Sonification of Bacterial Chemotaxis: Listening for Patterns in the Response of Swimming Bacteria to Chemical Stimuli

Chemotaxis describes the movement of a bacterial population toward or away from chemical stimuli in order to enhance their chance of survival.  Bacteria such as Escherichia coli swim about their environment through the use of flagella, tail-like appendages that protrude out of the cell. Flagella can rotate clockwise or counter-clockwise. Counter-clockwise motion propels the E. coli forward in a straight line or run, while clockwise motion results in a change of direction or tumble. This alternating series of runs and tumbles results in a random walk typical of a diffusive process.  When swimming toward increasing concentrations of a chemoattractant individual bacteria extend their runs and bias the overall migration of the population toward the chemoattractant source. Chemotaxis plays an important role in the pathogenesis of infection, nitrogen fixation in leguminous plants, and bioremediation of polluted groundwater.  Sonification is the use of sound to represent data and allows one to also hear the data rather than just see it graphically.  The human ear is trained to detect patterns and slight differences in frequency or pitch. By using the video as an “instrument” and mapping it to sound, events that might be visually chaotic may become sonically organized. This sound may give rise to patterns, allowing one to not only see bacterial chemotaxis, but to listen to it as well. Thus, sonification may be an effective way to detect a bacterial population’s response to chemical stimuli. Sonification of data can shed new light on relationships and patterns in the data that may not have been realized otherwise. Time lapse images using dark field microscopy of an HCB1 E. coli population were taken.  The bacterial responses to varying concentrations of α-methylaspartate, a chemoattractant, were captured.  These time lapse images were then sonified by mapping position to frequency of sound.  Sound maps of the data reflected the presence of varying concentrations of attractant.  Sound maps showed the change in frequency of the “turns” and runs of the bacteria.  As the bacteria “turned” less often with an increase in attractant, the sound maps became sparser.  These preliminary results demonstrate sonification as a method to screen for chemotactic responses and the promise to study bacterial responses to a wide array of chemical stimuli in real time.