Sulphadiazine belongs to sulfonamides that inhibit enzymes in the folic acid pathway, thereby inhibiting bacterial thymidine synthesis. The sulfonamide blocks the conversion of para-aminobenzoic acid (PABA) to dihydrofolic acid (DFA). Trimethoprim blocks the conversion of DFA to tetrahydrofolic acid by inhibiting dihydrofolate reductase. Sulfonamides have a broad-spectrum antimicrobial effect against Gram-positive bacteria including most Streptococcus spp., Staphylococcus spp., Actinomyces spp. and against many gram-negative organisms including Brucella spp., E. coli, Haemophilus spp., Klebsiella spp., Pasteurella spp., Proteus spp., Salmonella spp., Actinobacillus spp.