Extended-spectrum -lactamase blaCTX-M-1 gene carried on an IncI1 plasmid in multidrug-resistant Salmonella enterica serovar Typhimurium DT104 in cattle in France

Sir, Since the 1980s, multidrug-resistant Salmonella enterica serovar Typhimurium definitive phage type (DT) 104 has been extensively reported as a cause of infections in humans and cattle due to the dissemination of clonal isolates carrying the Salmonella genomic island 1 (SGI1). SGI1 is mostly responsible for the resistance to a core group of five antimicrobials, including ampicillin/amoxicillin, chloramphenicol/florfenicol, streptomycin/spectinomycin, sulphonamides and tetracyclines (ACSSuT phenotype).1 In addition, the wide dissemination of plasmidic extended-spectrum β-lactamases (ESBLs) in humans and animals may lead to the emergence of S. enterica carrying both SGI1 and ESBL genes. This was reported for the first time in 2007 in blaTEM-52-carrying Salmonella Agona and Typhimurium from poultry and humans, and in blaCTX-M-1-carrying Salmonella Typhimurium from humans, poultry and domestic animals, with both genes located on IncI1 conjugative plasmids.2–4 Yet, this combination has never been described in bovine isolates of S. enterica, despite the epidemic of multidrug-resistant Salmonella Typhimurium DT104 in this animal species and the significant prevalence of ESBLs in Escherichia coli isolates in cattle, especially in France.5 Here, we describe a Salmonella Typhimurium strain (isolate 25008) recovered in 2010 from faeces of a diarrhoeic calf and collected through the RESAPATH network, which carries out surveillance of antimicrobial resistance in animal infections in France (www.resapath.anses.fr). This strain displayed the typical pentaresistance conferred by SGI1 and the presence of the island was assessed by PCRs.6 It was integrated at the specific location described for all other published isolates, as proved by the detection of the chromosomal junctions of the trmE (also called thdF left junction) and int2 (right junction) genes of the retron sequence. Furthermore, PCR mapping of SGI1 confirmed the classical organization of the SGI1 backbone and of its complex integron In104, responsible for the ACSSuT phenotype.6 In addition to the two classical SGI1 integron cassette arrays of 1000 and 1200 bp, an additional 1600 bp product was also detected by integron PCR using primers CS1 and CS2,6 which carried the dfrA17 and aadA5 cassettes, as shown by sequencing. In addition, strain 25008 was resistant to ceftiofur but susceptible to cefoxitin, with a typical double-disc synergy suggesting ESBL production, which was confirmed by PCR detection and sequencing of a blaCTX-M-1 β-lactamase gene preceded by the ISEcp1 element. This resistance to ceftiofur was transferred by conjugation to an E. coli recipient strain K12 J5, along with sulphonamide and trimethoprim resistances. The dfrA17 and aadA5 gene cassettes and the blaCTX-M-1 gene were also detected in the transconjugant (TC) strain, suggesting that they were located on the same plasmid. The blaCTX-M-1 plasmid transferred in TC-25008 belonged to the IncI1 incompatibility group and to the ST3 group previously detected in avian E. coli isolates, as shown by PCR-based replicon typing and plasmid multilocus sequence typing, respectively.7,8 Since SGI1-carrying Salmonella Typhimurium isolates from humans and healthy poultry were recently reported to harbour ST3/IncI1 blaCTX-M-1-positive plasmids, we performed EcoRI restriction analysis and showed that plasmids from TC-25008 and the human TCs (TC-05-9280 and TC-08-843) shared highly similar restriction profiles (Figure 1a).4 In parallel, PulseNet standard PFGE of XbaI-digested chromosomal DNA carried out on these three SGI1-carrying strains also demonstrated an identical XTYM-1 PFGE profile (Figure 1b), which is the most prevalent one for Salmonella Typhimurium DT104 strains in France. (a) Restriction analysis (EcoRI) of plasmids extracted from E. coli TCs. 1, TC-25008; 2, TC-05-9280; 3, TC-08-843; M, molecular weight marker IV (Roche Diagnostics, Meylan, France). (b) XbaI-PFGE profiles of the S. enterica serovar Typhimurium strains. 1, 25008; 2, 05-9280; 3, 08-843; M, lambda ladder (Bio-Rad, Marnes la Coquette, France). In this study, we report the first description of multidrug resistance island SGI1 together with ESBL production in Salmonella Typhimurium isolated from cattle, an animal reservoir that mainly supported the epidemic dissemination of the multidrug-resistant DT104 clone but remains nearly free of Salmonella producing ESBLs.3 Indeed, the current literature strongly indicates that ESBL genes carried by IncI1 plasmids are significantly associated with the avian reservoir of E. coli and Salmonella in Europe.4 Consequently, in addition to a transfer from poultry to humans, our data now suggest the passage of the same blaCTX-M-1-carrying IncI1 plasmid from poultry to cattle. This hypothesis is further supported by the fact that blaCTX-M-1-carrying IncI1 plasmids (including this one) were detected in >10% of the faecal E. coli from healthy poultry in France, suggesting a strong prevalence of these plasmids in the gut flora of the poultry population.9 Poultry was also abundantly documented as an important reservoir of S. enterica of various serovars carrying blaCTX-M-1 on IncI1 plasmids in Europe and, therefore, might be a starting point for ESBL dissemination to humans and other food animals, such as cattle.3,10 These data also further highlight the huge success of ESBL-carrying plasmids in disseminating antimicrobial resistance among different bacterial backgrounds and animal environments. More specifically, considering the important prevalence of SGI1-carrying Salmonella Typhimurium isolates in cattle, the association and emergence of highly diffusible ESBL gene-carrying IncI1 plasmids in these strains is of concern, and further surveillance of this genotype is warranted, both in humans and animals. This work was funded by the Agence Nationale de Sécurité Sanitaire (Anses). None to declare. We wish to thank all the veterinary laboratories participating in the RESAPATH network and, more specifically, the veterinary laboratory from Meurthe-et-Moselle (54) from which strain #25008 was obtained. We thank also Estelle Saras, Christiane Brunet and Véronique Métayer for their skilled assistance, and Dr Alessandra Carattoli for providing the plasmid incompatibility group control strains.