Imagine looking down through a microscope and seeing a big mass of bacterial cells, writhing in sync, churning in circles. You can almost hear a buzz of activity. The micron-sized organisms migrate across a plate of agar, gobbling up the nutrient-rich media, recalling the frenetic activity of bees in a hive.
What you see through the microscope is called “bacterial swarming”: a phenomenon unique to a few types of bacteria, where they move in unison across a solid medium at higher speed than normal, in patterns that resemble turbulent movement seen on much bigger scales, like in smoke or flowing water. Swarming is not merely a type of behavior—in certain cases, swarming bacteria become larger in size, secrete a slimy surfactant, or grow more than one flagellum, the wiry, tail-like structure that helps them move. The surfactant reduces surface tension and creates a fluid interface above the solid nutrient agar, which helps the bacteria to move quickly. Bacteria normally swim under the influence of chemicals, pursuing attractive ones and avoiding distasteful ones. While swarming, bacteria need no chemical carrots or sticks; the physical and behavioral changes allow them to glide across plate, sometimes creating beautiful patterns.
While in a swarm, bacteria attain a level of coordination and apparent cooperation that’s unusual for unicellular life. Like the bees in a hive, the cells in a swarm assume some degree of specialization. “Leader cells” at the edge of the swarm are morphologically different from those in the interior. In rod-shaped E.Coli, these cells forgo replication, become more elongated, and line up with each other. Bacteria at the interior of the swarm are constantly hustling to squeeze through and move towards the edge of the colony. The beating flagella provide the mechanical force for the bacteria to move. As they jostle for limited space, these bacterial cells collide with each other, and the collisions align the cells such that they all move in a cohesive manner. Some researchers have also hypothesized that the collisions make the system chaotic and have the potential to generate tiny vortices—properties that are associated with turbulence on the macroscopic scale.
Scientists still don’t know why exactly bacteria swarm. Recent studies have suggested that it helps protect against both antibiotics and other, competing microbial communities. Researchers have also found that some swarming interfere with the formation of biofilms—tough, slimy agglomerations of bacterial cells—and that this connection may be used to prevent certain nasty bacterial infections. . When a bacteria like P.aeruginosa forms a biofilm, the individual bacteria become more tolerant to antibiotics and the biofilm matrix makes it harder for certain antibiotics to penetrate and act on the entire colony. But bacteria like P.aeruginosa have evolved to make a tradeoff and choose to either swarm or produce a biofilm. In a recent study, researchers created the right conditions for these bacteria to swarm in large quantities, and found that it impaired their biofilm-producing prowess. This has encouraged researchers to investigate how to coax the bacteria to swarm so it will be less likely to form biofilms and cause infection.
Daniel Kearns, a microbiologist at Indiana University in Bloomington says that the number of swarming bacteria known to science is “grossly underestimated”. He says that over time, lab researchers have grown and cultured mostly those bacteria that don’t show swarming characteristics, because it is very difficult to study individual bacterial colonies when they are collectively swarming towards the edge of the nutrient plate; in fact, researchers had done everything they could to inhibit such behavior. Hence the number of bacteria that can actually swarm in nature is much larger than what we currently know. There may be a lot more yet to learn about why and how bacteria engage in this unusual behavior.
Manasi Vaidya is a science journalist based in New York City who covers biology and health. Follow her on Twitter at @manasivaidya22.