| Integron, a genetic element of gene capture and expression in Gram-negative bacteria, has been showed to play an important role in acquired antibiotic resistance of bacteria. Integron consists of an integrase gene (intI), a integrase specific recombination site (attI), and gene cassettes with varied number. One gene cassette usually contains a open reading frame encoding antibiotic resistance and a specific recombination site called attC (or 59-base element). Gene cassette could be integrated into an integron with recombination between attI and attC sites by integrase (IntI) , or excised from an integron and become a free circle molecule.More Shigella clinic isolates recently become resistant to the antibiotics used commonly in the past decades, and some strains even resist to multiple-drugs. Past studies showed that conjugative resistance plasmids could confer resistance to many antibiotics among Shigella strains. Lately several investigations reported class 1 and 2 integron in Shigella strains and their association to antibiotic resistance. In this study, we examined class 1 and 2 integron in Shigella sonnei and Shigella flexneri isolates from Hangzhou, China, to investigate their distribution and determine their gene cassette arrays and their location in genome. A atypical class 1 integron without 3' conserved region was foundto prevail among Shigella flexneri isolates, and the excision models of it's gene cassette were charactered by inverse PCR. Moreover the roles of gene cassettes in the atypical class 1 integron and class2 integron to confer resistance were discussed. Among 64 isolates of Shigella sonnei and Shigella flexneri employed this study, 59 were isolated from sporadic patients, 4 from patients of two outbreak events, and 1 from well water sample involved in one outbreak event, respectively, during 1998 to 2002.1. Characteristic of typical class 1 integronTypical class 1 integron includes 5' conserved region (intI1), varied region (gene cassettes), and 3' conserved region (qacE 1 and sull). 6 out of 64 isolates of Shigella sonnei and Shigella flexneri were positive with all markers of the three region (9.4%). All these PCR products of varied regions were sequenced. A aadA2 cassette, encoding aminoglycoside adenylyltransferase, was found in 2 isolates of Shigella sonnei, and a two cassettes array of dfrA1, encoding dihydrofolate reductase, and aadAla in one isolate of Shigella sonnei, and another two cassettes array of dfrA1 7 and aadA5 in two isolates of Shigella sonnei and Shigella flexneri y-variant each, and a dfrV cassette in one isolate of Shigella flexneri 6, respectively. In two of these isolates with typical class 1 integron, it was showed that the integrons were both harbored in plasmids after intll Southenbloting both on their genome DNA and plasmid DNA.2. Characteristic of atypical class 1 integronDuring the detection of typical class 1 integron using PCR, we found that among 30 isolates, most of which belonged to Shigella flexneri, all were positive for intll but negative for their varied regions, and most of them were also negative for 3' conserved region except three isolates positive. Based on the result, it is hypothesized that there might be a atypical class 1 integron without 3' conserved region in these isolates. Their intlls were located on choromosome after Southenbloting with intllprobe on genome and plasmid DNAs extracted from 4 of these isolates. One 3.4 kb BamH I and Pst Irestricted fragment with intll positive was cloned from Shigella flexneri 2a isolate C101-1 and sequenced. Sequence analysis showed the fragment with a array of intll, attI site, two cassettes of blaoxA-30 and aadA1, encoding oxacillinase and aminoglycoside adenylyltransferase respectively, and partial sequence of IS1, replacing the 3' conserved region usually found in typical class 1 integron. The gene cassettes regions were examined in all these 30 isolates by PCR with a pair of primers annealing to the sequences of intll and IS1, and PCR products with same length were amplif...
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