Abstract
Salmonella spp. is one of the most important food-borne pathogens causing digestive tract and invasive infections in both humans and animals. Extended-spectrum β-lactamases (ESBLs) especially the CTX-M-type ESBLs are increasingly being reported worldwide and in China. These studies seldom focused on Salmonella isolates from food-producing animals. The aim of this study was to characterize the antimicrobial resistance profiles, serotypes and ESBLs and in particular, CTX-M producing Salmonella isolates from chickens and pigs in China. Salmonella isolates were identified by API20E system and polymerase chain reaction (PCR) assay; serotypes were determined using slide agglutination with hyperimmune sera; antimicrobial susceptibility was tested using the ager dilution method; the prevalence of ESBLs and PMQR genes were screened by PCR; CTX-M-producing isolates were further characterized by conjugation along with genetic relatedness and plasmid replicon type. In total, 159 Salmonella strains were identified, among which 95 strains were Salmonella enterica serovar Typhimurium, 63 strains were S. enterica serovar Indiana, and 1 strain was S. enterica serovar Enteritidis. All of these isolates presented multi-drug resistant phenotypes. Forty-five isolates carried blaCTX-M genes, the most common subtype was CTX-M-27(34), followed by CTX-M-65(7) and CTX-M-14(4). Most blaCTX-M genes were transmitted by non-typeable or IncN/IncFIB/IncP/IncA/C/IncHI2 plasmids with sizes ranging from 80 to 280 kb. In particular, all the 14 non-typeable plasmids were carrying blaCTX-M-27 gene and had a similar size. PFGE profiles indicated that CTX-M-positive isolates were clonally related among the same serotype, whilst the isolates of different serotypes were genetically divergent. This suggested that both clonal spread of resistant strains and horizontal transmission of the resistance plasmids contributed to the dissemination of blaCTX-M-9G-positive Salmonella isolates. The presence and spread of CTX-M, especially the CTX-M-27 in S. enterica serovars Typhimurium and Indiana from food-producing animals poses a potential threat for public health. Control strategies to limit the dissemination of these strains through the food chain are necessary.
Highlights
The emergence of antibiotic-resistant bacteria has become a serious challenge in human and veterinary medicine globally and poses a serious public health threat
Three serotypes were identified in the 159 Salmonella isolates and this accounted for 95 strains of S. enterica serovar Typhimurium, 63 strains of S. enterica serovar Indiana, and 1 strain of S. enterica serovar Enteritidis
We examined Salmonella isolates recovered from food-producing animals to determine their serotypes, antimicrobial susceptibility phenotypes, and genotypes
Summary
The emergence of antibiotic-resistant bacteria has become a serious challenge in human and veterinary medicine globally and poses a serious public health threat. Non-typhoidal Salmonella (NTS) are a leading cause of bacterial diarrhea worldwide and one of the most important foodborne pathogens. The majorities of human infections by NTS are associated with food product consumption of meat, eggs, milk, seafood, and other fresh products derived from animals (Foley and Lynne, 2008). In the animal husbandry industry, antibiotics are used both for therapy and as growth promoters. Animals are always a large repository of resistant bacteria. Especially poultry and pigs have been shown to be major environmental reservoirs of food-borne NTS (Vo et al, 2006)
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