A sulfonamide antibiotic; inhibits growth of E. coli (MIC = 10 μg/ml) and clinical MRSA isolates (MICs = 25-50 μg/ml); inhibits growth of MRSA in vivo in mice when administered in combination with trimethoprim (MIC = 0.8 μg/ml, ED50s = 6.4 and 9.6 mg/kg for two MRSA strains); acts by inhibiting DHPS; inhibits recombinant P. carinii DHPS (IC50 = 23 nM; Ki = 7.5 nM) and folate biosynthesis in situ by 48.6%; decreases recurrent infection in a mouse model of urinary tract infection with E. coli when administered in combination with trimethoprim