The adsorption events and movements of individual E. coli and B. subtilis bacteria were detected in real time using ultramicroelectrodes(UMEs). Electrochemical detection of single bacteria was enabled by their inhibition of diffusive flux of a redox species when near the UME. With the UME biased for steady state oxidation of the redox species, each bacteria-electrode collision event at the UME resulted in a transient in the oxidation current. The two bacteria species show differences in their collision transient shapes, with E. coli showing "step" shaped transients and B. subtilis showing predominantly "blip" (spike) shaped transients. Simultaneous fluorescence microscopy of the UME surface during bacteria detection experiments allowed each current transient in the electrochemical data to be correlated with a specific collision event of fluorescently labeled bacteria. It was established that step-shaped transients observed for E. coli were due to adsorption of bacteria to the UME surface, while blip-shaped transients observed for B. subtilis were due to short-lived bounce events of the bacteria. The frequency of recorded transients varied linearly over bacteria concentrations of 1-10 fM for E. coli and 15-60 fM for B. subtilis, establishing the blocking collision method as a route for in situ determinations of bacteria concentration. This work demonstrates for the first time differences in blocking collision transient shapes between two bacterial analytes, and suggests applications of the method for selective electrochemical bacteria sensing. (c) 2018 Elsevier Ltd. All rights reserved.