Animals Fight Bacteria Using Stolen Defense Systems

Matt Pinski/University of Washington

Humans aren’t the only ones on the planet to be engaging in a battle against bacteria; these organisms are also constantly fighting each other over scarce resources. They’re some of the oldest inhabitants of Earth, so they’ve had a while to evolve the sophisticated antibacterial weaponry used to kill the competition. Some bacteria, for example, inject deadly enzymes into rival cells that rapidly degrade their protective walls, causing the contents of the cell to spill out.

While it can take millions of years for such sophisticated defense mechanisms to evolve, it doesn’t take them long to spread in bacterial communities. That’s because they can play pass-the-parcel with their genes in a process called horizontal gene transfer. Now, new research has suggested that some animals have joined in on this gene swapping party, and have acquired their own set of antibacterial weaponry from certain microbes. This, the researchers say, could be helping the animals kill off unwanted pathogens growing in or on them. The work has been published in Nature.

Scientists from the University of Washington happened upon this intriguing and unexpected discovery while scouring existing archives for potential antibacterial toxins.

“When we started digging into genome databases, we were surprised to find that toxin genes we thought were present only in bacteria were also in several animals,” study author Matt Daughterty said in a news-release. “We immediately started wondering why they were there.”

More specifically, they found that genes for a group of bacterial cell wall-degrading toxins, tae genes, had been transferred to animals on at least six occasions. Furthermore, the resulting genes have been preserved in the recipients for hundreds of millions of years. This was particularly surprising since, despite being common among bacteria, horizontal gene transfer between bacteria and more complex organisms is a rare occurrence.

The organisms possessing this set of stolen genes were diverse, but one particular species caught their eye- the deer tick Ixodes scapularis. That’s because these arthropods are known to transmit the bacterium responsible for Lyme disease, Borrelia burgdoferi. Ticks can harbor pathogens such as this in their guts, which are then transmitted through their saliva when they take a blood meal from a susceptible animal.

Interestingly, they found that the toxins from these acquired genes were present in both the saliva and the guts of the ticks, and that the toxins limit the proliferation of B. burgdoferi. When they used genetic manipulation to reduce toxin production in the ticks, the bacteria significantly increased in abundance.

The researchers are now continuing their work by examining how these toxins might influence the transmission of Lyme disease. They would also like to investigate the functions of the toxins in animals other than ticks, and whether other bacterial defense systems have been also been transferred to other organisms.

[Via University of Washington Health Sciences/UW Madison and Nature]

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