Effects of chaotic fluid mixing on the blooming of microbe colonies

We present experiments on the blooming behavior of colonies of bacteria in a time-periodic, oscillating vortex array flow. The flow gives rise to chaotic mixing with coexisting regions of ordered and chaotic regions; the size of the “islands” of ordered behavior depends on the frequency and amplitude of the time-dependent perturbation. The microbes used are B. subtilis and E. coli, both with the GFP mutation to enable fluorescence imaging. The fluid is LB broth with or without methyl cellulose (used for slow flows), and the entire system is held at a temperature of approximately 37 degrees C. With imposed flows, the bacteria typically bloom in 5-7 hours. We discuss how the blooming patterns relate to the mixing in the flow, particularly the structure of the chaotic and ordered regions in the flow. We also explore cases in which the bacterial colonies form streamers and consider how the streamer patterns relate to the fluid mixing.