Scientists Decode Genes for Drug-Resistant Staph

Findings might lead to new antibiotics against this growing threat

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By Amanda Gardner
HealthDay Reporter

TUESDAY, Feb. 28, 2006 (HealthDay News) -- Researchers have succeeded in deciphering the genetic make-up of a major strain of drug-resistant, potentially deadly staph bacteria in the United States.

Genes responsible for the virulence of community-acquired methicillin-resistant Staphylococcus aureus (MRSA) in this country appear to be taken from another, less toxic, bacterium.

"It has, in effect, borrowed genetic characteristics from an otherwise rather benign organism and, in so doing, acquired an extra degree of lethality," explained Dr. Pascal James Imperato, chairman of the department of preventive medicine and community health and director of the master of public health program at the State University of New York Downstate Medical Center in New York City.

Imperato was not involved in the study, which appears in the Feb. 29 online issue of The Lancet.

The newly mapped genes could be used as markers to track the spread of this particular strain in both hospital and community settings, and to investigate more effective therapies to fight it.

S. aureus can be found on the skin or in the nose of about 30 percent to 40 percent of the U.S. population, experts say. Most of the organisms are benign, but occasionally the bacteria can cause infections, especially skin infections, of varying degrees of severity. They can also trigger life-threatening infections such as toxic shock syndrome.

MRSA is resistant to methicillin and other members of the class of widely used antibiotics that includes penicillin, oxacillin and amoxicillin. According to an accompanying editorial, scientists have more recently found strains of the bug with resistance to all known antibiotics.

These infections have historically broken out among patients in hospitals. Increasingly, however, staph infections and drug-resistant staph infections are cropping up within the wider community. They were responsible for infecting a group of children in Minnesota in the late 1990s, and have also been linked to the "flesh-eating disease."

This particular strain of MRSA, called USA300, can cause particularly nasty infections and is becoming more prevalent. According to the editorial, in 1992 less than 3 percent of S. aureus organisms tested were MRSA, whereas now 40 percent in the United Kingdom are. The figure is higher in the United States and Japan.

This community-acquired version of the organism is genetically distinct from the hospital-acquired version, and contains a toxin called Panton-Valentine leukocidin.

"We're dealing in the community with a slightly different organism than what we deal with in hospitals," Imperato said. Community-acquired MRSA has tended to be more susceptible to a number of antibiotics, whereas the hospital version has over time become resistant to these drugs, he explained.

Now, however, community-acquired MRSA is exhibiting some of the more virulent characteristics previously associated with hospital-based strains, Imperato said.

USA300 was first isolated in September 2000, and has since been implicated in outbreaks of skin and soft tissue infections in healthy individuals in at least 21 U.S. states, Canada and Europe.

The University of California, San Francisco, researchers sequenced the genome of USA300, and compared it with sequences of 10 other staphylococcal strains.

They discovered that USA300 has incorporated part of the genome of another germ, Staphylococcus epidermidis. These additions appear to enable USA300 to multiply in host tissue without encountering the wrath of the immune system.

The gene acquisition seems to continue a historical trend. Hospital- and community-acquired MRSA were first formed by taking genetic elements from other staphylococci, the researchers noted.

Unfortunately, the trend could continue.

"The problem is the current genetic make-up may not be stable over the long term," Imperato said. "It can undergo changes."

More information

For more on community-associated MRSA, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Pascal James Imperato, M.D., distinguished service professor and chairman, department of preventive medicine and community health, and director, master of public health program, State University of New York Downstate Medical Center, New York City; Feb. 28, 2006, The Lancet online

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