In recent years, magnesium alloys have attracted significant attention as a new class of biodegradable implant materials, because of their superior biocompatibility and low modulus. We describe here the inhibition of bacterial colonisation and biofilm formation on surfaces of Mg-2Zn-2Gd alloy with different grain structures (~44 µm to ~710 nm) obtained via multiaxial forging. Surface energy and contact angle measurements using goniometer and wettability were assessed with water, SBF, n-Hexane and DMEM. The higher surface energy of ultrafine-grained Mg-2Zn-2Gd alloy led to the release of more Mg+2 ions at an early stage, which consequently increased the pH of fluid in the vicinity of the implant, therefore producing unfavourable environment for the survival of Escherichia coli (E. coli) bacteria. This led to damage of bacterial cell walls thereby reducing their adhesion. In summary, the study underscores that grain size had a significant impact on antimicrobial behaviour. © 2016 Informa UK Limited, trading as Taylor & Francis Group.