Reported in the journal Science, the discovery comes after 120 years of research into the disease, which causes abortions in livestock and fevers in humans.
The researchers say that two other bacteria, including a species that attacks plants, sense light using the same type of protein structure, and at least 94 more species possess the code for it in their DNA.
These bacteria have been very well studied for years, and no one knew they could sense light. Now it seems like it's a common thing rather than being an anomaly," said lead author Trevor Swartz, who initiated the study.
The ubiquity of the structure suggests that light may play a much more important role in bacterial life than has previously been recognized. Since the recurrent structure can be paired with a variety of signalling proteins, it provides organisms with immense versatility in the ways they use light, Swartz said.
"We have bumped into an entirely new family of light receptors in nature," said co-author Roberto Bogomolni, Professor of Chemistry and Biochemistry at the university.
The receptor molecule contains a light-sensing region known as an LOV domain because it resembles similar units in other proteins that sense light, oxygen, or voltage, said co-author and longtime collaborator Winslow Briggs, of the Carnegie Institution of Washington.
The researchers selected four species whose DNA contained genes for the structure: Brucella melitensis, B. abortus, Pseudomonas syringae (a well-studied plant pathogen), and Erythrobacter litoralis, a common bacterium in sea water. They spliced the genes into Escherichia coli, a lab-friendly bacterium that is easy to work with, and grew the bacteria a darkened lab and tracked molecular activity with radioactive tracers.
When the researchers flashed with a strobe light, the LOV domains immediately changed shape, forming a temporary bond in a process that Bogomolni likened to the opening of a hinge. When open, the hinge exposes the rest of the protein and activates it. When darkness returns, the bonds break and the LOV domain swings shut.
According to the researchers, whenever Brucella comes under attack from a host's immune system, light received by the LOV domain activates the bacterium's counter-defences, allowing it to reproduce rapidly and making it highly virulent. In the dark, without these defences activated, Brucella's division rate drops by 90 per cent.
When the researchers grew experimental Brucella in the light but with the LOV domain disabled, they found an identical drop in division rate.
Swartz and Bogomolni are still unsure as to how the regulation of virulence by light benefits the pathogen. They suspect that it allows the bacterium to determine when it is outside of its hostlike when a cow's infected foetus has been aborted and is lying in the field. By increasing its virulence under those circumstances, the bacteria are more likely to survive and infect a new host.
The researchers believe that understanding how bacteria respond to light could lead to therapeutic advances.
"We are beginning to understand what we like to call the molecular calisthenics that go on. But we still don't know what is happening at the cellular level, and that will require years of research to find out," Bogomolni said.