Active Surveillance On Salmonella. Spp In Food And Study On A New Mechanisam Of Resistance Gene Dissemination

ObjectiveTo find out the contamination level and characteristics of Salmonella.spp in 7 kinds of food in the areas of Chinese Foodborne Pathogens Surveillance Network in 2002. To set up the basic database of the isolates' serotypes and antibiotic resistance patterns and to achieve active surveillance on microbiological risk in food. To find out the presence of class I integron and cassette in Salmonella.spp in China and the relation between cassette-integron system and resistance patterns. And to illuminate the horizontal transfer characteristic of resistance gene among bacteria.MethodsIdentify the received isolates by standard methods and serotype the Salmonella isolates and take the antimicrobial susceptibility testing for 14 antibiotics. Detect class I integrons and cassettes in multiresistant Salmonella isolates and draw structural organizations of gene cassettes in integron by using PCR and DNA sequencing. Detect the horizontal transfer of integron among bacteria by plamid conjugation experiments.Results196 strains of Salmonella were isolated from 5521 food samples of 7 kinds, which were randomly sampled from the areas of Chinese Foodborne Pathogens Surveillance Network. Most isolates came from raw meat. S.derby, S.agona and S.enteritidis were the top three serotypes. And 118 isolates showed resistance to at least one of the 14 antibiotics tested and 47.5 percent of all isolates were resistant to more than three antibiotics. The antibiotics with the top three resistant rates were Streptomycin(46.5%), Tetracycline (33.8%) and Sulfonamides (21.4%). All the Salmonella isolates were susceptible to Amikacin and Ceftriaxone.Class I integrons, including 7 kinds of integron profiles, were identified among 25 of all 71 multiresistant Salmonella isolates. Integron-positive srains contained one or more cassettes of 7 cassettes tested, but some of them were "silent gene" which didn't encode resistance. By using primers of the inserted resistance genes in combination with those pf the conserved segments, we determined the content and order of cassettes in the integrons. In plamid conjugation experiments, class I integrons were transferred to transconjugants from 9 donor strains, followed by some of transferred resistance patterns.ConclusionWe obtained the epidemiological data of Salmonella from 7 kinds of food on the base of Chinese Foodborne Pathogens Surveillance Network in 2002, and build resistance baseline of foodborne Salmonella which revealed severe antibiotic resistance status in China. Through the study on cassette-integron system in foodborne multiresistant Salmonella in China, we obtained the dataincluding the relations between presence of integrons and resistance patterns, the distribution of resistance genes on integrons, the horizontal transfer characteristic of integrons. These findings provided sound data for continuous active surveillance on Salmonella in China and for comparative research. The results of this research is also helpful to deeper research on cassette-integron system and to establishment of reasonable measures for preventing and controlling foodborne diseases and bacteria antibiotic resistance.