Prevalence and antimicrobial resistance of Salmonella spp. in retail whole chickens in Beijing City

ObjectiveTo investigate the antimicrobial resistance characteristics of Salmonella spp. isolated from retail whole chickens in Beijing， analyze the prevalence and genomic characteristics of cefotaxime （CTX）-resistant strains.MethodsSalmonella strains were isolated from retail whole chickens in urban Beijing， and were subjected to antimicrobial susceptibility testing. CTX-resistant strains were selected for serum-hemagglutination analysis and whole-genome sequencing， and bioinformatics methods were used to analyze the genomic characteristics of the strains.ResultsThe contamination rate was 67.8% （67/87）. A total of 180 Salmonella strains were isolated from 87 samples， of which 98.9% （178/180） of the strains showed resistance to at least one antibiotic， with nalidixic acid （86.1%， 155/180） being the most resistant one， and 69.4% （125/180） of the strains were multi-drug resistant （MDR）. Genomic analysis of 37 CTX-resistant strains identified 6 distinct serotypes and 6 sequence types （ST）， with Kentucky/ST198 （16/37， 43.2%） and Indiana/ST17 （13/37， 35.1%） being the predominant serotype/ST combinations. Core genome multilocus sequence typing （cgMLST） revealed high genetic diversity， which correlated well with serotypes and ST types. The predominant ESBL-encoding gene was blaCTX-M-55（51.4%， 19/37）. Quinolones resistance was primarily mediated by aac（6'）-Ⅰb-cr （29.7%， 11/37） and chromosomal point mutations， among them 75.7% of the strains notably harbored four mutations simultaneously at sites in GyrA （S83F， D87N/D87G） and ParC （T57S， S80I/S80R）. Additionally， three imipenem- resistant Salmonella Indiana strains harbored the blaNDM-5 gene.ConclusionSalmonella contamination in retail whole chickens in urban Beijing is highly widespread， accompanied by a high prevalence of antimicrobial resistance. Most CTX-resistant strains harbored multiple resistant mechanisms. Notably， the detection of carbapenem resistance in MDR Salmonella Indiana poses new risks and challenges for public health and clinical infection treatment.