Bacterial range expansion and the Fisher speed: a discrepancy in nutrient-rich media

Abstract

Bacterial motility, while one of the most well-studied phenomena in microbiology, is still poorly understood in its relation to macroscopic phenomena such as range expansion. In particular, the Fisher speed, a theoretical relation of macroscopic motility to planktonic (cellular) motility, has not been rigorously tested in chemotactic bacteria. Both the swimming of individual bacteria and the bacterial colony’s overall spread were observed in five species of bacteria in low-density agar using light sheet fluorescence microscopy and cell phone imaging, respectively. Subsequent image analysis indicated a discrepancy between the predicted Fisher speed and the observed range expansion speed. These results are in agreement with recent findings for the commonly studied bacterium E. coli, and can likely be explained by chemotactic effects driving macroscopic motility. These results may be important in understanding how bacteria can colonize complex structured environments such as the vertebrate gut.