Legionella pneumophila is a gram negative bacterium (this refers to the characteristics of the bacterial cell wall and is important in determining which antibiotics will be most useful to fight it) and is the causative agent in Legionnaires’ disease.
Once the bacterium has entered the body it invades cells called macrophages. Macrophages are cells of the immune system which normally scavenge cells debris and engulf invading pathogens so they can be destroyed by other cells.
The Legionella pneumophila bacterium, however, is able to replicate itself inside the macrophage instead of being destroyed.
The bacterium is transmitted via tiny airborne droplets that are released when water is disturbed and are then inhaled into the lungs.
The most common sources are cooling towers (suspected to be the case in the current Scottish outbreak), ice machines, hot-tubs, showers, air conditioning systems and any complex potable water system or cooling system.
Once infection has taken hold (between 2-10 days), the immune response initially causes fever, chills, muscle aches, headaches and a cough, similar to flu symptoms.
More serious consequences of infection include kidney and liver damage. Middle aged, elderly people, smokers, and patients with chronic lung conditions have an elevated risk of contracting the disease.
Antibiotics that have good intercellular penetration are most effective against the bacterium, such as tetracyclines or erythromycin.
Posted by Jenny Gawley 
