Enzyme amplified colorimetric sensing methods provide visual readouts without the need of instrumentation. However, these methods have issues due to the enzymes high cost, instability, extraction, and purification. Recently, it has been observed that metal oxide NPs mimic many natural enzymes. Fe3O4 magnetic NPs are now known to mimic the horseradish peroxidase enzyme that oxidizes chromogenic substrates such as TMB, ABTS, OPD into colored products. In this regard, we have designed and developed dopamine-functionalized iron oxide (Dop-Fe3O4) nanoparticles and exploited their enzyme mimicking ability to develop a simple colorimetric bacterial sensing strategy. These nanoparticles catalyse the oxidation of a chromogenic substrate in the presence of H2O2 into a green colored product. The catalytic activity of the nanoparticles is inhibited in the presence of bacteria, providing naked eye detection of bacteria at 104 cfu mL−1 and by spectrophotometric detection down to 102 cfu mL−1.