Tricks of bacterial infection come to light

Sunlight makes a vicious strain of bacteria even more dangerous, a new study has found.

A team of researchers, including Stanford scientists, has discovered that certain single-celled infectious bacteria can tell the difference between light and dark, and actually increase their infectiousness 10-fold when hit by sunlight.

This is the first time light has been shown to change the course of a bacterial disease. And these particular bacteria are probably not alone: As many as one-third of other bacterial species may react to light by producing physiological or chemical changes.

The study was published in the Aug. 24 issue of Science.

Brucella, the bacteria that cause the infectious disease brucellosis, and more than 100 other kinds of bacteria contain proteins originally thought to be functional only in plants, according to Winslow Briggs, one of the researchers and director emeritus of the Carnegie Institution Department of Plant Biology. "Many of these bacteria have been pretty well studied, but nobody has ever showed light responses in them," said Trevor Swartz, the study's lead author and a former postdoctoral fellow in Briggs' lab. "In Brucella, we showed that light actually is controlling infection."

Brucellosis, also known as Malta fever, afflicts both livestock and humans, often causing spontaneous abortions in cattle. More than 500,000 new cases of human brucellosis are diagnosed every year, mostly in developing countries, where drinking or eating unpasteurized dairy products is more common. Because of restrictions on raw milk in the United States, brucellosis cases here are rare. Infected humans suffer from flu-like symptoms such as fever or joint pain, sometimes lingering for weeks or months. More severe infections can result in chronic pain or meningitis and encephalitis—inflammations of the spinal cord and brain—according to the Centers for Disease Control and Prevention. (The disease is rarely fatal in people.)

About 10 years ago, Briggs found that these plant proteins respond to the blue wavelengths in sunlight. Blue-light-sensing proteins play a variety of roles in plants, Briggs said, such as guiding plants' growth toward sunlight. When sunlight hits the plant, these proteins change shape, and this shape-shifting sends cues inside the plant cells that ultimately change the direction of the plant's growth.

Briggs and Swartz were surprised to find the DNA sequence for the light-sensing proteins in many bacterial genomes. Of the more than 500 bacterial species whose genomes have been sequenced to date, 12 percent have one or more proteins that look like light-sensing proteins in plants, Briggs said. The researchers wondered whether these proteins would also respond to light. Along with Mary Beth Mudgett of Stanford's Department of Biology and Roberto Bogomolni of the University of California-Santa Cruz, they cast light on the potential light-sensing proteins from Brucella and three other kinds of bacteria, including Psuedomonas, a plant pathogen. To their surprise, these proteins changed shape in response to light, similar to the changes they saw when the plant proteins were exposed to light.

The researchers were originally stumped by the appearance of light-sensitive proteins in bacteria, organisms not previously thought to care about light or dark. "The question was, what the blazes is it doing in Brucella?" Briggs said. "That's where we had to hook up with people who were handling Brucella, because it's a very dangerous pathogen."

Researchers Fernando Goldbaum and Gastón Paris of the Laboratory of Structural and Molecular Immunology at the Leloir Institute in Buenos Aires mixed equal amounts of Brucella with macrophages. "Macrophages are these cells that circulate around and chomp up stray material that shouldn't be there, like bacteria," Briggs said.

Macrophages swallow bacteria whole and then chew them up internally, like a snake digesting its prey. But, in the human body, successfully infectious bacteria, such as Brucella, survive this digestion and multiply inside the macrophage cells, eventually bursting out and killing them. Trying to figure out what the proteins might be doing in the bacteria, the researchers put dishes with a mixture of macrophages and bacteria in the light, and wrapped others in foil to keep them in the dark. After 24 hours, the researchers found that the amount of light-exposed Brucella still alive in the dishes outstripped those grown in the dark by almost 10 to 1. Somehow, the change in the blue-light-sensing protein triggered by the light allowed the Brucella to escape being destroyed.

Swartz said this study gives rise to many more questions. "How does this mechanism work in the cell? This protein gets activated—does it activate another protein? Then how does it increase infection?" Swartz asks. "And why is it important for the bacteria to have this function? The work we've done is just the tip of the iceberg."

"This is a really striking result," said Sean Crosson, assistant professor of biochemistry and molecular biology at the University of Chicago, who studies similar proteins in the non-infectious bacteria Caulobacter. "It's going to change the way we think about several species."

Swartz said he suspects that this protein helps to tell Brucella if it is inside or outside its host. But he acknowledged that the bacteria's ability to sense light may serve other functions. Briggs has another hypothesis: that Brucella may use the light-sensing protein as a way to boost its virulence into high gear once it has been consumed by macrophages in the blood. "Your blood is circulating very close to the surface of your skin," he said. "Put a flashlight up to your hand—a lot of light shines through. My hypothesis is that as macrophages get close to the skin they get enough light to activate some of these proteins. That pumps up the bacteria's virulence, and my guess is that it does that by turning on defense machinery in the bacteria, to defend itself against the macrophages."

Swartz and Briggs said the finding that light regulates Brucella's virulence might open doors for future potential therapies against brucellosis and other bacterial diseases. In many cases, it is unknown how infectious bacteria dodge the human immune cells that are trying to kill them. "Understanding the chain of events and proteins involved from absorption of light to activation of bacterial defenses should provide new targets for drugs to fight the diseases," Briggs said, but he added that these light-triggered events remain a mystery.

The paper's other authors are Tong-Tseung Tseng of the Carnegie Institution; Jung-Gun Kim of the Department of Biology at Stanford; Marcus Frederickson of UC-Santa Cruz; Diego Comerci and Rodolfo Ugalde, both of the Universidad Nacional de San Martín (Argentina); and Gireesh Rajashekara and Gary A. Splitter, both of the University of Wisconsin-Madison.

Funding for the study was provided by the National Science Foundation and the National Institutes of Health.

Rachel Tompa is a science-writing intern at the Stanford News Service.

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