Filopon Didier, Mérieau Annabelle, Bernot Gilles, Comet Jean-Paul, Leberre Rozenne, Guery Benoit, Polack Benoit, Guespin-Michel Janine
GREPI EA 2938 CHU de Grenoble, BP 217, 38043 Grenoble cedex 9, France.
BMC Bioinformatics. 2006 May 30;7:272. doi: 10.1186/1471-2105-7-272.
Pseudomonas aeruginosa, an opportunistic pathogen, is often encountered in chronic lung diseases such as cystic fibrosis or chronic obstructive pneumonia, as well as acute settings like mechanical ventilation acquired pneumonia or neutropenic patients. It is a major cause of mortality and morbidity in these diseases. In lungs, P. aeruginosa settles in a biofilm mode of growth with the secretion of exopolysaccharides in which it is encapsulated, enhancing its antibiotic resistance and contributing to the respiratory deficiency of patients. However, bacteria must first multiply to a high density and display a cytotoxic phenotype to avoid the host's defences. A virulence determinant implicated in this step of infection is the type III secretion system (TTSS), allowing toxin injection directly into host cells. At the beginning of the infection, most strains isolated from patients' lungs possess an inducible TTSS allowing toxins injection or secretion upon in vivo or in vitro activation signals. As the infection persists most of the bacteria permanently loose this capacity, although no mutations have been evidenced. We name "non inducible" this phenotype. As suggested by the presence of a positive feedback circuit in the regulatory network controlling TTSS expression, it may be due to an epigenetic switch allowing heritable phenotypic modifications without genotype's mutations.
Using the generalised logical method, we designed a minimal model of the TTSS regulatory network that could support the epigenetic hypothesis, and studied its dynamics which helped to define a discriminating experimental scenario sufficient to validate the epigenetic hypothesis. A mathematical framework based on formal methods from computer science allowed a rigorous validation and certification of parameters of this model leading to epigenetic behaviour. Then, we demonstrated that a non inducible strain of P. aeruginosa can stably acquire the capacity to be induced by calcium depletion for the TTSS after a short pulse of a regulatory protein. Finally, the increased cytotoxicity of a strain after this epigenetic switch was demonstrated in vivo in an acute pulmonary infection model.
These results may offer new perspectives for therapeutic strategies to prevent lethal infections by P. aeruginosa by reverting the epigenetic inducibility of type III cytotoxicity.
铜绿假单胞菌是一种机会致病菌，常见于慢性肺部疾病，如囊性纤维化或慢性阻塞性肺炎，以及急性病症，如机械通气相关性肺炎或中性粒细胞减少患者。它是这些疾病中导致死亡和发病的主要原因。在肺部，铜绿假单胞菌以生物膜生长模式定居，分泌胞外多糖将自身包裹其中，增强其抗生素抗性，并导致患者呼吸功能不全。然而，细菌必须首先大量繁殖并表现出细胞毒性表型以逃避宿主防御。参与感染这一步骤的一个毒力决定因素是III型分泌系统（TTSS），它可将毒素直接注入宿主细胞。在感染初期，从患者肺部分离出的大多数菌株具有可诱导的TTSS，可在体内或体外激活信号后注入或分泌毒素。随着感染持续，大多数细菌会永久丧失这种能力，尽管未发现基因突变。我们将这种表型称为“不可诱导型”。正如控制TTSS表达的调控网络中存在正反馈回路所暗示的那样，这可能是由于一种表观遗传开关导致可遗传的表型修饰而无基因型突变。
我们使用广义逻辑方法设计了一个TTSS调控网络的最小模型，该模型能够支持表观遗传假说，并研究了其动态变化，这有助于确定一个足以验证表观遗传假说的判别性实验方案。基于计算机科学形式方法的数学框架对该模型的参数进行了严格验证和认证，从而得出表观遗传行为。然后，我们证明了一株不可诱导型铜绿假单胞菌在短暂脉冲的调控蛋白作用后，能够稳定获得因钙耗竭而诱导TTSS的能力。最后，在急性肺部感染模型中证实了这种表观遗传开关后菌株细胞毒性的增加。
这些结果可能为通过恢复III型细胞毒性的表观遗传诱导性来预防铜绿假单胞菌致死性感染的治疗策略提供新的视角。
