Studies
of electron transfer at the population level veil the nature
of the cell itself; however, in situ probing of the electron transfer
dynamics of individual cells is still challenging. Here we propose
label-free structural color microscopy for this aim. We demonstrate
that Shewanella oneidensis MR-1 cells show unique
structural color scattering, changing with the redox state of cytochrome
complexes in the outer membrane. It enables quantitatively and noninvasive
studies of electron transfer in single microbial cells during bioelectrochemical
activities, such as extracellular electron transfer (EET) on a transparent
single-layer graphene electrode. Increasing the applied potential
leads to the associated EET current, accompanied by more oxidized
cytochromes. The high spatiotemporal resolution of the proposed method
not only demonstrates the large diversity in EET activity among microbial
cells but also reveals the subcellular asymmetric distribution of
active cytochromes in a single cell. We anticipate that it provides
a potential platform for further exploring the electron transfer mechanism
of subcellular structure.