Abstract: To investigate the antimicrobial resistance and the prevalence and tranmission of extended-spectrum β-lactamase CTX-M,a total of 199 samples were collected from geese and the environment from 10 geese farms in Jiangmen and Yangjiang of Guangdong.Isolation and identification of the Escherichia coli (E.coli) were performed by MALDI-TOF-MS.Antibiotic susceptibility test of E.coli strains was characterized using agar dilution method.The bla_CTX-M gene and its genetic environment were detected by PCR analysis among cefotaxime-resistant E.coli isolates.Pulsed-field gel electrophoresis (PFGE),conjugation experiments and plasmid replicon types were conducted to determine the transmission of bla_CTX-M gene.A total of 196 E.coli strains were obtained and antimicrobial susceptibility testing showed that over half of these isolates were resistant to ampicilin,florfenicol,doxycycline and streptomycin.Meanwhile,10% to 25% were third-generation cephalosporins-resistant,and 49 (24.6%) isolates showed resistance to cefotaxime.For comparison,resistance rates of all the tested drugs among E.coli strains from Yangjiang and geese were generally higher than the ones from Jiangmen and environment,respectively,especially for cefotaxime and ceftiofur (P<0.05).Among the cefotaxime-resistant E.coli strains,19 possessed bla_CTX-Mgene,including bla_CTX-M-55(n=17),bla_CTX-M-27(n=1) and bla_CTX-M-65(n=1).All of the bla_CTX-M-carrying strains showed multidrug-resistant phenotype against 5 to 11 kinds of antimicrobial agents.The ISEcp1-bla_CTX-M-55-orf477 structure was found in all of the 17 bla_CTX-M-55-carrying E.coli strains with three different spacer sequences between ISEcp1 and bla_CTX-M-55,while the ISEcp1-bla_CTX-M-27/65-IS903 structure was identified in 2 isolates harboring bla_CTX-M-9G.Ten pulsed-field gel electrophoresis (PFGE) patterns were found in 19 bla_CTX-M-positive E.coli strains,with a dominant profile (47.4%),including isolates from various sources,indicated the clonal transmission of bla_CTX-M-positive isolates in local regions.The bla_CTX-Mgene was successfully transferred among 12 isolates (63.2%) carrying bla_CTX-M,and IncFⅡ plasmid (n=10) was the most prevalent replicon identified among these bla_CTX-M-carrying transconjugants,followed by IncHⅠ2 (n=2).The doxycycline- and florfenicol-resistant phenotypes were co-transferred with bla_CTX-M gene.In conclusion,our study revealed that it was serious for the antimicrobial resistance of E.coli strains in Yangjiang and a relatively high prevalence of bla_CTX-M gene,mainly bla_CTX-M-55 gene,was found.The clonal spread and horizontal transfer via plasmids and ISEcp1 element were major factors contributing to the dissemination of bla_CTX-M gene in the geese farms in Guangdong.It should be further monitored and attached more attention.

Key words: geese farm; Escherichia coli; antimicrobial resistance; bla_CTX-M gene; the third-generation cephalosporins