Legionella pneumophila, Cause of Legionnaire's Disease

Studies attribute the vast existence of Legionella pneumophila as discussed on the habitat page, to the relationships it has established with other microorganisms.  Aquatic biofilms, inhabited by bacteria, algae, protozoa and other microorganisms in which these bacteria usually reside, increase the availability of nutrients. In these areas the bacteria thrive due to the high concentrations of rust and organic particles which provides the bacteria with iron and nitrogen that are needed for growth and protection. 

Within the biofilm niches, Legionella pneumophila live in symbiosis with amoeba and ciliated protozoa.  They reproduce and survive as intercellular parasites within these organisms.  (They cannot multiply extracellularly.) But the relationships that Legionella pneumophila have established with these organisms has required some very unique adaptations because generally protozoa phagocytose (engulf and use for nutrients) bacteria! (To find more information about this, visit the Pathogen Proliferation page.)  Similarily, in order to proliferate within amoeba, Legionella pneumophila has had to make changes.  For example, studies show that its membrane lipid content is altered when Legionella pneumophila occupy these organisms, and its protein profile changes as well.  Although a few other bacteria species have  revealed their ability to multiply slightly within amoeba, Legionella pneumophila (and other members of its genus) are the only bacteria that have demonstrated their ability to proliferate.

It is through these relationships that this species of bacteria is able to survive so well within its environments.  In studies, amoeba-grown bacteria have illustrated their ability to be extremely resistant to chemical disinfectants and biocide treatments.  The symbiosis has also resulted in a great increase in the bacteria’s resistance to fluctuations in temperature, pH and exposure to oxidizing agents.  It has also been observed that when protozoa release vesicles containing these bacteria, that they are again highly resistant to biocides. 

It is through these relationships that these organisms are transmitted to humans!