The Anti-Sigma Factor TcdC Modulates Hypervirulence in an Epidemic BI/NAP1/027 Clinical Isolate of Clostridium difficile

Nosocomial infections are increasingly being recognised as a major patient safety issue. The modern hospital environment and associated health care practices have provided a niche for the rapid evolution of microbial pathogens that are well adapted to surviving and proliferating in this setting, after which they can infect susceptible patients. This is clearly the case for bacterial pathogens such as Methicillin Resistant Staphylococcus aureus (MRSA) and Vancomycin Resistant Enterococcus (VRE) species, both of which have acquired resistance to antimicrobial agents as well as enhanced survival and virulence properties that present serious therapeutic dilemmas for treating physicians. It has recently become apparent that the spore-forming bacterium Clostridium difficile also falls within this category. Since 2000, there has been a striking increase in C. difficile nosocomial infections worldwide, predominantly due to the emergence of epidemic or hypervirulent isolates that appear to possess extended antibiotic resistance and virulence properties. Various hypotheses have been proposed for the emergence of these strains, and for their persistence and increased virulence, but supportive experimental data are lacking. Here we describe a genetic approach using isogenic strains to identify a factor linked to the development of hypervirulence in C. difficile. This study provides evidence that a naturally occurring mutation in a negative regulator of toxin production, the anti-sigma factor TcdC, is an important factor in the development of hypervirulence in epidemic C. difficile isolates, presumably because the mutation leads to significantly increased toxin production, a contentious hypothesis until now. These results have important implications for C. difficile pathogenesis and virulence since they suggest that strains carrying a similar mutation have the inherent potential to develop a hypervirulent phenotype.

医院获得性感染（nosocomial infections）正日益被视为重大患者安全议题。现代医院环境及配套医疗实践，为微生物病原体的快速演化提供了适宜生态位：这些病原体可很好地适配该环境并存活、增殖，进而感染易感患者。耐甲氧西林金黄色葡萄球菌（Methicillin Resistant Staphylococcus aureus，MRSA）、耐万古霉素肠球菌（Vancomycin Resistant Enterococcus，VRE）等细菌性病原体便是典型案例——二者均获得了抗菌药物耐药性，同时具备增强的存活能力与毒力特性，给临床医师带来了严峻的治疗困境。近期研究表明，产芽孢艰难梭菌（Clostridium difficile）也属于这一类病原体。自2000年以来，全球范围内艰难梭菌医院获得性感染数量显著上升，这主要源于流行株或高毒力分离株的出现，这类菌株似乎具备广谱抗菌药物耐药性与毒力特性。针对这些菌株的出现、持续传播与毒力增强，学界已提出多种假说，但尚缺乏支持性实验数据。本研究描述了一种利用同基因菌株（isogenic strains）的遗传学方法，用以鉴定与艰难梭菌高毒力形成相关的因子。研究证实，毒素产生的负调控因子——抗σ因子（anti-sigma factor）TcdC的自然突变，是流行株艰难梭菌分离株发展为高毒力菌株的重要因素，推测该突变可导致毒素产生量显著升高——这一假说迄今为止仍存在争议。上述结果对艰难梭菌的致病机制与毒力研究具有重要意义，因为它们表明携带类似突变的菌株具备发展为高毒力表型的内在潜能。