A pharmaceutical composition comprising an antimicrobial agent and an enhancer of an antimicrobial agent, wherein the enhancer of an antimicrobial agent is an inhibitor of gene, that by inactivating the gene product potentiates the effectiveness of the antimicrobial agent. In some embodiments, the pharmaceutical composition further comprises a pharmaceutically acceptable carrier. In some embodiments, the antimicrobial agent is an antimicrobial peptide such as a polymyxin, for example but not limited to colistin. In some embodiments of the present invention provides methods to treat and/or prevent infection of a subject with a microorganism by administering a pharmaceutical composition comprising an antimicrobial agent and an enhancer of an antimicrobial agent. In some embodiments, the present invention provides methods to inhibit growth of a microorganism by administering a pharmaceutical composition comprising an antimicrobial agent and an enhancer of an antimicrobial agent.