Molecular characterization of plasmids encoding CTX-M-15 β-lactamases from Escherichia coli strains in the United Kingdom

Objectives: The UK, like other countries worldwide, has a growing problem with CTX-M β-lactamase-producing Escherichia coli. Five major clonally related strains have been identified among CTX-M-15 producers. We characterize here the plasmids from clonal strains A and D. Methods: Plasmids were extracted and transformed into E. coli DH5α; conjugative mating was attempted on agar. MICs were determined by agar dilution. β-Lactamases were typed by isoelectric focusing; antibiotic resistance genes and integrons were identified by PCR and sequenced. Plasmid incompatibility groups were determined by replicon PCR. Results: bla CTX-M-15 was carried by a 150 kb plasmid in strain A and a 70 kb plasmid in strain D. Conjugative transfer of cefotaxime resistance was only achieved from strain D; plasmids from both strains were transferred by transformation. The plasmid from strain A additionally carried bla TEM-1 (variably), bla OXA-1, aac(6′)-Ib-cr and tet (A), as well as a class 1 integron with the gene cassettes aad A5 and dfr 17; the plasmid from strain D carried bla TEM-1 consistently, also bla OXA-1, aac(6′)-Ib-cr, aac3-IIa and tet (A). Both plasmids belonged to incompatibility group FII. Conclusions: bla CTX-M-15 was plasmid-mediated in both strains A and D and was linked to other antibiotic resistance genes including aac(6′)-Ib-cr, which encodes an acetyltransferase, not previously found in Europe, acting on both aminoglycosides and some fluoroquinolones. Although the plasmids from the two strains differed in size, both were related and conferred similar multi-drug resistance phenotypes, suggesting that they may share a similar genetic scaffold. Both shared features with plasmids encoding CTX-M-15 β-lactamases in E. coli from Canada and India.