environmental Betaproteobacteria were the original source of these genetic elements. Because these elements appear to be readily mobilized, their lateral transfer into human commensals and pathogens was inevitable, especially given that Betaproteobacteria carrying class 1 integrons are common in natural environments that intersect with the human food chain. The strong selection pressure imposed by the human use of antimicrobial compounds then ensured their fixation and global spread into new species.
authors.affiliation: Department of Biological Sciences, Macquarie University, Sydney, NSW 2109, Australia. michael.gillings@mq.edu.au
authors.forename: Michael
authors.initials: M
authors.lastname: Gillings
authors.forename: Yan
authors.initials: Y
authors.lastname: Boucher
authors.forename: Maurizio
authors.initials: M
authors.lastname: Labbate
authors.forename: Andrew
authors.initials: A
authors.lastname: Holmes
authors.forename: Samyuktha
authors.initials: S
authors.lastname: Krishnan
authors.forename: Marita
authors.initials: M
authors.lastname: Holley
authors.forename: H W
authors.initials: HW
authors.lastname: Stokes
mesh_terms: D020562:Betaproteobacteria / Q000235:genetics* / Q000302:isolation & purification; D002876:Chromosomes, Bacterial; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D004352:Drug Resistance, Microbial / Q000235:genetics*; D019143:Evolution, Molecular; D005618:Fresh Water / Q000382:microbiology*; D023061:Gene Order; D022761:Gene Transfer, Horizontal; D032023:Integrons; D008969:Molecular Sequence Data; D010802:Phylogeny; D010957:Plasmids; D017422:Sequence Analysis, DNA; D012988:Soil Microbiology
chemical_list: D004269:DNA, Bacterial