How bacteria create flower art

When sticky bacteria meet roaming bacteria in a petri dish, friction between the two can cause flower patterns to blossom.

Escherichia coli bacteria growing on a substance similar to Jell-O called agar tend to stick to the surface, and colonies of the microbes don’t spread very far. But colonies of Acinetobacter baylyi expand in rapidly growing circles as the bacteria crawl on hairlike pili over the agar’s surface. Neither type of microbe is very exciting to look at on its own, says Lev Tsimring, a theoretical physicist at the University of California, San Diego. But “when we mixed them together, we saw these absolutely mind-blowing structures growing.”

Physical interactions between the two types of bacteria create floral patterns, he and colleagues found. Mobile A. baylyi “pushes E. coli in front of it, sort of like a snowplow,” Tsimring says. But sticky E. coli dig in their heels, holding back a wave of A. baylyi like an elastic band wrapped around a balloon, he says. In some places where there are fewer E. coli forming a barrier, the more agile bacteria break through, painting petals as they shove their reluctant neighbors forward. Those breakthroughs tend to happen at fairly regular intervals, creating relatively symmetrical blossoms.

The petal shape that forms depends on how fast A. baylyi bacteria move, how well E. coli is stuck to the surface and the proportions of each type of bacteria at the colony edges. (E. coli must outnumber A. baylyi or the speedy bacteria will blow right past their more sedentary partners.) Tsimring’s team described the math behind the blooms January 14 in eLife.

Such equations have been used to explain how reactions between chemicals might produce Turing patterns — regular, repeating patterns found in nature, such as spots or stripes on animals’ coats (SN: 12/21/15). The new research shows that, in addition to chemicals, scientists should consider how mechanical forces help shape patterns too, Tsimring says.

Understanding how bacteria grow in mixed company may shed light on how biofilms form, enabling researchers to devise better ways to disrupt these communities composed of different types of microbes. Some biofilms have been linked to stubborn infections (SN: 5/20/16).