Quorum sensing

When the Hawaiian bobtail squid rises from the sand each evening to hunt, it swims cloaked from detection by predators and prey because it casts no moonlight shadow, a trick it owes to compartments full of glowing bacteria on its underside. It tunes these mini-floodlights to the right intensity using light sensors on its back, adjusting a shutter on its body to match the moonlight perfectly (1).

Although this example of symbiosis is a marvel on its own, it yielded an insight far more profound as scientists looked closer: that bacteria, seemingly the most basic and solitary of life forms, are, in fact, communicating with each other. They are counting themselves, their cousin species, and the unrelated “others” in their vicinity and changing their behavior as a group in response to the results of this census. Researchers who study this behavior, known as quorum sensing, believe this ability is the origin of multicellularity and communal behavior. It may also yield important practical benefits, such as new approaches for combating drug-resistant bacterial infections in humans.

One of the first insights into quorum sensing came from J. Woodland Hastings of Harvard University, who noticed that Vibrio fischeri, the bacteria that live in the squid, and a related species, Vibrio harveyi, would not light up until they reached a certain threshold population density, thus avoiding wasting their resources on making light when illumination would be ineffective (2). (Indeed, the squid dumps most of its bacteria at dawn when it buries itself again, and the bacteria spend the day rebuilding their quorum to relight at night.)

As researchers probed deeper, they discovered the molecular mechanisms behind the phenomenon. Bacteria produce and detect small signal molecules with a specificity that allows them to distinguish and count their own, their cousins, and outsiders. When their receptors detect enough of a particular signal—indicating that sufficient cells are present—they trigger a response cascade, turning on or off a set of perhaps 200 genes, many of which have unknown function.

Advances in molecular biology tools have allowed researchers to monitor behaviors more subtle than bright blue luminescence. This ability has revealed that such “talk” is the rule rather than the exception in bacterial life (3). Bacteria control traits including antibiotic production, nutrient storage, the formation of spores, and more using quorum sensing.

Researchers have even found examples of evolutionary cat-and-mouse in the bacterial messages. Some bacteria have evolved to degrade other species’ signaling molecules to mess up the count. Some have cloaked themselves so that others cannot detect them, and some have evolved quorum-sensing signals made of antibiotics to which they have resistance—great for counting themselves but lethal to other species.

One of the key behaviors bacteria invoke in response to a quorum of their own species is to mount a pathogenic attack—turning on genes that produce toxins, lyse other cells, and otherwise cause infection. Researchers therefore reason that drugs that target quorum-sensing behavior could be the next generation of antibiotics, especially for antibiotic-resistant infections. One way this might work is to deliver a drug that jams the quorum-sensing system so the attack is never triggered, buying time for the immune system to respond to the bacteria.

Such applied questions are at the frontier of the field, along with researchers’ growing understanding of the extent and diversity of quorum sensing throughout the microbial world—far beyond an ethereally glowing squid.