Analysis of infectious factors of Bacteroides fragilis in the compromised host

对脆弱类杆菌YCH46菌株的基因组分析表明，该菌株通过广泛扩增参与多糖结合、降解和运输的基因，获得了极高的利用多糖的能力。参与表面结构合成的基因，如衣壳多糖和外膜蛋白，通过启动子的可逆DNA倒置，以开关的方式调节表达。我们在脆弱芽孢杆菌YCH46菌株中发现了一个独特的27kb基因簇，命名为SGU(唾液糖共轭利用)基因簇，它由13个负责唾液酸糖结合物摄取和降解的基因组成，位于唾液酸酶基因(NanH3)下游区域。聚合酶链式反应分析表明，在所测试的脆弱芽孢杆菌菌株中，sgu基因簇中几乎所有的区域都是保守的。转录分析的结果使我们能够根据不同碳源的表达模式，将SGU簇中的基因分为三个调控单位。一种小鼠腹内脓肿形成的模型显示，SGU基因簇中的nanH3、ESTA和EST被认为在脆弱芽孢杆菌的脓肿形成中起重要作用。用同源菌株的无细胞提取物体外甲基化质粒DNA，可以克服脆弱芽孢杆菌的限制障碍。

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.