| Summary: | 碩士 === 國立臺灣大學 === 獸醫學研究所 === 103 === Abstract
Fimbriae are surface appendages of Salmonella and play an important role in infecting host cell by initially adhering to surface of host cells, such as M cell. There are several fimbrial gene clusters within the genome of Salmonella, while type 1 fimbriae encoded by fim are the most commonly observed fimbrial appendages among these. Type 1 fimbriae mediates adherence to a variety of cells such as erythrocytes, leukocytes, respiratory cells, intestinal cells, and fungal cells. Moreover, type 1 fimbriae has been documented to be associated with virulence. In fact, 80% of the Salmonella isolates express type 1 fimbriae may suggest this fimbrial type play an important role at some stage in its pathogenesis. Infections in swine are always associated with either Salmonella enterica serovar Choleraesuis (S. Choleraesuis) or Salmonella enterica serovar Typhimurium (S. Typhimurium). Pigs infected with S. Choleraesuis usually exhibit diarrhea, disseminated intravascular coagulation (DIC), necrotic hepatitis, granulomatous encephalitis, interstitial pneumonia, and necrotic/ ulcerative colitis, while salmonellosis manifested as enterocolitis is primarily ascribed to S. Typhimurium. In contrast to S. Typhimurium, with most of the isolates produce type 1 fimbriae and is a serovar with a broad host range, previous examination in our laboratory has revealed that almost 97% of S. Choleraesuis isolates did not exhibit type 1 fimbriae in vitro. It is possible that the evolutionary pressure encountered by S. Choleraesuis during specific host swine adaption may contribute to such phenotype. This phenotype may somehow benefit S. Choleraesuis to sustain systemically. There was no transcriptional defect within the fim genes. The recombinant plasmids possessing the insert DNA with either structural or regulatory elements of fim were constructed and transformed into the non-type 1 fimbrial expressing S. Choleraesuis. Our results revealed that fimH gene may be responsible for the expression of type 1 fimbriae. There were 6 amino acid variations between the FimH peptide of type 1 fimbrial expressing S. Choleraesuis and that of non-type 1 fimbrial expressing ones. Site-directed mutagenesis was therefore performed on such positions of fimH and it was demonstrated that changing amino acid from glycine to valine at the position of 63 could confer a non-type 1 fimbrial expressing S. Choleraesuis to produce fimbrial appendages. Amino acid variation of FimH may deter its polypeptide to properly interact with other Fim subunit to assemble an intact fimbrial shaft. The correlation between such specific amino acid variations within the fimH and pathogenesis of S. Choleraesuis warrants further investigations.
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