Current interest in antimicrobial coatings is driven by an urgent need for more effective strategies to control microbial infection. In this study, antimicrobial nanoclays were prepared by ion-exchange of sodium montmorillonite (MMT) with silver ions which have been previously reported to exhibit biocidal activity. The extent of ion-exchange achieved was estimated by X-ray photoelectron spectroscopy. The silver-modified nanoclay (AgMMT) fully inhibited growth of Gram-negative bacterium Escherichia coli DH5 alpha (E. coli) over 24 h; annealing AgMMT under typical conditions used to prepare polymer composites did not reduce its antimicrobial efficacy. However, powder coatings of AgMMT dispersed in epoxy/polyester resin exhibited no antimicrobial effect on E. coli. This is believed to be caused by poor wetting of the polymer coating, which restricted the diffusion of silver ions from the coating.