Emergence of extended-spectrum beta-lactamases in clinical isolates of Salmonella enterica in Tehran, Iran.

The purpose of the current study was to investigate the presence and molecular type(s) of extended-spectrum beta-lactamases (ESBLs) in Salmonella spp. isolates obtained from patients with diarrhea in hospitals of Tehran, Iran. Over a period of 17 months, 129 Salmonella spp. were isolated from fecal samples and tested for susceptibility using the Kirby-Bauer disk diffusion method; then, screening for ESBL-producing isolates and determination of their minimum inhibitory concentrations were carried out using the combined disk method and standard agar dilution method, respectively. The presence and type of ESBL-encoding genes were determined by PCR and sequence analysis. The isolates were all identified as Salmonella enterica of different serovars. The highest resistance in the collected Salmonella isolates was to nalidixic acid (45.7%), followed by tetracycline (43.4%), trimethoprim-sulfamethoxazole (36.4%), ampicillin (15.5%), and chloramphenicol (14.7%). All the isolates were susceptible to ciprofloxacin, gentamicin, and cefoxitin. Three S. enterica isolates were resistant to ampicillin, piperacillin, ceftazidime, cefotaxime, ceftriaxone, cefpodoxime, cephalothin, and aztreonam. PCR and DNA sequencing revealed that two of the three isolates harbored both a bla(CTX-M-15) and a bla(TEM) gene while the third one carried only a bla(CTX-M-15) gene. This is the first study providing structural data for a CTX-M-type beta-lactamase produced by Salmonella isolates recovered in Iran.