Identification And Characterization Of Class 1 Integron And Salmonella Genomic Island 1 Among Salmonella Strains From Healthy Humans

Antibiotic resistance has become a topic of world-wide concern. The incidence of bacrerial infection and the increasing antibiotic-resistance ability have threatened human's health. As a main one of antibiotic-resistance mechanisms for infectious pathogen the integron and gene cassette system is well researched by specialists in recent years. 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. SJS1-SJS45 were from 2005 and SJS46-SJS58 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.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.