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Study Finds Staph a Quick-Change Artist

Bacterium can acquire genes for resistance

MONDAY, July 9, 2001 (HealthDayNews) -- Strains of Staphylococcus aureus readily acquire new genes from their bacterial relatives, says a new study. The finding raises public health concerns about how quickly common bacteria can become harmful to humans and resistant to the drugs used to fight them.

A research team led by Dr. James Musser, chief of the Laboratory of Human Bacterial Pathogenesis at the National Institute of Allergy and Infectious Diseases, found that 22 percent of S. aureus's genome was made up of genes that didn't directly contribute to the bacterium's basic life processes.

These so-called "contingency" genes, says Musser, lie dormant until the bacterium's survival is challenged. At that point, they snap into action, giving the bacterium a greater chance of survival.

The work appears in the the July 17 issue of the Proceedings of the National Academy of Sciences.

S. aureus is found in air, food, water, on surfaces, or on people and animals. Various strains of the bacterium can be found in the nose and throat and on the hair and skin of more than 50 percent of people who are healthy. However, some strains can cause minor infections like pimples or other skin conditions, while others can cause increasingly serious problems, including food poisoning, blood infections or pneumonia. In rare cases involving drug-resistant strains, some S. aureus infections can be fatal.

Musser's team set out to determine whether methicillin-resistant strains of S. aureus had evolved from a single ancestral strain with resistant genes, or whether different resistant strains had independently evolved multiple times.

The researchers examined 36 strains of S. aureus found in humans, cows, and sheep, using a technique called DNA microarray analysis, which detects the presence of thousands of genes simultaneously.

This technology, says Musser, has allowed researchers to better understand how both methicillin-resistant S. aureus and strains related to toxic shock syndrome (TSS) have evolved.

But more importantly, Musser found that a gene called mec has "jumped" into distinct S. aureus genomes at least five times -- conclusive evidence that resistance has evolved independently in multiple strains over history.

"[It] indicates that the ability of this organism, which is a major human pathogen, to acquire drug-resistant traits will undoubtedly continue well into the future," says Musser.

According to Dr. Donald Low, the microbiologist-in-chief at Mount Sinai Hospital in Toronto, mecs "come into S. aureus to cause methicillin-resistance, and it's done it more than one time, and it can do it again."

Low says this study also sheds light on the outbreak of TSS in the 1980s, suggesting that the hyper-absorbent tampons introduced at that time altered the host environment within the female reproductive system, triggering at least one of the genes in the S. aureus strains responsible for the condition.

Low says a worst-case scenario would involve a disease-causing strain of S. aureus with a lot of genetic variation. "The worst possible thing that can happen is that you find a drug to treat such a virulent pathogen, and into it comes a gene which confers resistance," says Low.

"What happens if a vancomycin-resistance [gene] gets into S. aureus that is also resistant to all of these other antibiotics, taking away the single most important last-line drug that we have to treat infectious due to this [bacterium]?"

Musser adds that better surveillance strategies are needed to determine which resistant strains are most likely to become widespread in the community, in hospitals, or on farms. "From a public health standpoint, we have the need to be ever-vigilant and study the ability of strains to pick new genes and new combinations of genes," he says.

Depending on the strain, S. aureus produces certain toxins. "It very well may be that from a vaccine development standpoint, we will need to formulate the vaccines to maximize coverage of the most abundant strains or the strains that are making the broadest array of toxins," says Musser.

What To Do

Both the Food and Drug Administration and the Centers for Disease Control and Prevention provide information about S. aureus.

You can learn about antibiotic resistance from this article in Scientific American.

SOURCES: Interviews with James Musser, M.D., Ph.D., chief, Laboratory of Human Baterial Pathgenesis, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, Hamilton, Mont.; Donald E. Low, M.D., microbiologist-in-chief, Department of Microbiology, Toronto Medical Laboratories & Mount Sinai Hospital, Toronto; July 17, 2001, Proceedings of the National Academy of Sciences
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