Analysis of infectious factors of Bacteroides fragilis in the compromised host

对脆弱拟杆菌YCH46的基因组分析表明，该菌株通过大量扩增参与多糖结合、降解和运输的基因，获得了非常高的利用多糖的能力。参与表面结构合成的基因的表达，如荚膜多糖和外膜蛋白，是通过启动子的可逆DNA倒置来调节的。我们发现了一个独特的27 kb的基因簇，被命名为sgu（唾液糖缀合物利用）基因簇，该基因簇由13个负责唾液糖缀合物的摄取和降解的基因组成，位于脆弱B.菌株YCH46唾液酸酶基因（nanH3）的下游区域。PCR分析表明，sgu基因簇的几乎所有区域在脆弱芽孢杆菌中都是保守的。转录分析结果将sgu基因簇中的基因根据对不同碳源的表达模式划分为三个调控单元。小鼠腹腔内脓肿形成模型显示，sgu基因簇中的nanH3、estA和estS在脆弱芽孢杆菌的脓肿形成中起重要作用。利用同源菌株的无细胞提取物对质粒DNA进行甲基化处理，可以克服脆弱芽孢杆菌的限制性屏障。

Genomic analysis of Bacteroides fragilis strain YCH46 revealed that this strain has acquired an exceptionally high capability to utilize polysaccharides through extensive amplification of the genes involved in polysaccharide binding, degradation and transport. The expression of genes involving in the synthesis of surface architectures, such as capsular polysaccharides and outer membrane proteins are regulated in an on-off switching through reversible DNA inversions of promoters. We identified a unique 27-kb gene cluster, designated as sgu (sialoglycoconjugate utilization) gene cluster, which consists of 13 genes responsible for the uptake and degradation of sialoglycoconjugates and is located on the region downstream of the sialidase gene (nanH3) in B. fragilis strain YCH46. PCR analysis demonstrated that almost all of the regions in the sgu gene cluster were conserved among the B. fragilis strains tested. Results of transcriptional analysis enabled classification of the genes in the sgu cluster into three regulatory units on the basis of expression patterns in response to various kinds of carbon sources. A mouse model of intraabdominal abscess formation revealed that nanH3, estA, and estS in the sgu gene cluster were thought to play an important role on abscess formation of B. fragilis. The restriction barrier of B. fragilis could be overcome by methylation of plasmid DNA in vitro with a cell-free extract from the homologous strain.