A new front is opening in the never-ending war between germs and humans.
As befits this information age, the battle is to control the enemy’s communications systems. The enemies in this case are potentially lethal bacteria such as E. coli, salmonella and Vibrio cholerae, the bug that causes cholera.
Researchers have discovered in recent years that these and other infectious microbes need to exchange messages with each other in order to be dangerous. They are harmless if they can’t communicate.
These organisms have developed what researchers call a “bacterial language”–a set of chemical signals that enable them to take a head count, rather like a sergeant calling the roll of his platoon. The bacteria don’t attack until they sense that their numbers are sufficient.
The messages are hormone-like molecules that certain microbes can send and receive–saying, in effect, “I’m here,” and responding, “So am I.”
Researchers figure they may be able to prevent or cure disease if they can jam the bacterial communication network–say, by blocking the apparatus that receives messages, known as a “receptor”–on the surface of the microbes.
“Some bacteria both speak and understand a common chemical language,” said Jeffrey Stein, chief scientist at Quorex Pharmaceuticals, a drug company in Carlsbad, Calif. Stein likened it to a system of wireless communication.
In a recent report in the Proceedings of the National Academy of Science, Bonnie Bassler, a molecular biologist at Princeton University, in Princeton, N.J., said E. coli and salmonella bacteria wait until their numbers reach a critical mass before they start to release the poisonous toxins that have sickened or killed hundreds of people who ate contaminated food.
“If bacteria started producing toxins as soon as the infection began, it would be like waving a flag to alert the host’s immune system,” Bassler wrote. “If the bacteria are in small numbers, they don’t stand a chance, but if they wait until they reach high cell densities, then they have a much better chance of establishing an infection.”
Scientists call this bacterial communication system “quorum sensing.” That’s because it works a bit like a quorum in human society, where it takes a certain number of people to get a meeting going.
“Quorum sensing enables bacteria to coordinate their behavior, to act like multi-cellular organisms and to acquire the benefits of cooperative activity,” Bassler said.
New weapons and tactics to counter infectious microorganisms are becoming crucial because these little creatures keep developing resistance to existing drugs. For example, one such microbe, Staphylococcus aureus, resists all but one potent antibiotic, vancomycin, and even that line of defense is crumbling.
“There is a lot of interest in new drugs that turn off that [staphylococcus] system,” said Stein, whose company is working to develop and patent such remedies. “We’re developing compounds that interfere with molecular signaling [by] turning off receptors. This is a new concept, a fundamentally new class of anti-microbial tools.”
The phenomenon of signaling molecules was discovered in the 1970s in two sea-dwelling bacteria, Vibrio fischeri and Vibrio harveyi, which emit a blue glow when their population reaches a certain density.
Since then, more than 30 species of bacteria have been found to exchange messages this way.
In addition to its potential for preventing disease, the discovery of bacterial communication may help explain how complex, many-celled organisms developed.
For its first 3 billion years, Earth was inhabited by single-celled microorganisms such as bacteria. Finally, about 600 million years ago, cells began to join in larger unions, such as algae and fungi, presumably because they were better able to find food and protect themselves.
“Multi-cellular organisms need to signal each other to cooperate and coordinate their defenses,” Stein said.
According to biology professor Barrie Bycroft of Nottingham University, in Nottingham, England, “chemical signaling between single-celled organisms is likely to have been a key evolutionary step in the development of all multi-cellular higher organisms.”
In a sense, therefore, modern civilization–with all its complex laws and rules–can trace its ancestry back to those little quorum-sensing bacteria, which learned to cooperate and coordinate their activities for their common good.
