Types of Legionella

Legionella is a type of bacteria that is known for causing a serious type of pneumonia called Legionnaires disease, or a milder illness called Pontiac fever. It is called gram-negative, because of the way it appears when it is stained with a certain type of laboratory dye. Gram-negative bacteria have a particular cell wall structure that can make them harder to treat with certain antibiotics. The bacteria were first identified in 1976 when an outbreak of severe pneumonia occurred among attendees of American Legion convention in Philadelphia. Since then, Legionella has been recognized as a significant public health concern and outbreaks have occurred worldwide. There are over 50 species of Legionella bacteria but only a few are known to cause the human disease. The most common types of Legionella that cause illnesses in humans are Legionella pneumophila. This is the most common cause of Legionnaires disease and is responsible for up to 90% of reported cases. It has 16 different serogroups, the serogroup 1 being the most common cause of the infection.

Non-pneumophila Legionella species are typically associated with milder forms of the illness such as Pontiac fever. There are other Legionella species that have been known to cause human illness, which are various and way beyond the scope of this course. It is worth noting that not all Legionella species are pathogenic, some are harmless to humans. However, any water source or system that may contain Legionella bacteria should be treated with caution and managed carefully to reduce the risk of Legionella infection.