| Summary: | 碩士 === 高雄醫學大學 === 醫學研究所碩士班 === 93 === Integrons and gene cassettes are known as a horizontal transfer mechanism of acquisition and dissemination of resistance genes among bacteria. They play important roles in the development and variation of multiple antimicrobial resistances of bacteria. Most integrons from clinical isolates belong to class 1 and their location on plasmids and transposons may contribute to the spread of antibiotic resistance genes and even integrons themselves. However, it has not yet been elucidated that the movement ability of integrons themselves and the relations between the movement and structures of integrons.
Several studies have reported that some class 1 integrons are bounded by 25-bp inverted repeats, designated IRi and IRt, at the 5’ and 3’ ends, respectively. Furthermore, tni genes required for transpositions were present downstream the gene cassettes. This suggests integrons may be related to transposons and move by transposition. This study was to characterize the structures of class 1 integrons in clinical isolates of Escherichia coli to understand the structural variety and evolutionary lineage of class 1 integrons.
In this study, inverted repeats IRi and IRt, at the outer boundaries of class 1 integrons, were detected in 88 clinical isolates of E. coli collected in 1993 and 2002 by polymerase chain reaction. Then, the fragments of class 1 integrons between the sul1 gene and the IRt were amplified, followed by sequencing, sequence analysis and restriction fragment length polymorphism to confirm the structures of class 1 integrons.
Of the 88 E. coli isolates, the inverted repeats IRi and IRt were present in 90% and 62.5 % of the isolates, respectively. The sequences neighboring the IRi end of class 1 integrons were more conserved than those neighboring the IRt end. The structures of class 1 integrons downstream the sul1 gene have been characterized in 57 E. coli isolates. Among the four transposition genes (tni A, B, Q, R) contained in a complete tni module, tniA and tniB frequently existed, but the other two genes tniQ and tniR were not found. However, most of the tniA and tniB genes had sequence deletions. Four types of insertion sequences were found associated with class 1 integrons, including IS26, IS6100, IS1326 and IS1353, IS26 were the most prevalent among them. Besides, several resistance genes not contained in cassettes were present downstream the sul1, including a macrolide resistance operon consisting of the genes mphR(A), mrx and mphA, dfr19 for trimethoprim resistance and strA-strB for streptomycin resistance.
The present study reveals that class 1 integrons carry the characteristic features of transposons. In addition, the structural variations of class 1 integrons are still ongoing, especially at 3’ end, thus may affect the mobility of integrons.
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