Shigella is an important cause of mild to severe diarrheal disease with high incidence in developing countries. The Israeli National Reference Center for Shigella (NRCS) performs routine surveillance of antimicrobial resistance on isolates received from all over the country (hospitals and community health centers). Resistance to third generation cephalosporins is infrequent amongst Shigella sp. This work aimed to assess the prevalence of Extended Spectrumb-lactamases (ESBL) producing strains and to identify the predominant enzymes involved. During a period of 4 years (2009-2012) a total of 4,452 Shigella isolates were tested for antibiotic resistance by the disk diffusion method according to the CLSI guidelines. 77 isolates (1.7%) were found resistant to ceftriaxone and screened for the production of ESBLs by the double disk synergy test (ceftazidime or cefotaxime with\without clavulanic acid). Among these isolates, 48 displayed an ESBL phenotype (1%). Further characterization was performed by Vitek2 system. ESBL phenotype was observed among Shigella sonnei, Shigella flexneri and Shigella boydii but not among Shigella dysenteriae. Molecular analysis detected CTX-M-15 as the prevalent b-lactamase among these strains, and only 3 isolates carried CTX-M-3 gene. PFGE analysis revealed clonal relatedness in some extent within the different serogroups. This is the first report confirming the presence of CTX-M-15 among Shigella in Israel. Though clonal expansion was observed at least in some of the strains, the presence of CTX-M-15 across the different serogroups indicates horizontal transfer. Further testing must be performed in order to determine dissemination of ESBLs among Shigella strains.
