Multilocus Sequence Typing And Antimicrobial Resistance Of Salmonella Isolates Of Different Origins

Salmonella is one of the most important foodborne pathogens causing fever, enteritis, septicemia in humans and animals. Salmonellosis is one of the most widespread foodborne bacterial illnesses in humans and animals. Usually, it is the top cause of food poisoning caused by bacteria throughout the world. Salmonella contamination in the food chain has a significant economic impact due to the costs of medical care and lost productivity. Human salmonellosis is most often attributed to the consumption of contaminated foods such as poultry, beef, pork, eggs, milk, seafood, and fresh produce. Direct contact with infected animals has also been associated with salmonellosis. In recent years, antimicrobial resistant Salmonella spp are increasing due to the extensive use or inappropriate use of antimicrobial agents in both humans and food animals. The increasingly drug resistance of Salmonella has become a significant issue in public health throughout the world.There are a number of ways in which isolates of Salmonella may be distinguished from each other. Isolates can be separated based on phenotypic characterization, such as antibiotic resistance patterns and serotyping, or by genotypic methods, such as molecular typing techniques. Moreover, multilocus sequence typing basing on the sequence of the DNA in microbial identification has more advantages on the typing ability, resolution, repeatability, comparability and sensitivity, due to the traditional biochemical characterization and serotyping having not been effective for homology analysis. A study was conducted to investigate the status and prevalence of Salmonella in feeds, poulty, pork and chicken meat. Multilocus sequence typing was conducted to deterimine genetic lineages and the source of Salmonella isolates. Meanwhile, antimicrobial resistance of Salmonella isolates of different sources was included in this study.A total of182strains of Salmonella were isolated from the1837samples from2010to2011, and the detection rate of Salmonella was9.9%. The detection rate of chicken meat was high up to26%.28serotypes were identified among the182strains, the main serotype varies due to the source of samples, the main serotype of strains isolated from pork is SalmonellaDerby, the main serotype of strains isolated from poulty and chicken meat is Salmonella Enteritidis, and the main serotype of strains isolated from feeds is Salmonella Senftenberg.According to the previously studies published,7housekeeping genes including aroC, dnaN, hemD, hisD, pure, sucA and thrA were selected and their primers for amplification and sequencing were synthesized.182strains of Salmonella from four different sources and73strains of human isolates were analyzed using MLST method. The genotyping results of MLST showed that there were37STs in these isolates and5new STs were revealed. The37STs were divided into one clonal complex, two doublets, a pair of DLV and others singletons.The clonal complex comprised five STs, including ST-19,ST-34,ST-99,ST-725and NEW4, the ancestor of this comlex is ST-19. All255strains of Salmonella were divided into36serotypes and37STs, it could conclude that the discriminatory power of MLST was close to that of the serotyping method, most serotypes obtained in this study could correlate with one ST.All of255Salmonella isolates were tested for their antimicrobial resistance to21antibiotics, the result revealed that the highest percentage of resistance to the antibiotics was nalidixic acid(41.6%), followed by tetracycline(41.2%). Resistance to cephalosporins and quinolones (except nalidixic acid) is low, all below5%. Resistance to other antibiotics were various. One hundred and sixty five isolates (64.7%) were resistant to at least two antibiotics. Different sources of Salmonella isolates had different antimicrobial resistance ratio, strains from poulty showed the highest antimicrobial resistance ratio(100%), followed by strains from chicken meat(75%) and pork(67.8%), and then strains from humans, antimicrobial resistance ratio was60.3%. Salmonella isolates from chicken meat commonly displayed multidrug resistance (MDR) to>5antimicrobials, with the ratio of44.7%, moreover, two strains of Salmonella isolates showed resistance to all of the antibiotics tested. The antimicrobial resistances of isolates with different serotypes were diverse, in this study, Salmonella Enteritidis isolates were found with the highest antimicrobial resistance ratio, followed by Salmonella Typhimurium, Salmonella Meleagridis, Salmonella Muenster and Salmonella Derby, while Salmonella Agona with the lowest antimicrobial resistance ratio.