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Last Updated: Wednesday, 13 July, 2005, 00:55 GMT 01:55 UK
Chemical 'protects MRSA bacteria'
MRSA infection can kill
A chemical which gives the bacterium responsible for MRSA its characteristic golden pigment helps it ward off the human immune system, US scientists say.

They believe the molecule neutralises chemicals the human body releases to destroy the staphylococcus bacterium.

The University of California team says targeting this molecule could offer ways to stop antibiotic-resistant forms of the bug, including MRSA.

The study is reported in the Journal of Experimental Medicine.

Instead of attempting to kill the bacteria directly with standard antibiotics, a treatment strategy to inhibit the Staph pigment would disarm the pathogen
Dr Victor Nizet

Staphylococcus aureus is the leading cause of human infections in the skin and soft tissues, bones and joints, abscesses and normal heart valves.

It particularly flourishes in the hospital setting, producing bloodstream and surgical wound infections, including MRSA.

The bacteria are named after the golden pigment that coats their surface, which is caused by molecules, called carotenoids.

Carrot chemicals

These molecules, similar to those that give carrots their orange colour, have long been touted for their antioxidant properties, helping to minimise the damage inflicted on the body's tissues by charged particles called free radicals.

The San Diego study found that Staph bacteria use carotenoids to inactivate chemicals released by immune system cells that are lethal to most bacteria.

The researchers produced a mutant strain of Staph that could not produce carotenoids and was white instead of the normal gold.

It was less able to defend itself against attack by the immune system and could no longer form an abscess when injected into the skin of mice.

Next, they tested golden and white strains of Staph on blood samples taken from a human patient with a disease rendering the key immune cells incapable of producing their normal attack chemicals.

In this case, the nonpigmented strain was as effective at producing disease as its golden cousin.

New strategy

Finally, the scientists showed that adding carotenoids to a different, normally colourless strain of streptococcus bacteria boosted their ability to resist immune system attack and produce infection.

When injected into the skin of a mouse, the bacteria triggered the formation of a larger ulcer than produced by the colourless version.

Lead researcher Dr Victor Nizet said: "The discovery of the critical role played by golden pigment in protecting against infection provides a novel target for treatment of serious Staph infections including those produced by antibiotic-resistant MRSA.

"Instead of attempting to kill the bacteria directly with standard antibiotics, a treatment strategy to inhibit the Staph pigment would disarm the pathogen, making it susceptible to clearance by our normal immune defences."

Dr Mark Enright, an expert on bacterial infections at the University of Bath, agreed the research did raise the promise of new ways to combat Staph infection.

However, he said the bacteria were extremely adaptable and might be able to compensate for any attempt to disable its defences.

He also stressed that many infections were caused by Staph variants that had little pigmentation - although it was possible that pigmentation was only switched on once the bacteria had invaded the blood.

Dr Enright told the BBC News website: "Staph has a large number of virulence factors and toxins and to target just one of these may prove inadequate."

Q&A: MRSA 'superbugs'
24 Feb 05 |  Health

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