Abstract：
Bacteroides fragilis is an obligately anaerobic Gram-negative bacterium and a major colonizer of the human large colon where Bacteroides is a predominant genus. During the growth of an individual clonal population, an astonishing number of reversible DNA inversion events occur, driving within-strain diversity. Additionally, the B. fragilis pan-genome contains a large pool of diverse polysaccharide biosynthesis loci, DNA restriction/modification systems and polysaccharide utilization loci, which generates remarkable between-strain diversity. Diversity clearly contributes to the success of B. fragilis within its normal habitat of the gastrointestinal (GI) tract and during infection in the extra-intestinal host environment. Within the GI tract, B. fragilis is usually symbiotic, for example providing localized nutrients for the gut epithelium, but B. fragilis within the GI tract may not always be benign. Metalloprotease toxin production is strongly associated with colorectal cancer. B. fragilis is unique amongst bacteria; some strains export a protein >99 % structurally similar to human ubiquitin and antigenically cross-reactive, which suggests a link to autoimmune diseases. B. fragilis is not a primary invasive enteric pathogen; however, if colonic contents contaminate the extra-intestinal host environment, it successfully adapts to this new habitat and causes infection; classically peritoneal infection arising from rupture of an inflamed appendix or GI surgery, which if untreated, can progress to bacteraemia and death. In this review selected aspects of B. fragilis adaptation to the different habitats of the GI tract and the extra-intestinal host environment are considered, along with the considerable challenges faced when studying this highly variable bacterium.
摘要：
脆弱拟杆菌是一种专性厌氧革兰氏阴性细菌，是人类大结肠的主要定殖者，其中拟杆菌是主要属。在个体克隆种群的生长过程中，发生了惊人数量的可逆DNA倒置事件，驱动菌株内多样性。此外,脆弱芽孢杆菌的泛基因组包含大量不同的多糖生物合成基因座，DNA限制/修饰系统和多糖利用基因座，产生显著的菌株间多样性。多样性显然有助于脆弱芽孢杆菌在其胃肠道（GI）正常栖息地内以及在肠外宿主环境中感染期间的成功。在胃肠道内,脆弱芽孢杆菌通常是共生的，例如为肠道上皮提供局部营养，但是胃肠道内的脆弱芽孢杆菌可能并不总是良性的。金属蛋白酶毒素的产生与结直肠癌密切相关。B.fragilis是独特的细菌;一些菌株输出的蛋白质>99％结构相似的人泛素和抗原性交叉反应，这表明与自身免疫性疾病有关。脆弱芽孢杆菌不是主要的侵入性肠道病原体；然而，如果结肠内容物污染肠外宿主环境，它成功地适应了这个新的栖息地并引起感染;典型的腹膜感染是由发炎的阑尾或胃肠道手术破裂引起的，如果不治疗，会进展为菌血症和死亡。在这篇综述中，考虑了脆弱芽孢杆菌对胃肠道不同栖息地和肠外宿主环境的适应的选定方面，以及研究这种高度可变的细菌时面临的相当大的挑战。
