Vancomycin is mainly used as an antibacterial agent of last resort, but recently vancomycin-resistant bacterial strains have been emerging. Although new antimicrobials have been developed in order to overcome drug-resistant bacteria, many are structurally complex beta-lactams or quinolones. In this study, we aimed to create new anti-drug-resistance antibacterials which can be synthesized in a few steps from inexpensive starting materials. Since sulfa drugs function as p-aminobenzoic acid mimics and inhibit dihydropteroate synthase (DHPS) in the folate pathway, we hypothesized that sulfa derivatives would act as folate metabolite-mimics and inhibit bacterial folate metabolism. Screening of our sulfonanilide libraries, including benzenesulfonanilide-type cyclooxygenase-1-selective inhibitors, led us to discover benzenesulfonanilides with potent anti-methicillin-resistant Staphylococcus aureus (MRSA)/vancomycin-resistant Enterococcus (VRE) activity, that is, N-3,5-bis(trifluoromethyl) phenyl-3,5-dichlorobenzenesulfonanilide (16b) [MIC = 0.5 mu g/mL (MRSA), 1.0 mu g/mL (VRE)], and 3,5-bis(trifluoromethyl)-N-(3,5-dichlorophenyl) benzenesulfonanilide (16c) [MIC = 0.5 mu g/mL (MRSA), 1.0 mu g/mL (VRE)]. These compounds are more active than vancomycin [MIC = 2.0 mu g/mL (MRSA), 125 mu g/mL (VRE)], but do not possess an amino group, which is essential for DHPS inhibition by sulfa drugs. These results suggested that the mechanism of antibacterial action of compounds 16b and 16c is different from that of sulfa drugs. We also confirmed the activity of these compounds against clinical isolates of Gram-positive bacteria. (C) 2008 Elsevier Ltd. All rights reserved.