Objectives: To characterize plasmids encoding extended-spectrum β-lactamases (ESBLs) from a recent UK collection of clinical Escherichia coli isolates.
Methods: The isolates comprised 118 ESBL producers referred from 54 laboratories. Plasmids were transferred by electroporation, and their incompatibility groups, associated addiction systems and resistance genes with the flanking genetic environments were identified by PCR or sequencing.
Results: Seventy isolates had plasmids encoding CTX-M-15 (n = 53), CTX-M-14 (n =9), CTX-M-27 (n = 1), CTX-M-3 (n = 2) and SHV-12 (n = 5) ESBLs that were transformable; non-transformable ESBLs were mainly CTX-M enzymes (42/48). Most transformable bla(CTX-M-15) genes (43/53) were harboured on single replicon or multireplicon IncF plasmids, with IncFIA4-FIB1-FII31 (n = 11) and IncFIA1-FII2 (n = 15) being most frequent; the latter included eight pEK499 plasmids, typical of UK epidemic strain A. Plasmids harbouring bla(CTX-M-14) belonged variously to IncF, IncI1 and IncHI2 types, and 16 encoding CTX-M or SHV enzymes were non-typeable. Only IncF plasmid types carried the addiction systems sought and those with bla(CTX-M-15) frequently harboured bla(OXA-1) and aac(6')-Ib-cr, and often transferred trimethoprim and tetracycline resistance; those with bla(CTX-M-14) encoded trimethoprim, sulphonamide, streptomycin and tetracycline resistance. Most ESBL genes were associated with the well-known mobile elements ISEcp1 and IS26, but nearly half (23/55) of the ISEcp1 sequences upstream of bla(CTX-M-15) were interrupted by an IS26 at various positions.
Conclusions: Most ESBLs (70/118) were encoded by transformable plasmids, although a sizable minority could not be transformed. The majority of transformable plasmids (51/70; 72.9%) were diverse multiresistant IncF types possessing multiple addiction systems. The spread of bla(CTX-M-15) can be attributed not just to clonal expansion, but also to the horizontal dissemination of related plasmids.