IS1R-Mediated Plasticity of IncL/M Plasmids Leads to the Insertion ofblaOXA-48into the Escherichia coli Chromosome

Abstract

ABSTRACTThe OXA-48 carbapenemase is mainly encoded by ∼62-kb IncL/M plasmids. However, chromosome-mediated genes have been observed inEscherichia coliisolates. In this work, we investigated the genetic environment of OXA-48 in members of the familyEnterobacteriaceae(n= 22) to understand how the OXA-48-encoding gene is transferred into theE. colichromosome. The OXA-48-encoding gene was located within intact Tn1999.2transposons in the ∼62-kb plasmids or within a truncated variant of Tn1999.2for the OXA-48-encoding genes located in the chromosomes ofE. colibacteria. The analysis of the Tn1999.2genetic environment revealed an inverted orientation of the transposon in five ∼62-kb plasmids (5/14 [35%]) and in all chromosome inserts (n= 8). The sequencing of pRA35 plasmid showed that this orientation of Tn1999.2and the acquisition of an IS1Rinsertion sequence generated a 21.9-kb IS1R-based composite transposon encoding OXA-48 and designated Tn6237. The sequencing of a chromosomal insert encoding OXA-48 also revealed this new transposon in theE. colichromosome. PCR mapping showed the presence of this element in all strains harboring an OXA-48-encoding chromosomal insert. However, different insertion sites of this transposon were observed in theE. colichromosome. Overall, these findings indicate a plasticity of the OXA-48 genetic environment mediated by IS1Rinsertion sequences. The insertion sequences can induce the transfer of the OXA-encoding gene intoE. colichromosomes and thereby promote its persistence and expression at low levels.