Linkage of acquired quinolone resistance (qnrS1) and metallo-beta-lactamase (blaVIM-1) genes in multiple species of Enterobacteriaceae from Bolzano, Italy

Objectives Twenty-four of 209 oxyimino-cephalosporin- and/or aztreonam-resistant Enterobacteriaceae collected around Bolzano had reduced susceptibility or resistance to carbapenems and gave positive metallo-ß-lactamase (MBL) tests. Their resistance mechanisms were investigated. Methods Resistances were identified by Vitek 2 and MIC tests and isolates were genotyped by PFGE. Resistance genes were identified by PCR and sequencing, and plasmids were transferred by conjugation and/or transformation. Plasmid-borne genes were identified by Southern blotting, and their genetic surroundings were investigated by PCR mapping. Results The 24 isolates with positive EDTA/imipenem synergy tests had blaVIM-1 carried on 40–150 kb plasmids. Imipenem MICs ranged from 2 to >32 mg/L, while those of meropenem and ertapenem were lower. The isolates included a clonal cluster of 10 Klebsiella pneumoniae, two other K. pneumoniae isolates, and diverse isolates of Escherichia coli (seven), Klebsiella oxytoca (three) and Citrobacter freundii (two). Six MBL producers were aztreonam-susceptible; the 18 aztreonam-resistant isolates had co-resident extended-spectrum ß-lactamases. blaVIM-1 occurred as the first cassette in class 1 integrons, with aacA4 as the second cassette. Quinolone resistance gene qnrS1 was detected in 21 of 24 (87.5%) blaVIM-1-positive isolates versus 14 of 185 (7.6%) blaVIM-negative isolates (P < 0.0001), with 13 of the latter belonging to a clonal cluster of E. coli. qnrS1 was located on the same plasmids as blaVIM-1 and aacA4, but was not closely linked, as judged by PCR mapping. Conclusions blaVIM-1 has become disseminated among enterobacteria in a small Italian town. The frequent association of genes conferring carbapenem, aminoglycoside and quinolone resistance on single plasmids will facilitate co-selection.