The pseudomonas aeruginosa bacterium causes hospital-acquired pneumonia, a condition most often contracted by patients already under intensive care.

The bacterium, which has become increasingly resistant to antibiotics, kills half the patients it infects.

Writing in the journal Nature Medicine, scientists at the University of California, San Francisco, and the Medical College of Wisconsin describe how they targeted a species of the bacterium that is also responsible for the untreatable lung inflammations which devastate the lives of half a million people with cystic fibrosis.

Sufferers are never able to clear their lungs of the microbes.

The bacterium belongs to the same group of bacteria as e. coli, salmonella, and the microbe responsible for plague, all of which may be candidates for the same immunisation strategy developed by the scientific team.

Deadly toxins

Professor Jeanine Wiener-Kronish, one of the senior researchers, said: "All these species utilise the same combination of proteins to deliver deadly toxins to host cells.

"We have developed an antibody against one of these proteins and shown that it blocks pseudomonas from injecting toxins into lung cells.

"This antibody may well provide immunisation against these other bacteria species too."

The bacterium employs what is known as a type III secretory system to infect lung cells.

A combination of at least 25 bacterial proteins work in concert to allow the microbe access to penetrate the outer surface of the host cell and inject toxins into it.

The toxins are particularly difficult for the cell to defend against because they wipe out the primary line of defence - the macrophages that would normally engulf such invaders.

The research team developed antibodies to five of the proteins in the bacterium's toxin delivery system and tested the ability of each to block the deadly process.

Invading microbe foiled

One of the five, an antibody to the bacterium's PcrV antigen, prevented the invading microbe from destroying the lung cell macrophages.

As a result, the cells could mount a defence against the bacteria and prevent delivery of the toxin.

Most importantly, lung cells were protected, the researchers report.

The scientists do not yet know the precise role of the bacterial protein they have blocked.