Extended-spectrum β-lactamase-conferring transferable resistance to different antimicrobial agents inEnterobacteriaceae isolated from bloodstream infections

Twenty (18.5 %) out of 108 clinical isolates of the familyEnterobacteriaceae responsible for bloodstream infection were extended-spectrum β-lactamase (ESBL)-positive in two screening tests, the double disk synergy test and theOxoid Combination Disk method. Eleven out of the 20 ESBL-positive isolates transferred oxyimino-β-lactam resistance toE. coli K12 C600 recipient strain with a frequency of 10−8–10−1 per donor cell. PCR analysis revealed that the majority of the transconjugants (9 of 11) express CTX-M-type β-lactamases. Donor strains and their transconjugants displayed susceptibility patterns typical of ESBL producers. They were resistant to oxyimino-β-lactams but susceptible to clavulanic acid and carbapenems. Resistances to aminoglycosides, tetracycline and mercuric chloride were, in some cases, co-transferred with oxyimino-β-lactam resistance, suggesting that various resistance determinants were carried by the same conjugative plasmids.