With all that exercise, Bassler is athlete trim. She curls her legs up under her on the chair when she talks, and when she wants to make an important point she leans in and whispers, as if confiding a secret. Or she stands up and shouts, if that's what it takes.

Her communication skills serve Bassler well in her teaching, something she considers an essential part of her work. She is director of graduate studies in the molecular biology department at Princeton and also teaches the molecular biology course for non-science majors. "For many of these kids, it's the first class they've taken that isn't subjective," she says. Unlike literature classes or creative writing workshops, in science—she teaches them—"there's a right answer and a wrong answer." Without a class like hers, she says, many of them may graduate from college without understanding that or anything about science. And that, in turn, will make it more difficult for them to address scientific debates as adults, and as citizens.

Back when Bassler was working on barnacles, while at Johns Hopkins University, she attended a meeting of Navy-sponsored researchers where Mike Silverman, a geneticist at the Agouron Institute, a nonprofit research organization in La Jolla, California, presented his research.

Silverman's focus was a marine bacterium called Vibrio fischeri, which has the ability to light up, or bioluminesce. It is found, among other places, in the "light organ" on the underside of the bobtail squid, with which the bacteria live symbiotically. The squid hunts at night in shallow water, and when moonlight threatens to make it visible to predators by virtue of its shadow on the sand below, the bacteria bioluminesce and the light organ glows. Goodbye shadow.

This clever masquerade depends on an unusual property of V. fischeri, discovered in the 1970s. When grown in the laboratory, the bacteria don't glow until their population passes a critical threshold. Then they light up simultaneously.

But if bacteria are loners, seeking nothing more than nutrients and an opportunity to reproduce, how do they know their population has exceeded the threshold to light up and then do so in concert?

Silverman figured out that this light-emitting behavior represents an amazing feat of self-recognition on the part of the bacteria. He discovered the mechanism underlying how the bacteria produce and release a chemical signal that their fellows can detect. As the population grows, this chemical accumulates, and the bacteria detect it when it reaches a certain peak. When the level gets high enough, the lights go on.

The general reaction at that Navy research meeting, Bassler recalls, was "So what?" His findings were seen as an oddity in an odd organism, nothing more. But Bassler was fascinated. "I was a biochemist, Silverman was a geneticist. I didn't know any molecular biology or genetics. I didn't really know what a gene or a transcription factor was—nothing! But I knew I wanted to work on these Vibrios. I ran up to him—literally ran—after his talk, and said, 'You have to take me on as your postdoc.'" Despite her lack of experience, he did take her on. "I still don't understand that," she says, laughing.

Silverman, now retired in Jackson Hole, Wyoming, remembers it differently. "She came from a good lab, and I ran a small operation and I needed her," he says. "From the time she arrived, she worked hard. I would come into the lab at night, and there she was. And she would be there again in the morning." She soon asked for more responsibility, and he gave it to her. "In Wyoming, they say give a horse its head—let it run," he says.

In Silverman's lab, she began working on Vibrio harveyi, a species closely related to V. fischeri and also bioluminescent—again, as long as its population is large enough. Bassler's job was to learn how the bacteria produce and monitor the population-indicating chemical signal.

She started making mutants of the bacteria—to try to knock out the signal so that they wouldn't illuminate. But she couldn't get them to go dark. Sometimes they were dim, but they always lit up. That led to the first of a series of discoveries that, she says, were then difficult to imagine—but seem perfectly obvious in hindsight. V. harveyi, it turned out, has not one chemical signal but two. "You knock one out and the other one works, and they still light up," she says.

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