Bacteria 'See' the Light
THURSDAY Aug. 23, 2007 -- At least one type of bacteria may be able to sense light and then strengthen itself to survive, researchers report.
The bacteria Brucella abortus causes serious illness in animals and flu-like fevers in humans, and it appears to rely on its ability to sense light to maximize its virulence once it's out of a host.
"It wasn't expected that Brucella had any response to light. Why would it need light?" said study co-author Roberto Bogomolni, chairman of biochemistry at the University of California, Santa Cruz. "But we found that it does, in fact, activate with light, and it increases the virulence when activated by light."
The finding is published in the Aug. 24 issue of Science.
"Bacteria adapt their behavior to the environment. In that way, light is an important environmental signal that tells the bacterium where they are. They use this information in deciding how to behave," explained the co-author of an accompanying editorial, John Kennis, from the Faculty of Sciences at Vrije Universiteit, in Amsterdam, the Netherlands.
"When Brucella has infected an animal or human, it's inside the body where it is dark. However, when the bacterium is ejected from the host, it finds itself in the outside world where there is light. It then must infect the next victim, so the signal 'I see light' means 'I have to become infectious again,' and so Brucella has evolved a mechanism by which detection of light turns on its virulence response," Kennis explained.
Kennis said this light-sensing ability is akin to a "very primitive eye."
Bogomolni said the researchers first thought to look for light-sensing ability in Brucella because the bacterium's genome contained proteins similar to those that direct the movement of plants to light. These proteins have been dubbed the "LOV" domain, because they can detect light, oxygen and/or voltage. In the case of Brucella, light activates an enzyme called histidine kinase that causes Brucella to replicate rapidly.
Brucella isn't the only bacteria to contain LOV-domain proteins. According to the researchers, more than 100 different bacteria contain these proteins. However, their function in other bacteria is currently unknown.
For example, said Kennis, "E. coli also has a light-sensing protein. We don't know its function yet, but it may tell E. coli whether it's in our gut or outside, in the light."
Bogomolni said that because histidine kinases are only found in bacteria -- not in humans or animals -- turning these enzymes off is a "very tempting drug design target." Theoretically, such a drug would destroy bacteria but leave humans or animals unharmed.
"If we could do something to prevent the expression of the histidine kinase, we might impede the transition to virulence," said Philip Tierno, director of clinical microbiology at New York University Medical Center.
"We may be running out of the ability to use antibiotics, so these other avenues take on a very important position in the fight against disease," Tierno added.
To learn more about Brucella bacteria, go to the U.S. Centers for Disease Control and Prevention.
Posted: August 2007