Development of Stationary Bands During Escherichia Coli Swimming

Abstract

Bacteria have evolved to have different mechanisms of motility depending on the environment. On surfaces bacteria can locomote by swarming, a phenotype that is cell density dependent and requires cell coordination. As a result of the cell coordination several strains form stationary patterns. In a low agar environment, cells swim independently through the medium. Bacteria can respond to gradients of chemoeffectors in the environment and will localize to the steepest part of the gradient, creating traveling chemotaxis rings. The formation of stationary patterns, however, is rarely reported for swimming bacteria. Here we uncover a number of mutant strains in Escherichia coli K-12 that create a stationary bull’s eye pattern in low percentage agar. This pattern consists of repeating regions of high and low cell density. We identified two mutations that can result in formation of this phenotype: one is a disruption of the transcription factor LrhA and the other is insertion of IS elements in the intergenic region between the divergently transcribed genes flhD and uspC. While mutations in these genes have been identified by others and shown to increase motility in TB swim plates at 30° C, we observed that these mutants form bull's eye patterns in swim plates made from LB and incubated at 37 °C. This pattern can form in a variety of lrhA disruptions, however, the location of the IS element in the intergenic uspC-flhD region correlates with the formation of either a bull’s pattern or chemotaxis rings. IS elements that disrupt the flhD promoter correlate with chemotaxis patterns, and those outside of the promoter region correspond with banding pattern formation. We identify the chemoreceptors necessary for pattern formation and show that the global regulator (p)ppGpp is essential for pattern formation.