| Summary: | 碩士 === 國立嘉義大學 === 動物科學系碩士班 === 92 === ABSTRACT
There are over 2,500 serotypes of Salmonella spp, but only eight serotypes of non-typhoid Salmonella harboring virulence plasmids. S. Choleraesuis is a common pathogen in swine and causes serious system infection in human. In recent years, multi drug resistant Salmonella spp. had frequently been reported because antibiotics have been abused in therapy of bacterial infection and animal feed. The purpose of this study concentrated on S. Choleraesuis in two directions. First, using the method of suppression subtractive hybridization to determine the genomic DNA variation between S. Choleraesuis and S. Typhimurium, we identified specific genes for S. Choleraesuis and S. Typhimurium respectively. Currently, there were 25 specific genes in S. Typhimurium but absent in S. Choleraesuis. The functions of these genes included cell metabolism, gene repair and virulence. In contrast, there were 12 specific genes in S. Choleraesuis but absent in S. Typhimurium. 66.6% of these genes belonged to transposon and prophage. This suggests that the evolution of S. Choleraesuis involves of horizontal gene transfers. Another project was the expression of resistance gene (ampC) for cetriaxone. This resistance gene structure carried by a transposon-like mobile element, which contains four genes tnpA-ampC-blc-sugE, was located on a 138-kb plasmid of S. Choleraesuis SC-B67. The result showed that either S. Choleraesuis or E. coli harbored 138 kb plasmid, neither strains showed inducible resistance and their MICs (minimal inhibitory concentration) were slightly different (8 µg/ml for S. Choleraesuis and 16 µg/ml for E. coli). When recombinant plasmid DNA containing DNA fragment of ampC or tnpA-ampC gene structure was transferred into competent E. coli, the resistance was inducible. Different gene structure and mutation also effected the expression of ampC, MIC of E. coli, containing tnpA-ampC gene structure, increased up to 128 µg/ml. MICs of transformed E. coli with ampC gene structure differed among three strains. Two of them, the MIC were 16 µg/ml and 0.064 µg/ml respectively and then induced up to ＞256 µg/ml. The MIC of remaining one, a non-inducible strain, was decreased to 0.023 µg/ml. After nucleotide and amino acid sequence analysis, two former strains didn’t have any mutation. But the last strain had a point deletion, resulting in a frameshift mutation, on the ampC gene. Expression of ampC may be regulated by different gene structure and mutation of the functional gene. Due to variation in resistance pattern correlated to gene structure, therefore, we hypothesize that other two genes, blc and sugE, and some genes in the chromosome of E. coli can play a role in regulation of such induction.
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