Current Research

The bacterium Legionella pneumophila and related species, normal inhabitants of freshwater and moist soil water environments, are also intracellular parasites of protozoa. In these environments, protozoa survive by eating bacteria for food. However, when protozoa ingest Legionella, the bacterium deploys mechanisms to take over the protozoan cell parasitizing it for nutrients and energy to support its intracellular lifestyle.

The intracellular developmental cycle is also unique in that it alternates between replicative forms and transmissive phase forms that eventually mature into cyst-like forms. Bacterial cyst-like forms are released upon lysis of the spent protozoan host cell and are able to survive amid environmental stresses (e.g. desiccation, chlorine, biocides) until the next protozoan host cell is encountered.

Legionella and protozoa are also commonly found in man-made water handling systems (e.g. cooling towers, air conditioning units, hot water heaters) that are part of the city infrastructure. Warm ambient temperatures promote increased production of Legionella cyst-like forms within these reservoirs that, when coupled with aerosolization, can be inadvertently inhaled by susceptible individuals leading to the manifestation of bacterial pneumonia termed Legionnaires’ disease. This is caused by successful infection of lung macrophages by Legionella. Recent outbreaks attributed to aerosolization of Legionella bacteria include Quebec City (2012) and Toronto (2005).

Uptake of Legionella is facilitated by phagocytic mechanisms common to both protozoa and lung macrophages. Along with an assortment of contributing virulence factors, successful infection of both protozoa and macrophages is enabled by a Dot/Icm Type IV Secretion system that, analogous to a medical syringe, delivers effector molecules into the host cytoplasm to affect remodeling of the host cell environment to establish a replicative niche. Expression of these virulence traits is coordinated with cyst formation.

However, research has shown that certain Legionella gene products are indispensable for survival in protozoa but dispensable for survival in human macrophages and vice versa, indicating that not all survival mechanisms are shared.