Photodynamic control of bacterial motility by means of azobenzene molecules

Abstract

We demonstrate that azobenzene dye molecules, such as Methyl Orange (MO) and Acid Yellow 9 (AY9), can be used to dynamically control the motility of flagellar bacteria E. coli. We show that the speed of bacteria can be changed by light up to +23% and -45%, for different dyes. Indeed, these molecules have opposite effect: when photoexcited, the bacterial speed increases for MO and decreases for AY9. In order to understand the corresponding mechanism, the concentration dependence, toxicity, viscosity and pH of used solutions were analyzed. The viscosity and pH were found to be unchanged for both molecules in all photoexcitation conditions. The toxicity of the irradiated state of MO was found to be lower compared to its state in the dark at equilibrium. In contrast, for AY9, both states were nontoxic at the fixed pH 7, but its irradiated state was less toxic when the pH dropped with the increase of its concentration (due to the acid effect of the molecule). Our results show that the chemical interaction with bacteria is at the origin of the results obtained with MO, while the proton pump inhibition could be responsible for those with AY9.