FimH上的胺基酸變異可能是造成豬霍亂沙門氏菌 不表現第一型線毛的原因

Abstract

線毛是沙門氏菌表面毛髮樣的蛋白結構物，在細菌感染宿主時扮演了吸附宿主細胞如M細胞表面的重要角色。 沙門氏菌擁有數個線毛基因組，而最常見的是表現第一型線毛的fim基因組。第一型線毛可以吸附不同細胞，例如紅血球、白血球、呼吸道上皮細胞、腸道上皮細胞以及酵母菌等。根據研究，第一型線毛與細菌的致病機制有關，而且80%的沙門氏菌都會表現第一型線毛，表示第一型線毛可能在細菌感染的過程中扮演一個重要的角色。豬隻的沙門氏菌症由豬霍亂沙門氏菌以及鼠傷寒沙門氏菌兩種血清型，其中豬隻感染豬霍亂沙門氏菌會導致下痢、敗血症、四肢肢端發紺、泛發性血管內血液凝固症、壞死性肝炎、肉芽腫性腦炎、間質性肺炎、壞死性腸炎等症狀等。而鼠傷寒沙門氏菌主要造成豬隻的胃腸炎。感染宿主廣泛的鼠傷寒沙門氏菌分離株絕大部分都會產生第一型線毛，但是根據本實驗室目前的研究顯示大部分(97%)豬隻分離的細菌不表現第一型線毛。呈現如此的線毛表線型或許在演化上與豬霍亂沙門氏菌感染易造成全身性感染有關。本實驗把豬霍亂沙門氏菌第一型線毛表現株的線毛基因組 fim gene cluster，依照功能做成分段的重組基因，轉形回不表現第一型線毛之豬霍亂沙門氏菌，分析其不表現線毛的原因為何? 對於可能影響的因子做深入的探討。 在S. Choleraesuis 中 fim gene cluster並沒有發現轉錄以及轉譯的功能缺損，本實以分子選殖 (molecular clonin)的方式將重組質體以電穿孔的方式轉型回不表現第一型線毛之豬霍亂沙門氏菌，完成 fimH 6個點突變之後，結果在fimH G63V恢復第一型線毛的表現。推測是主要結構蛋白 FimH 可能有出現胺基酸的變異，造成其無法送出細胞表面外以至於第一型線毛無法生成，可能是該血清型在演化過程中的結果。

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.