Complete nucleotide sequence of CTX-M-15-plasmids from clinical Escherichia coli isolates: Insertional events of transposons and insertion sequences

Background: CTX-M-producing Escherichia coli strains are regarded as major global pathogens. Methodology/Principal Findings: The nucleotide sequence of three plasmids (pEC_B24: 73801-bp; pEC_L8: 118525-bp and pEC_L46: 144871-bp) from Escherichia coli isolates obtained from patients with urinary tract infections and one plasmid (pEC_Bactec: 92970-bp) from an Escherichia coli strain isolated from the joint of a horse with arthritis were determined. Plasmid pEC_Bactec belongs to the IncI1 group and carries two resistance genes: bla_TEM-1 and bla_CTX-M-15. It shares more than 90% homology with a previously published bla_CTX-M-plasmid from E. coli of human origin. Plasmid pEC_B24 belongs to the IncFII group whereas plasmids pEC_L8 and pEC_L46 represent a fusion of two replicons of type FII and FIA. On the pEC_B24 backbone, two resistance genes, bla_TEM-1 and bla_CTX-M-15, were found. Six resistance genes, bla_TEM-1, bla_CTX-M-15, blaOXA-1, aac6'-lb-cr, tetA and catB4, were detected on the pEC_L8 backbone. The same antimicrobial drug resistance genes, with the exception of tetA, were also identified on the pEC_L46 backbone. Genome analysis of all 4 plasmids studied provides evidence of a seemingly frequent transposition event of the bla_CTX-M-15-ISEcp1 element. This element seems to have a preferred insertion site at the tnpA gene of a bla_TEM-carrying Tn3-like transposon, the latter itself being inserted by a transposition event. The IS26-composite transposon, which contains the bla_OXA-1, aac6'-lb-cr and catB4 genes, was inserted into plasmids pEC_L8 and pEC_L46 by homologous recombination rather than a transposition event. Results obtained for pEC_L46 indicated that IS26 also plays an important role in structural rearrangements of the plasmid backbone and seems to facilitate the mobilisation of fragments from other plasmids. Conclusions: Collectively, these data suggests that IS26 together with ISEcp1 could play a critical role in the evolution of diverse multiresistant plasmids found in clinical Enterobacteriaceae. © 2010 Smet et al.