Understanding the mechanisms employed by Legionella to survive in diverse environments

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In water and moist soil environments, unicellular organisms (termed protozoa) eat bacteria for food. One type of bacterium, 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 featuring 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 immunocompromised humans, they can cause a respiratory infection called Legionnaires' disease (LD). I use a diverse range of molecular biological tools in complementary research approaches to elucidate the interactions between the host and Legionella bacteria. In particular, this research is intended to investigate the regulatory mechanisms controlling CLF formation in protozoa, and the results will lead to improved water quality strategies. Reservoirs of Legionella bacteria include natural and man-made freshwater systems, and recently it was discovered that Legionella bacteria can undergo their lifecycle in free-living nematodes normally present in moist soil environments. This research will further examine the Legionella life-cycle in nematodes to investigate whether nematodes can retain and disperse Legionella bacteria within the soil. If this is the case, then free-living nematodes may serve both as reservoir and an amplifier of Legionella in the natural environment. This ecological factor was not previously considered in the etiology of Legionnaire's disease, and thereby investigating the role of the nematode host will have a dramatic impact on the current understanding of the regulatory mechanisms used by Legionella bacteria to adapt and survive in diverse environments.