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

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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.