Ecological role of soil nematodes in retention and dispersion of the environmental pathogens legionella pneumophila and legionella longbeachae

在水和潮湿的土壤环境中，单细胞生物(称为原生动物)以细菌为食物。一种名为军团菌的细菌是原生动物的寄生虫，当被摄取时，它会将原生动物的宿主细胞作为其生命周期的食物来源。军团菌的生命周期是独特的，因为它在两种细菌形式之间交替：复制形式(RF)和囊状形式(CLF)。当食物充足时，RFs主动复制自己，当细菌感觉到食物变得稀缺时，RFs分化为CLF，这是一种休眠形式，具有加固的细胞壁，以确保在环境中生存。耗尽的原生动物宿主细胞破裂，将CLF释放到环境中，在那里它们将被重复生命周期的原生动物摄取。CLF的传染性比RFS强得多，目前人们认为，当CLF被雾化在水滴中并被人类无意中吸入时，会导致一种称为军团病(LD)的呼吸道感染。感染这种疾病的风险很高的人往往免疫功能受损，而健康的人风险较低。最近的报告记录了人们在接触园艺盆栽土壤后发生LD的情况，这导致了一种建议，即将传染性军团菌传播给易感人群的另一种和/或替代途径可能来自土壤中自然存在的线虫。这种1毫米长的土壤线虫是非寄生的，以细菌为食物。这项研究旨在研究控制CLF形成的分子调控机制，其结果将导致改进控制措施、检测和治疗策略。此外，这项研究将确定军团菌是否能在营养丰富的线虫消化道内生存和复制，如果是，则调查土壤线虫是否能在土壤中保留和分散军团菌细菌。自由生活的线虫在自然环境中既可以作为军团菌的储蓄者，也可以作为军团菌的增殖者，这一暗示突出了以前在军团病的病因学中没有考虑的生态因素，从而对目前对该疾病的理解造成了巨大的影响。

In water and moist soil environments, unicellular organisms (termed protozoa) eat bacteria for food. One type of bacteria, Legionella, is a parasite of protozoa and when ingested, uses the protozoan host cell as a food source for its lifecycle. The Legionella lifecycle is unique because it alternates between two bacterial forms: the replicative form (RF) and the cyst-like form (CLF). When food is abundant, RFs actively replicate themselves and when the bacteria sense that food is becoming scarce, the RFs differentiate into CLFs, a dormant form that has fortified cell walls to ensure survival in the environment. The spent protozoan host cell bursts open releasing the CLFs into the environment where they will be ingested by foraging protozoa repeating the life-cycle. The CLFs are much more infectious than the RFs, and it is currently believed that when CLFs are aerosolized in water droplets and inadvertently inhaled by humans, causes a respiratory infection called Legionnaires' disease (LD). Individuals at high risk of contracting the disease are very often immunocompromised whereas healthy individuals are at low risk. Recent reports documenting the occurrence of LD in people after exposure to garden potting soil has led to the proposal that an additional and/or alternative route for transmission of infectious Legionella bacteria to susceptible individuals may originate from nematodes that naturally exist in soil. The 1-mm long soil nematodes are non-parasitic and eat bacteria for food. This research is intended to investigate the molecular regulatory mechanisms controlling CLF formation of which results will lead to improved control measures, detection, and treatment strategies. In addition, this research will determine if Legionella can survive and replicate within the nutrient-rich nematode digestive tract, and if so, investigate whether the soil nematodes can retain and disperse Legionella bacteria within the soil. The implication that free-living nematodes may serve both as reservoir and an amplifier of Legionella in the natural environment highlights an ecological factor that was not previously considered in the etiology of Legionnaire's disease, and thereby poses a dramatic impact on the current understanding of the disease.