The present invention relates generally to the fields of bacteriology and mycology. More particularly, the present invention provides novel inhibitors of multidrug transport proteins that may be used in combination with existing antibacterial agent and/or antifungal agents to increase the toxic effects of the antimicrobial agents. More specifically, the present invention provides methods and compositions for enhancing the antibacterial action of fluoroquinolones by administering fluoroquinolones in combination with an inhibitor of multidrug transporters and of enhancing the antifungal action of azole antifungal agents by administering an azole antifungal agent in combination with an inhibitor of multidrug transporters. Compositions comprising indole, urea, quinoline or aromatic amide based inhibitors also are disclosed.