Puzzled by Legionella?

Module 1

Legionella is the waterborne bacterial pathogen that caused the outbreak of Legionnaires’ disease in Philadelphia in 1976. Legionella bacteria are characterized by their unique pinkish rod-like shapes when stained with the Gram stain. Observing these gram-negative bacteria in guinea pig samples was a clue that enabled McDade to identify this new bacterial species.

Since its discovery, more than 60 species of Legionella have been identified (Mercante and Winchell 2015; Special Pathogens Laboratory 2019). The species that was the source of the illness in 1976 was named Legionella pneumophila, as it was found in the lungs and caused pneumonia in its victims. Each Legionella species is unique and can be distinguished by certain characteristics. For example, you can distinguish the colonies of some Legionella species because they fluoresce under UV light. Some will fluoresce a blue-white color such as L. anisa, L. dumoffii, L. gormanii, and L. bozemanii, while others will fluoresce red such as L. rubrilucens.

Most Legionella infections are caused by the same species of Legionella that was found in the first cases in Philadelphia, Legionella pneumophila (Stout 2019). This Legionella species is responsible for as many as 90% of cases of Legionnaires’ disease.