Sulphadiazine is the most active sulphonamide against gram +ve (staphylococci and streptococci) and a wide range of gram -ve organisms (Pasteurella, Proteus and pseudomonas.)
Sulphadiazine is rapidly absorbed and execrated.
sulphadiazine is similar in structure to PABA
PABA is converted in the bacterial and protozoal cell into Folic acid which essential growth Factor.
sulphadiazine act by competitive antagonism with PABA, in the bacterial cell so prevent Folic acid Formation.
sulphadiazine is a bacteriostatic agent.
Trimethoprim is a bactericidal antibacterial agent
Trimethoprim act by preventing folic acid formation by blocks the conversion of dihydro folic Acid to tetrahydro folic acid by inhibiting dihydrofolate reductase.
The combination of sulphadiazine and Trimethoprim are bactericidal broad spectrum compound.
There is a great synergism between sulphadiazine and Trimethoprim.