Protein secretion pathways in the phylum Bacteroidetes

类杆菌属的成员是迄今为止人类肠道微生物群中最丰富的革兰氏阴性细菌。最近，我们用质谱仪检测了实验室条件下生长的类杆菌属中最常见的成员之一--脆弱杆菌的外膜蛋白质组和分泌组。虽然我们没有发现任何新的分泌途径，但我们发现这种生物使用的分泌策略与大肠杆菌等变形杆菌非常不同。脆弱芽孢杆菌缺乏在变形杆菌中广泛存在的许多途径(例如，II型、III型和IV型途径)，但同时产生多种I型分泌系统。脆弱芽孢杆菌与变形杆菌也有很大的不同，因为它产生大量暴露在细胞表面的脂蛋白。I型途径的底物和脂蛋白通过OM运输的机制尚不清楚。有趣的是，分泌组中发现的许多蛋白质与Genbank数据库中的蛋白质缺乏显著的同源性，推测它们具有新的功能。 我们的一些实验涉及使用产肠毒素(ETBF)的脆弱芽孢杆菌，这些菌株的特征是分泌一种名为脆性溶素(BFT)的毒素。尽管ETBF菌株会导致腹泻，并与结直肠癌有关，但它们的特征尚未很好。为了方便我们的研究，我们最近获得了四个ETBF毒株的完整基因组序列。序列分析表明，这些菌株在基因组水平上表现出相当大的变异。只有少量主要位于BFT致病岛(BFT PAI)和侧翼CTn86接合转座子的基因在所有四个菌株和第五个基因组已测序的菌株中保守。有趣的是，系统发育分析强烈表明，BFT PAI在进化过程中被非产毒(NTBF)菌株多次获得。在表型水平上，我们发现ETBF菌株比NTBF菌株NCTC 9343更不适合，并且对它产生的生长抑制蛋白敏感。ETBF菌株也比NTBF菌株更容易形成生物膜，这可能会促进肿瘤的形成。虽然ETBF菌株的基因组多样性增加了它们致病性不同的可能性，但我们的实验结果也表明，它们有共同的特性，这些特性是由非通用遗传元件的不同组合赋予的。

Members of the genus Bacteroides are by far the most abundant Gram-negative bacteria in the human gut microbiome. Recently we used mass spectrometry to examine the outer membrane (OM) proteome and secretome of one of the most commonly studied members of the Bacteroides genus, B. fragilis, grown under laboratory conditions. Although we did not identify any novel secretion pathways, we found that this organism uses a very different range of secretion strategies than Proteobacteria such as E. coli. B. fragilis lacks many of the pathways that are widespread among the Protebacteria (e.g., the type II, III and IV pathways) but produces multiple type I secretion systems simultaneously. B. fragilis also differs dramatically from Proteobacteria in that it produces a large number of lipoproteins that are exposed on the cell surface. The substrates of the type I pathways and the mechanism by which lipoproteins are transported across the OM are unknown. Interestingly, many of the proteins found in the secretome lack significant homology to proteins that are in the Genbank database and presumably have novel functions. Some of our experiments have involved the use of enterotoxigenic (ETBF) strains of B. fragilis that are distinguished by the secretion of a toxin called fragilysin (BFT). Although ETBF strains cause diarrhea and have been associated with colorectal cancer, they have not been well characterized. To facilitate our studies, we recently obtained complete genome sequences of four ETBF strains. The sequences show that these strains exhibit considerable variation at the genomic level. Only a small number of genes that are located primarily in the BFT pathogenicity island (BFT PAI) and the flanking CTn86 conjugative transposon are conserved in all four strains and a fifth strain whose genome was previously sequenced. Interestingly, phylogenetic analysis strongly suggests that the BFT PAI was acquired by non-toxigenic (NTBF) strains multiple times during the course of evolution. At the phenotypic level, we found that the ETBF strains were less fit than the NTBF strain NCTC 9343 and were susceptible to a growth-inhibitory protein that it produces. The ETBF strains also showed a greater tendency to form biofilms, which may promote tumor formation, than NTBF strains. Although the genomic diversity of ETBF strains raises the possibility that they vary in their pathogenicity, our experimental results also suggest that they share common properties that are conferred by different combinations of non-universal genetic elements.