Viruses Boost the Bite of Some Bacteria

Genes from resident viruses make some germs even nastier

TUESDAY, July 16, 2002 (HealthDayNews) -- Why are certain strains of bacteria more vicious than others?

The answer lies in the genes -- specifically, killer genes culled from resident viruses.

Scientists at the National Institute of Allergy and Infectious Diseases (NIAID) sequenced the genome of a particularly virulent strain of streptococcus bacteria, and found it contains a larger number of genes from bacteriophages, or viruses that inhabit bacteria.

The discovery, reported this week in the online edition of Proceedings of the National Academy of Sciences, is likely to lead to new therapies to combat a variety of serious infections, including strep throat, "flesh-eating" disease, scarlet fever, rheumatic fever, toxic shock syndrome and kidney ailments, the researchers say. Infections with this strain of streptococcus have unusually high death rates, they say.

Although sequencing bacteria genomes is not new, "this is the first paper that has sequenced a target strain for the major purpose of finding out exactly what makes it tick, what makes it a better pathogen," says Dr. James Musser. He is lead author of the study and chief of the laboratory of human bacterial pathogenesis at Rocky Mountain Laboratories in Hamilton, Mont. Rocky Mountain Laboratories are part of the NIAID.

Other experts agree the work is significant, but wonder whether it will change available treatments.

"I think what this does is help explain how a more serious form of that same bacteria could emerge, but I don't think it changes our approach," says Dr. Bruce Polsky, chief of the division of infectious diseases at St. Luke's-Roosevelt Hospital Center in New York City. "I think it's very useful in helping scientists and physicians understand how this occurs and why it occurs, and perhaps developing diagnostic methods to know if your patient has this particular bad type."

Musser and his colleagues sequenced the genome of a bacterium isolated from a patient with toxic shock syndrome. The bacterium belonged to the family of M3 strains of Group A Streptococcus (GAS).

Scientists have long been at a loss to explain why the M3 strains are so dangerous. "The reason we went after the M3 strains was because we and others have known for quite some time that these strains are unusually virulent and no one knew why," Musser says.

As it turned out, the M3 strains share about 90 percent of their genes with other, less virulent GAS strains. The remaining 10 percent, however, are actually genes from bacteriophages.

"Bacteria have their own set of viruses, just like humans, that live in and can be transmitted to different strains of the bacterium," Musser says. "What is unusual is that, in the case of the M3 strains, the bacterial viruses account for about 10 percent of the entire chromosome. That's a very large percentage."

Each of the viruses carries the genetic information for a new toxin.

"It's sort of like a warhead," Musser says. "By acquiring these new viruses, the bacterium can very rapidly develop a new set of virulence or disease-causing characteristics and that's what happened here. This has essentially made a bad pathogen into a really bad pathogen."

One of the toxins in the bacteria actually resembles an enzyme found in some snake venoms.

"We know exactly what's going on inside the pathogen, and we know exactly what at the molecular level permitted this organism to emerge in recent years," Musser explains. This should open the door for new treatments, he says.

"We're working on some new therapeutic and vaccine strategies on the basis of some of the findings in this manuscript, so there are some very rational things to do now that we have the genome sequence that may lead to accelerated development of new diagnoses, new therapies and control measures, and ultimately, we hope, vaccines," Musser says.

What To Do

For more information on bacteriophages, visit the Bacteriophage Ecology Group or the University of Leicester.

SOURCES: James Musser, M.D, Ph.D., chief, laboratory of human bacterial pathogenesis, Rocky Mountain Laboratories, NIAID, Hamilton, Mont.; Bruce Polsky, M.D., chief, division of infectious diseases, St. Luke's-Roosevelt Hospital Center, New York City; July 15, 2002, Proceedings of the National Academy of Sciences
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