| Objectives: To identify and characterize class 1 integron and salmonella genomic island 1 (SGI1) in Salmonella strains from healthy humans,and to analyze the relationship between integron or SGI1 and antibiotic resistance;Methods: Fifty-eight Salmonella strains isolated from healthy humans were tested for their susceptibility to 15 common antibiotics with disk diffusion method? according to Clinical and Laboratory Standard Institute (CLSI) criteria. S1-S45 were from 2005 and S46-S58 were from 2007. The presence of class 1 integrons were examined by PCR, the intI1 positive strains were tested for resistance gene cassettes and 3'segments, then the positive amplicons were sequenced and the sequences were blasted on NCBI (http://www.ncbi.nlm.nih.gov ). Plasmid conjugation with the mating method was used to detect the location of class 1 integrons and whether they could transfer to other bacterium. Sau-PCR was used to determine and characterize the possible genetic relationships of these Salmonella strains, then the results were analyzed by Quantity One software, and dendrograms were generated by the unweighted pair group method using arithmetic averages (UPGMA). The intI1 positive strains were tested for the presence of SGI1, and the positive amplicons were sequenced and analyzed. To detect the mobilization of the SGI1, the extrachromosome circle forms and the excision were examined, as well as conjugation and natural gene transformation.Results: 1 Of the 58 Salmonella strains, 51 strains (87.9%) were resistant to at least one of the antibiotics, and the others were susceptible to all the antibiotics. Two strains S13 and S49, both of which were S. Typhimurium, showed multidrug resistance; they were resistant to nine and six antibiotics. The most frequent resistance rate observed was to STR (82.8%), followed by TET (13.8%). All of the strains were sensitive to CEF, CRP, FOX, and AMK. In general, the resistance rate to antibiotics tested with the 2007 stains was higher than that of the 2005 strains. For example, the resistance to NAL increased by nearly 14 times, from 2.2% in 2005 to 30.8% in 2007. 2 Five of the 58 Salmonella strains were positive for class 1 integrons, they were S13 (S. Typhimurium), S27 (S. Westhampron), S28 (S. Newlands), S49 (S. Typhimurium), and S56 (S. Derby). Among these strains, S56 was obtained from a worker in food fields. The former three strains were from 2005 and the latter two from 2007,respectively. Three strains (S13, S49, and S56) were positive for cassette arrays of different lengths, with the amplicons of 2 000bp,1 000 and 1 200bp,1 000bp, whereas two strains (S27 and S28) had no products. Sequence analysis revealed that the 1 000bp harbored gene cassette aadA2, conferring resistance to spectinomycin and treptomycin, the 1 200bp amplicon harbored gene cassette blaPSE-1,conferring resistance to ampicillin, and the amplicon of 2 000bp harbored cassette array dfrA12-orfF-aadA2, conferring resistance to trimethoprim and streptomycin. The carriage of cassette arrays was corresponding to the resistance phenotype. 3 All the class 1 integrons detected were on plasmid, and could transfer to other bacterium. The transfer frequencies were in the range of 10?5 to 10?6 per recipient. 4 Sau-PCR yielded a complex banding pattern, and there were 12 clusters (named A-L) referring to 50% similarity, even the strains with the same serotype might be classified to different groups. The five intI1 gene-positive strains were subjected to four different groups: S27, S28, and S56 were subjected to three different groups: B, C, and A, respectively, Two MDR S. Typhimurium strains S13 and S49, both of which were positive of class 1 integrons, showed 100% similarity. There was no direct correlation between integron-positive and integron-negative strains. 5 SGI1 was detected in S49, with the aadA2, floR (coding for chloramphenicol and florfenicol resistance), tet(G) (coding for tetracycline resistance protein), and blaPSE-1 genes in its multidrug-resistant region, and it was located between thdF and a retronphage (int2). Some other gene clusters located in SGI1 were also detected, including int (coding for integrase), int-xis (coding for integrase and excisionase), S023 (coding for putative helicase), IS6100 (insertion sequence IS6100), IS6100-S044 (insertion sequence of IS6100 and S044). 6 Both of the extrachromosome circle form of SGI1 and the excision of SGI1 were detected. The SGI1 could not transfer to other bacteria with conjugation due to the absence of help plasmid R55, but they could transfer through natural gene transformation.Conclusion: 1 The antibiotic resistance rate of Salmonella strains from healthy humans were lower, so did the positive rate of class 1 integrons, which might because the lower antibiotic pressure. Both of the antibiotic resistance rate and the positive rate of class 1 integrons were increased likely in 2007 compared with 2005. As the healthy workers might have had little opportunity to come in contact with antibiotics, the resistant genes or class 1 integrons might have been transferred from other bacterial strains through the food chain. The resistance gene cassettes in class 1 integrons could transfer to other bacteria, resulting in the horizontally transference of antibiotic resistance. There was no direct correlation between the integron-positive and integron-negative strains. 2 To our knowledge, this is the first report on SGI1 in strains from healthy humans, and could transfer to other bacteria through natural gene transformation, leading to the transfer of multi-drug resistance.In a word, the presence of class 1 integrons and SGI1 in Salmonella strains from healthy humans suggests that widespread surveillance of antibiotic resistance should be conducted at the gene level and the mechanisms of antibiotic resistance should be studied, as these strains live symbiotically with other intestinal bacteria and could survive in vivo and transfer the resistance genes to other bacteria, and cause outbreaks when possible.
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