Expanded-Spectrum β-Lactamase PER-1 in an Environmental Aeromonas media Isolate from Switzerland

Abstract

Aeromonas spp. are environmental reservoirs of resistance determinants to different classes of antibiotic molecules (1, 3, 7). Intrinsic resistance to β-lactams among this genus may arise from the expression of chromosomal β-lactamases and/or efflux pumps (5). Acquired β-lactamases conferring resistance to narrow-spectrum β-lactams, such as OXA-type enzymes and the penicillinases TEM-1 and SHV-1, have been identified in that genus as well as in a single report of the extended-spectrum β-lactamase (ESBL) TEM-24 (2, 4). The aim of this study was to evaluate the prevalence of ESBLs among Aeromonas sp. strains recovered from several rivers, lakes, and activated sludges from the southern part of the Swiss Alps. Water samples were collected between 2002 and 2005 from the rivers Ticino and Vedeggio, from the lakes Cadagno and Lugano, and from activated sludges, all located in the Tessin county of Switzerland. The water samples were plated and growing bacteria identified by conventional biochemical methods (API-NE system; bioMerieux, Marcy-l'Etoile, France). All of the 50 Aeromonas sp. strains recovered were tested for ESBL detection by a synergy test using disks of ticarcillin-clavulanic acid, ceftazidime, cefotaxime, and aztreonam. The susceptibility testing of isolates was performed using Etest strips (AB Biodisk, Solna, Sweden). A single isolate, A72, recovered from an activated sludge located in Bioggio, displayed an ESBL phenotype. This isolate was resistant to most β-lactams, except cephamycins and carbapenems (Table ​(Table1).1). PCR amplifications performed as described previously (9), using primers specific for ESBL-encoding genes, followed by sequencing identified the blaPER-1 gene. Genotyping of isolate A72, according to the results of gyrB sequencing (10), identified Aeromonas media, a waterborne species known to cause diarrhea in humans (6). TABLE 1. MICs of antimicrobial agents for Aeromonas media A72, E. coli J53 harboring natural plasmid p72, and E. coli reference strain J53 A 70-kb plasmid (pAM) harboring the blaPER-1 gene was transferred by conjugation into the Escherichia coli J53 recipient strain, using a selection made of ceftazidime (10 μg/ml) and sodium azide (100 μg/ml). Transconjugant E. coli J53(pAM) showed an antimicrobial susceptibility pattern mirroring that of A. media A72 (Table ​(Table11). PCR mapping performed as described previously (9) identified the blaPER-1 gene as part of a Tn1213 composite transposon, as previously found (9). In addition, this Tn1213 structure was bracketed by the aphA6 and strA genes (encoding resistance to aminoglycosides) of transposon Tn5393d, as already observed in an Alcaligenes faecalis clinical isolate producing PER-1 recovered from Italy. In that latter isolate, the Tn5393d transposon was inserted into a Tn501 backbone carrying a mercury resistance transposon (8) that was not identified in A. media A72 by PCR mapping. Our study constitutes the first identification of an ESBL-producing environmental Aeromonas sp. strain. The occurrence of blaPER-1 on a conjugative and broad-host-range plasmid, inside a genetic context similar to that described for an A. faecalis clinical isolate recovered in a neighboring country, may indicate that Aeromonas spp. may act as vehicles for spreading such an antibiotic resistance determinant. These findings also strengthen the role of Aeromonas spp. as reservoirs or vectors of antimicrobial resistance determinants of clinical relevance, as recently exemplified by the identification of the plasmid-mediated quinolone resistance qnrS2 gene in Aeromonas punctata and A. media from the Seine River, Paris, France (1).