|Abstract||Antibacterial action of micro- and nanoscale ZnO is well-documented and has been thoroughly studied in recent years. Although many studies hypothesize several possible mechanisms involved, the fundamental nature of interactions leading to ZnO inhibiting bacterial growth are still not well identified. We investigated the nature of interactions between ZnO crystal surfaces and extracellular material putatively involved in bacterial growth inhibition. In this study, we used hydrothermally-grown ZnO microcrystals and commercial grade ZnO nanopowders as antibacterial agents against Staphylococcus aureus bacteria utilizing antimicrobial assays, optoelectronic probes, and electron microscopy. MIC and killing curve assays demonstrated that internalization of ZnO particles by the bacteria is not necessary for bacterial growth inhibition. Furthermore, while physical contact between bacteria cells and ZnO particles contributes to growth inhibition, antibacterial action is still observed without physical contact. These assays also revealed that the composition of growth media heavily influences the antibacterial action of ZnO. In conjunction with biological assays, electron microscopy of ZnO samples revealed that both bacteria and phosphate-rich growth media lead to the formation of new crystalline phases. Bacterial environments also damage ZnO crystalline surfaces. Photoluminescence and surface photovoltage experiments revealed substantial changes in ZnO excitonic behavior and surface optoelectronic process after interactions with growth media as well as bacteria. S. aureus interact with ZnO surfaces in a substantially different manner than any of the growth media used. Results of these optoelectronic studies agree with the observations from the biological assays.