Norwegian School of Veterinary Science, Department of Food Safety and Infection Biology, PO Box 8146 Dep, N-0033 Oslo, Norway.
BMC Microbiol. 2010 Nov 30;10:304. doi: 10.1186/1471-2180-10-304.
Bacillus cereus and the closely related Bacillus thuringiensis are Gram positive opportunistic pathogens that may cause food poisoning, and the three secreted pore-forming cytotoxins Hbl, Nhe and CytK have been implicated as the causative agents of diarrhoeal disease. It has been proposed that the Hbl toxin is secreted using the flagellar export apparatus (FEA) despite the presence of Sec-type signal peptides. As protein secretion is of key importance in virulence of a microorganism, the mechanisms by which these toxins are secreted were further investigated.
Sec-type signal peptides were identified in all toxin components, and secretion of Hbl component B was shown to be dependent on an intact Sec-type signal peptide sequence. Further indication that secretion of Hbl, Nhe and CytK is dependent on the Sec translocation pathway, the main pathway on which bacterial secretion relies, was suggested by the observed intracellular accumulation and reduced secretion of the toxins in cultures supplemented with the SecA inhibitor sodium azide. Although a FEA deficient strain (a flhA mutant) showed reduced toxin expression and reduced cytotoxicity, it readily secreted overexpressed Hbl B, showing that the FEA is not required for Hbl secretion. Thus, the concurrent lack of flagella and reduced toxin secretion in the FEA deficient strain may point towards the presence of a regulatory link between motility and virulence genes, rather than FEA-dependent toxin secretion.
The Hbl, Nhe and CytK toxins appear to be secreted using the Sec pathway, and the reduced Hbl expression of a FEA deficient strain was shown not to be due to a secretion defect.
蜡状芽孢杆菌和密切相关的苏云金芽孢杆菌是革兰氏阳性机会致病菌，可能导致食物中毒，而三种分泌的孔形成细胞毒素 Hbl、Nhe 和 CytK 被认为是腹泻病的致病因子。有人提出，尽管存在 Sec 型信号肽，但 Hbl 毒素是使用鞭毛出口装置 (FEA) 分泌的。由于蛋白质分泌对微生物的毒力至关重要，因此进一步研究了这些毒素的分泌机制。
在所有毒素成分中都鉴定出了 Sec 型信号肽，并且 Hbl 成分 B 的分泌被证明依赖于完整的 Sec 型信号肽序列。进一步表明 Hbl、Nhe 和 CytK 的分泌依赖于 Sec 易位途径，这是细菌分泌所依赖的主要途径，这是通过观察到在添加 SecA 抑制剂叠氮化钠的培养物中毒素的细胞内积累和分泌减少而得到的。尽管 FEA 缺陷菌株（flhA 突变体）显示出毒素表达减少和细胞毒性降低，但它很容易分泌过表达的 Hbl B，表明 FEA 不是 Hbl 分泌所必需的。因此，FEA 缺陷菌株中同时缺乏鞭毛和减少的毒素分泌可能表明运动和毒力基因之间存在调节联系，而不是 FEA 依赖的毒素分泌。
Hbl、Nhe 和 CytK 毒素似乎是通过 Sec 途径分泌的，并且 FEA 缺陷菌株中 Hbl 表达减少并不是由于分泌缺陷所致。
