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-beta-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 bla(VIM-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 beta-lactamases. bla(VIM-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%) bla(VIM-1)-positive isolates versus 14 of 185 (7.6%) bla(VIM)-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 bla(VIM-1) and aacA4, but was not closely linked, as judged by PCR mapping.
Conclusions: bla(VIM-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.