Bacteria Uses Sonar-Like Strategy to Probe Environment

An infection-causing bacteria uses a unique sonar-like strategy to scan its environment, similar to that used by bats when hunting in the dark, according to a new study.

The research solves a 70-year-old medical mystery and could lead to the development of new drugs that target bacterial infections without spurring antibiotic resistance.

Bacteria are microscopic single celled organisms, some of which are responsible for human infections and disease. The bacteria Enterococcus faecalis has interested researchers ever since 1934, when it was discovered that it could produce a substance called cytolysin that can poison or kill a broad range of organisms.

Scientist have since learned that the toxin is composed of two protein subunits, one small and one large, that are constantly being produced by the bacteria at low levels. It was observed that E. faecalis was somehow able to sense the presence of nearby target cells and ramp up toxin production in response, but until now, scientists were in the dark as to how this was accomplished.

The research team, led by Michael Gilmore, director of the Schepens Eye Research Institute at Harvard Medical School, solved the mystery when they discovered that the cytolysin subunits also act as a two-part environmental probe. Like armed sentries sent out in pairs, they scout the terrain, and when they come across an enemy, one will attack, while the smaller of the two rushes back with a message for reinforcements.

Here's how it works: If a target cell is present, the larger subunit will bind to it, leaving its smaller half unattended and free to report back to the bacterium.

The return signal kicks toxin production from its normally low levels into high gear.

In the absence of a target cell, the large subunits latch onto the smaller ones and keep them below the level needed to trigger an increase in toxin production.

Although the bacteria do not emit high pitched sounds the way bats do when they navigate in the dark, the basic principle is the same. "You send out a signal, and you look at the return signal to detect changes in the environment," Gilmore told LiveScience.

The study was detailed in a recent issue of the journal Science.

The ability to produce cytolysin probably evolved as a kind of lethal alarm system to help the bacteria establish islands of safety within their local environments and to keep out unwanted visitors, including other bacteria, scientists say. Over time, the adaptation may have evolved to become a useful weapon, giving the bacteria a means of obtaining nutrients -- found within other cells, and released only by rupturing them -- that would not otherwise be accessible.

The discovery could lead to the development of novel "toxin-inhibitor" drugs that could limit the severity of infections, Gilmore said. Because the bacteria would not be killed directly, there is less chance of them developing a resistance to the drugs. Such a development would be welcome news in hospitals, as more strains of bacteria are becoming increasingly resistant to even the most advanced antibiotics.