The colour purity and versatility of fabrication of one-dimensional photonic crystals (1D PhCs) make them ideal candidates for colorimetric sensing of a variety of analytes. For instance, the detection of bacterial contaminants in food via colorimetric sensors can be highly appealing, as most of the existing detection techniques are in general time-consuming and the read-out requires specialised personnel. Here, we present a colorimetric sensor based on hybrid plasmonic/photonic 1D crystals. We demonstrate that the modification of the silver plasmon resonance brought about by the effective silver/bacterium interaction can be translated into the visible spectral region, producing a change in the structural colour. In addition, we observe a superior colorimetric sensitivity against the Gram negative Escherichia coli compared to the Gram positive Micrococcus luteus, a result that we attribute to the more efficient electrostatic interaction and cellular adhesion between the silver surface and the Gram-negative bacteria outer membrane. This approach demonstrates that in principle an easy colorimetric detection of bacterial contaminants can be achieved through the use of bio-responsive plasmonic materials, such as silver, whose selective electrostatic interaction with bacterial cell wall is well-known and occurs without the need of chemical functionalisation.