The common water-borne bug that causes Legionnaire’s disease should not be able to infect people, according to Bath University researchers speaking today at the bi-annual meeting of the Society for General Microbiology at the University of East Anglia, UK. "According to normal evolutionary theories, Legionella pneumophila bacteria should never have learned how to cause diseases in humans, since it has no natural animal host," says Prof. Michael Brown of the Department of Pharmacy and Pharmacology at Bath University, UK. "Basically the bug is mistaking us for amoebae, and invading us one cell at a time."
The widespread water-loving bacteria can cause two diseases: either mild Pontiac fever, or a much more serious type of pneumonia called Legionnaires' disease, which kills about one in eight people infected, but can kill up to half of them.
The behavior of the Legionella bacteria depends upon how they grow. The Bath researchers found that the bacterium develops into a very different organism depending upon whether it is simply living freely in water, or as a biofilm (similar to the slime that forms down plugholes), or whether it has invaded and is living inside a simple single-celled creature such as an amoeba.
Each method of growth affects the ability of Legionella to cause disease and resist antibiotic drug treatments.
"We have found that typical laboratory methods of growing Legionella produce the less dangerous form of the bacterium, whereas growing Legionella inside amoebae produces a more aggressive strain similar to those that cause us lung disease and which are more resistant to antibiotics," said Professor Brown.
"Over the course of evolution, Legionella interacted with simple environmental organisms. It learned how to invade and destroy amoebae, and it uses the same abilities to invade and destroy human tissues, especially human lung cells," he added.
"We think that many other species of disease-causing bacteria learnt their tricks during billions of years of evolution alongside simple environmental organisms such as amoebae," says Professor Brown. "Indeed, amoebae can act as environmental reservoirs of various kinds of germs, including bovine tuberculosis.
"By studying the way these bacteria have evolved we will be able to understand some of their abilities to cause human and animal diseases, and perhaps start to work out ways of eliminating them from hospitals, hotels and other places."
[Contact: Professor Michael R W Brown]
13-Sep-2001
