Zinc oxide nanoparticles (ZnO NPs) induce morphological transformation of Escherichia coli from its native rod-shape of ≈2–4 μm to filamentous cells of 20–40 μm in length. The transient response can only be observed at up to 3.5 h proliferation, beyond which the cytotoxic effect is neutralized and the rod-shape is restored. The filamentation is part of the bacterium SOS response to the Trojan horse-type internalization of undissolved ZnO solids. In the absence of ZnO solids, no cell filamentation can be observed from the leached soluble zinc fraction or dissolved zinc salt.