What is Legionella?

Legionella are ubiquitous over the entire world found in a wide array of water systems. The bacteria are found primarily in association with unicellular protozoan and perhaps as members of biofilms. Many species are known to cause illness in humans, while others have only been isolated as environmental samples. The primary species isolated from patient material is Legionella pneumophila. The interaction of the bacteria with the host organism is the focus of numerous research programs and it serves as a model organism for facultative intracellular bacteria.
(Photo: Giemsa stain of Acanthamoeba polyphaga with L. pneumophila (dark rods) replicating within vacuoles)

Our Research Interest:

L. pneumophila can exist as a single organism in the environment for a long time. After contact with a host cell the bacteria is internalized and replicates in an intracellular compartment referred to as the replicative phagosome which avoid the typical phagosomal pathway and fusion with the lysosome. If the bacteria are inhaled into the respiratory tract of people, the bacteria are phagocytized by lung macrophages and replicate here similarly to in the natural protozoa host. In the ‘replicative’ form the Legionella multiplies as long as there are available nutrients. When nutrients become limited, the bacteria the ‘switch’ to a virulent, transmissible form.
At this point in the life cycle, the bacteria produce numerous new properties responsible necessary for transmission to a new host. The bacteria produce flagella for movement, and infect protozoa and macrophages more readily than replicative phase Legionella. A pigment is also produced at this point, that possibly provide protection in the environment. Transmissive phase bacteria are also more resistant to temperature and hydrogen peroxide than replicative phase bacteria.
Several factors play a role in the switch between phases. The two component system, LetA/LetS has an effect on flagella expression as well as interacts with the sigma factor RpoS and the type IV secretion system gene dotA. The global regulator,CsrA, also regulates expression on numerous genes responsible for the expression of phase – specific genes. csrB in turn regulates the activity of CsrA through binding of the protein. Finally, the Hfq protein functions regulates the switch through potential interactions with small regulatory RNAs and stabilization of mRNAs.
We are currently interested in the ecology of Legionella species and how environmental signals affect the ability of the bacteria to persist in the environment. Collaboration with the Savannah River National Lab is examining Legionella species found in man-made environments to determine how maintenance of these systems is affecting Legionella presence.