But a recent discovery could one day make deer ticks themselves vulnerable to the disease they transmit. Researchers seeking to discover how genes evolve inside bacteria stumbled across something they had not seen before: They found that the genes they thought were only present in bacteria were also inside more complex life forms — animals such as ticks and mites.

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The genes help bacteria develop a weapons system — toxins — used to wage war with competing bacteria. Deer ticks use these genes to produce a toxin that keeps Lyme from replicating to the point that it kills them.

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"This result was particularly surprising because the only role so far attributed to these toxins involved competition between bacteria," Joseph D. Mougous, a professor in the biology department at the University of Washington School of Medicine in Seattle, wrote in an e-mail response to questions. "We were shocked to see them – it is very rare to find this many related bacterial genes transferred in independent events to such a wide range of animals."

The report on how animals pilfered the defense systems of bacteria was published Monday in the journal Nature. It was supported and funded by the National Institutes of Health, the Defense Threat Reduction Agency, the Biotechnology and Biological Sciences Research Council and the Howard Hughes Medical Institute.

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In a statement about the research, which Mougous led, the University of Washington said the 14 scientists who participated "immediately recognized the potential medical importance of the toxin in ... the deer tick" because it is the main, and perhaps only, vector of Lyme disease.

"We were excited to see this in the deer tick, given the increasing prevalence of Lyme disease in North America," said co-author Seemay Chou, a postdoctoral researcher in the Mougous group.

Deer ticks lug around pathogens in their guts, which slip into the flesh of animals they bite through saliva. The researchers set out to determine how Lyme is transferred, and what would happen if they "used genetic strategies to reduce production of the toxin in ticks," the statement said. They saw that when production of toxin was reduced, "the Lyme disease pathogen rose significantly" in ticks.

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If Lyme disease rose to levels deer ticks can't tolerate, they would get a taste of their own medicine. Or poison.

The research was conducted in labs across the United States and in Great Britain. Mougous studied samples in a lab at the University of Washington that specializes in inter-bacterial competition. Harmit Malik, a co-author, made observations at the Fred Hutch Cancer Research Center, which specializes in evolutionary relationships. And Waldemar Volmer studied samples at his Newcastle University lab in Britain that specializes in the bacterial cell wall.

Frank Yang, who works on ticks and Lyme disease at Indiana University, and Christine Jacobs-Wagner, who focuses on bacterial cell biology at Yale University, also participated.

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"We are now following up on these results by looking into how these toxins influence Lyme disease transmission," said Chou.

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Mougous, a lead author of a study published Monday in the journal Nature, said inhibiting the tick's ability to regulate Lyme is a far-fetched hypothesis discussed by researchers as a possible medical benefit. Such a thing would probably require decades of study if it can ever be achieved.

The most meaningful finding is that genes known to jump only between microbes had found their way into more complex organisms such as mites and ticks through a process known as horizontal transfer. The researchers theorized that this first happened around 400 million years ago, allowing the animals to incorporate what they called a theft into their genome.

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"I would say the real benefit to the medical community is that the basic biology of ticks and the tick immune system are quite understudied," Mougous wrote in an e-mail. "The more we understand how these organisms modulate the levels of bacteria (and other pathogens for that matter) in association with them, the closer we are to identifying creative ways to intervene in transmission."