Figure 1.

Chemotactic excitation responses of swimming bacteria to repetitive step stimuli of photo-released aspartate. Arrows denote photolyzing near-UV light flash from the xenon flash lamp. (A) Change in aspartate concentration over the video microscope observation area with time. Each point represents the mean (±SE) of values from three independent caged HPTS photolysis experiments. Changes in fluorescence intensity were converted to estimates of photo-released aspartate (Methods) (B) Motile responses of E. coli, as measured by changes in population rcd. The bacterial population was allowed to adapt back to rcd_pre (dashed dotted line) between flashes. Dotted lines indicate the rcd for complete smooth swimming, obtained after mixing with the potent attractant l-serine (1 mM) (16).