Bacterial infections present one of the leading causes of mortality worldwide, resulting in an urgent need for sensitive, selective, cost-efficient, and easy-to-handle technologies to rapidly detect contaminations and infections with pathogens. The presented research reports a fully functional chemical-detection principle, addressing all of the above-mentioned requirements for a successful biosensing device. With the examples of Escherichia coli and Neisseria gonorrheae, we present an electrochemical biosensor based on the bacterial expression of cytochrome c oxidase for the selective detection of clinically relevant concentrations within seconds after pathogen immobilization. The generality of the biochemical reaction, as well as the easy substitution of target-specific antibodies make this concept applicable to a large number of different pathogenic bacteria. The successful transfer of this semidirect detection principle onto inexpensive, screen-printed electrodes for portable devices represents a potential major advance in the field of biosensor development.