IncI1 Plasmid Carrying Extended-Spectrum-␤-Lactamase Gene bla CTX-M-1 in Salmonella enterica Isolates from Poultry and Humans in France, 2003 to 2008

International audienceWe report the dissemination of a conjugative IncI1 plasmid carrying bla CTX-M-1 , conferring resistance to extended-spectrum cephalosporins, in Salmonella enterica isolates from poultry and humans in France from 2003 to 2008. By IncI1 plasmid subtyping, this plasmid was shown to be genetically related to that found in Escherichia coli isolates from healthy poultry in France. Food-producing animals are the primary reservoir of zoo-notic pathogens, and the prevalence of resistance to extended-spectrum cephalosporins (ESCs) in Escherichia coli and Salmonella enterica has increased in recent years. In Belgium and France, the emergence of resistance to ESCs, due to extended-spectrum ␤-lactamases (ESBLs), in E. coli and S. enterica from animal (mainly cattle and poultry) and human origins (1, 3, 6, 7, 10, 11, 14, 15, 17) has been reported. Resistance in these bacteria was reported to be conferred mainly by the ESBL genes bla CTX-M-2 , bla CTX-M-9 , bla CTX-M-15 , and bla TEM-52 (1, 3, 6, 11, 14, 15, 17). In addition, the occurrence of CTX-M-1 ESBL resistance in E. coli isolates recovered from food animals (cattle, poultry, and swine) in France was recently reported (7, 10, 11). The ESBL resistance genes were shown to reside on large conjugative plasmids of the IncI1 or IncHI2 incompatibility group (3, 5, 6). Since 2003, a number of S. enterica strains showing resistance to ESCs by production of an ESBL not previously detected in Salmonella in France and with various additional resistances to other antibiotic families have been isolated from poultry (n ϭ 1) and from humans (n ϭ 9) in France (Table 1). The human isolates consisted of seven serovar Typhimurium (including a monophasic variant) strains, one serovar London strain, and one serovar Newport strain, and the avian isolate was of serovar Llandoff. The purpose of the present study was to identify the ESBL gene and to characterize the plasmid(s) carrying this gene. For this, in addition to other methods, we applied the recently described plasmid multilocus sequence typing method for IncI1 plasmids carrying ESBL genes (5). The strains studied are shown in Table 1. Antibiotic susceptibility testing was done by the disc diffusion method, and MICs of ceftriaxone and ceftazidime were determined by Etest a