Abstract: The present invention relates to antimicrobial compositions and methods for their use. In particular, the compositions comprise an antimicrobial agent and a nanostructured liquid crystal carrier, wherein the antimicrobial agent is contained within the nanostructured liquid crystal carrier, and wherein the nanostructured liquid crystal carrier potentiates the activity of the antimicrobial agent. Antimicrobial agents encompassed by the present invention include cationic antibiotics, antimicrobial peptides, and antifungal agents. Nanostructured liquid crystal carriers encompassed by the present invention include those formed from an amphiphilic lipid such as monoolein and phytantriol. Antimicrobial compositions encompassed by the present invention can be used for the treatment or prevention of a microbial infection, such as that caused by a Gram-negative bacterium, including where the microbial infection forms part of a biofilm.