Two stationary kinds of bacteria can move when mixed

WASHINGTON — Strand a fish on a tree stump, and neither swims away. But mixing two kinds of soil bacteria that are stationary on dry surfaces allows the combo — by means not yet clear— to expand unusually quickly, multiplying and oozing as a colony across a firm laboratory agar surface.

Over generations, the pair gets faster, Lucy McCully of the University of Massachusetts Dartmouth said August 6 at the 2nd American Society for Microbiology Conference on Experimental Microbial Evolution. She and coauthor Mark Silby are interested in what happens when bacterial species mingle and evolve together. Bacteria often get studied in colonies of single kinds, but real soil is a stew of many ingredients.

In McCully’s lab tests, neither Pseudomonas fluorescens Pf0-1 nor Pedobacter sp. V48 can move much at all without water. The Pseudomonas has a swimmer’s flagella but the researchers see no evidence of mobility in Pedobacter. Yet when colonies overlap on a lab dish, they spawn combinations that balloon out unusually fast. Such speed suggests that the movement is not due to mere multiplication.

Over generations of commingling, evolution in one partner matters to the other. Researchers know this because combining one of the new speedy forms with the ancestral version of its partner, lowers the speed. In repetitions of the pairing, the Pedobacter develops the speed boost first and then something in Pseudomonas changes, enhancing speed even more.