On Tomorrow’s Table, Pamela Ronald shares a breakthrough in the study of bacterial communication. Although bacteria have been known to use a limited chemical vocabulary, for the first time they have been observed to use a protein as a signalling mechanism. Ronald writes, “Ax21 is a small protein. It is made inside the bacterial cell, processed to generate a shorter signal and then secreted outside the bacterium.” In the species studied, perception of Ax21 caused nearly 500 genes—ten percent of the bacterium’s genome—to change expression. Thus galvanized, individual bacteria assemble into “elaborate protective bunkers” called biofilms, producing “a virulent arsenal including ‘effectors’ that are shot directly into the host to disrupt host defenses.” As Ronald says, “this process transforms the bacteria from a benign organism to a fierce invader.” While the bacterium in question infects a rice plant, Ax21 and similar proteins may play the same role in other pathogens—including those that infect animals and humans. But we multicellular types are not defenseless: Ronald and her team have shown, for the first time, that a host’s immune receptors can overhear the microbial call-to-arms and prepare for war. 2011 Nobel Laureate Bruce Beutler also discusses Ronald’s work as it relates to his own, beginning at 40:45 in his Nobel Lecture of Dec. 7.