The scanning electrochemical microscope (SECM) was used to probe the redox activity of individual purple bacteria (Rhodobacter sphaeroides). The approaches developed in our previous studies of mammalian cells were expanded to measure the rates and investigate the pathway of transmembrane charge transfer in bacteria. The two groups of redox mediators (i.e., hydrophilic and hydrophobic redox species) were used to shuttle the electrons between the SECM tip electrode in solution and the redox centers inside the cell. The analysis of the dependencies of the measured rate constant on formal potential and concentration of mediator species in solution yielded information about the permeability of the outer cell membrane to different ionic species and intracellular redox properties. The maps of redox reactivity of the cell surface were obtained with a micrometer or submicrometer spatial resolution.