The social lives of bacteria
In the early 1990s, a team of physicists led by Eshel Ben-Jacob at Tel Aviv University discovered a species of soil bacteria with a startling ability. The bacterium, named Paenibacillus vortex, is not a simple, solitary organism. When cultured on a semi-solid agar surface in a petri dish, populations of these microbes, numbering from 100 billion to a trillion cells, begin to act as a single, coordinated entity. They form vast colonies with branching architectures that can span the entire dish.
The collective behavior depends on the vortices—dense, swirling aggregates of bacteria that rotate around a common center. These vortices, each a bright yellow dot in a stained colony, act as the building blocks and foragers for the larger structure. Driven by their flagella, the individual bacteria within a vortex move at speeds of about 10 micrometers per second. The entire group rotates either clockwise or counter-clockwise, pushing the colony's boundaries outward in search of nutrients. This swarming intelligence allows the colony to expand efficiently and even split and reunite to navigate around obstacles or exploit scattered food sources.
From microbes to matter
Physicists study the patterns of P. vortex as a model for "active matter," collections of individual agents that consume energy to move and exert forces on each other. Though vastly different in scale, the principles governing the bacterial colony have parallels in other complex systems, from flocks of birds to the formation of galaxies. The bacteria follow simple rules based on local interactions. They communicate through chemical signals and physical links to form order from decentralized, individual actions.
The patterns are highly sensitive to environmental conditions. Ben-Jacob's lab found that by altering the toughness of the agar or introducing stresses like temperature swings, they could induce the bacteria to form different architectures. Analysis of the P. vortex genome revealed an unusually high number of genes related to two-component systems and transcription factors. This genetic toolkit allows the bacteria to perceive a wide range of environmental signals and alter their collective behavior in response, showing a form of microbial sociality. The entire colony is a multi-cellular organism, with different groups of cells performing specialized tasks to ensure the survival of the whole.