LOS ANGELES, July 11 (Xinhuanet)-- The golden pigment that coats the surface of staphylococcus is not just for decoration, rather, the molecules that give the bacteria its golden hue also help it resist killing by immune system, US scientists reported on Monday.

    The researchers at the University of California, San Diego School of Medicine have discovered that staphylococcus, or the "Staph" bacteria, use a protective golden armor to ward off the immune system, a finding with the potential to lead to new treatments for serious infections now increasingly resistant to standard antibiotics.

    The study, focusing on the major human pathogen Staphylococcus

    aureus and the characteristic yellow-orange color for which it is named, is published in the latest issue of The Journal of Experimental Medicine.

    Among the deadliest of all disease-causing organisms, "Staph" is the leading cause of human infections in the skin and soft tissues, bones and joints, abscesses and normal heart valves.

    It especially flourishes in the hospital setting, producing bloodstream and surgical wound infections. The spread of antibioticresistant strains of Staph, referred to as methicillin-resistant Staphylococcus aureus or MRSA, has reached epidemic proportions andposes a major threat to the public health.

    Staph's coloration reflects the production of molecules called

    carotenoids, similar to those present in carrots and other colorfulvegetables and fruits. Dietary carotenoids have long been touted for their antioxidant properties with hope that they could slow aging or fight off cancer.

    The scientists found that pathogenic Staph took advantage of the antioxidant effects of its carotenoid pigment to extend its ownlife, by inactivating chemicals deployed by neutrophils that are lethal to most bacteria. The team used a molecular genetic approachfor their studies, knocking out the genes for carotenoid synthesis

    to generate a mutant strain of Staph that appeared white in color instead of the normal gold.

    They also found that the nonpigmented Staph mutant became much

    more susceptible to oxidants such as hydrogen peroxide and singlet

    oxygen produced by neutrophils. Without its golden pigment, the Staph lost its ability to survive in human neutrophils or blood, and could no longer form an abscess when injected into the skin of

    experimental mice.

    The power of the antioxidant pigment in promoting bacterial survival was further established extending the protective properties to a different bacteria.

    Pigment-producing genes of Staph were cloned into a normally colorless strain of Streptococcus ("Strep") bacteria that then turned yellow in color. The Strep expressing the golden carotenoid

    pigment became more resistant to oxidant and neutrophil killing, and produced larger ulcers when injected into the skin of normal mice.

    "The discovery of the critical role played by golden pigment inprotecting against infection provides an novel target for treatmentof serious Staph infections including those produced by antibiotic-resistant MRSA," said Victor Nizet, an associate professor who led

    the study.

    "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 defenses," he said. Enditem