Originally trained as a biochemist at Johns Hopkins University, the blue-eyed, athletic Bassler walked into a lecture hall on a whim in the late 1980s to listen to a talk by geneticist Michael Silverman of the Agouron Institute in La Jolla, California. It was one of only a handful of talks that the notoriously reserved Silverman had given in 10 years. Bassler was riveted by what she heard. Silverman talked about how bacteria make light inside the inch-long luminescent squid that live in the shallow waters off the Hawaiian coast (see “Aliens of the Sea,” page 76). Infant squid cannot glow until they excrete a mucuslike net to entrap the ubiquitous luminescent bacteria floating in the water. The squid draw captured bacteria into their “light pouches,” where the bacteria are bathed in nutrients —a diet richer than what they can find outside in the sea. In return, the bacteria (Vibrio fischeri, a close relative of the cholera germ) produce a dim blue-green light that is directed downward through small reflective organs in the squid to shine into the water below. When the squid swim at the ocean surface at night, hunting for shrimp, they are invisible to predators below because they look like moonlight on the water. Both squid and bacteria benefit. “The host wants the light, the bacteria get fed,” Bassler says.
The glow of V. fischeri provides an instructive glimpse into the communal behavior of bacteria. Autoinducers (chemical signaling molecules that produce more of themselves inside the cell) control the switch that turns the light genes off and on. Each bacterium secretes a bit of this light-evoking substance into the environment. When a crowd of bacteria and their autoinducers become dense enough, the lights in all the bacteria switch on at once. “This counting of heads is called quorum sensing,” Bassler explains. More broadly, this is how bacteria coordinate their actions in large groups: When the local concentration of autoinducers gets high enough, the bacteria know a crowd is present, and they flip over from solitary mode to group behavior.
The autoinducer molecule that triggers bacterial glow is made by a protein called LuxI, which has a very focused effect. “The molecule that the LuxI protein makes is acylated homoserine lactone, or AHL,” Bassler says. “Each LuxI protein and the molecule it produces is species-specific. There are two kinds of bacteria, and each talks in a different language. Gram-negative bacteria [which have a thin cell wall surrounded by an outer membrane] use the AHLs as autoinducers, while gram-positives [which have a thick cell wall] use peptides. This is a very ancient split.” When the V. fischeri make enough AHL autoinducer—called AI-1 for short—the cells wink on. But that is far from the only autoinducer.
Working with a related bacterium, Vibrio harveyi, in the early 1990s, Bassler discovered another kind of chemical signal that a wide range of bacteria emit. In many species this chemical, called autoinducer 2 (AI-2) has properties of a waste product, says molecular biologist Stephen Winans of Cornell University. AI-2 is the by-product of a complex process of metabolism in these species. Not all bacteria create AI-2, however. According to Winans, eons ago one line of early bacteria began to break down waste products along a pathway leading to the excretion of AI-2; another line did not. The latter are the bacteria that eventually gave rise to eukaryotic organisms, including humans. “That’s why you don’t excrete AI-2,” Winans says.
But Bassler found that AI-2 is much more than a waste product. “This little leftover molecule,” she says, got pressed into service as another bacterial language, one that can carry messages between different kinds of germs. Most forms of quorum sensing, including V. fischeri’s luminescence circuit, act as a private language—that is, each germ speaks only to others of its own kind. But AI-2 is a kind of bacterial Esperanto, Bassler determined. After she and her team purified the small AI-2 molecule and its protein receptor, they were able to show that the two form a lock-and-key structure, the telltale sign of a chemical signaling mechanism.
The big question was, what are different germs saying when they talk to each other? Bassler says that in some instances—such as in dental biofilms, in which some 600 species may be growing at a time—AI-2 is necessary for collective or cooperative behavior. First, though, the bacteria must be right next to each other to receive the signal, especially in a dynamic system like the mouth, where saliva is constantly washing across the teeth. The earliest colonists on freshly cleaned teeth, the streptococci, produce only low levels of AI-2; the fusobacteria produce moderate levels. The appallingly destructive germs love a very high level of AI-2, which sends them into overdrive. “They grow like gangbusters,” says Paul Kolenbrander of the National Institute of Dental and Craniofacial Research of the National Institutes of Health.
Quorum-sensing molecules also play an important part in bacterial virulence, or deadliness. If a lethal germ released toxic chemicals immediately after entering the host’s body, the immune system would quickly sense the toxin and go after the invader. So it pays for bacteria to wait, stealthily multiplying until the unwitting host is full of them. Then they can release their toxins all at once, overwhelming immunity and sickening or killing the host.
In their more recent work, Bassler and her colleagues are searching for ways to scramble the quorum-sensing signals of cholera germs. The researchers have demonstrated that in test tubes a particular chemical, called CAI-1, can induce deadly cholera cells to turn off their virulence genes.
Building on our understanding of how germs communicate, Naomi Balaban, a molecular biologist at Tufts University, has spent 17 years studying Staphylococcus aureus, a strain of bacterium that is the main cause of hospital-acquired infections. Antibiotic-resistant forms of S. aureus, known collectively as methicillin-resistant Staphylococcus aureus, or MRSA, have spread widely in hospitals throughout the world, forming long chains of infection. There are 19,000 MRSA-associated deaths in the United States alone each year.
Other forms of MRSA have begun to spread outside of hospitals; one strain, known as USA300, is especially deadly. It has infected and killed children and athletes, and no one knows where it came from or exactly how it spreads, though athletic locker rooms have been implicated in some cases. Like other forms of staph, USA300 can form invisible biofilms outside the body, making it almost impossible to eradicate. It is difficult to judge the actual prevalence of MRSA, since many staph infections do not get much more serious than a small pimple. Some cases do progress, though, and they may cause debilitating and almost untreatable soft-tissue infections like cellulitis and folliculitis, pneumonia, and often-fatal heart infections, or endocarditis. Another form of staph, Staphylococcus epidermidis, grows commonly in sheets of invisible biofilm on our skin, where it is normally benign. But if it is introduced into the body during a medical procedure—especially if a joint implant, catheter, or pacemaker is contaminated during insertion—both S. epidermidis and S. aureus can form dangerous biofilms that often cannot be treated without removal of the infected implant.
Balaban has discovered that all forms of staph, whether in a free-floating state or in a biofilm, have a complex form of chemical communication that can activate the agr (accessory gene regulator) system, producing a number of toxins. Somewhat controversially, Balaban also claims to have discovered another system that controls the agr system. The second system involves two proteins known as RNAIII activating protein (RAP) and TRAP, which Balaban calls “the most beautiful protein in the world.” TRAP is RAP’s target protein, Balaban says. It is found both on and within the staph cell. S. aureus secretes RAP into the environment, where the chemical collects and binds to the TRAP molecules on the cells. When enough RAP molecules adhere to enough target molecules, staph bacteria switch on their cell-to-cell communication and stress-response systems and begin producing the toxin that makes them so lethal. S. aureus bacteria, depending on their strain, can produce 40 or more different toxins. The toxins break down the cells in the host—which could very well be you—in order to release nutrients to the germs. That is why staph infections can be so destructive. When there are enough staph germs present, the host’s immune system is overwhelmed, and tissues are destroyed at a frightening rate, leading sometimes to shock and death.