Sulfonamides

Tetrahydrofolic acid (THF) is a coenzyme in the synthesis of purine bases and thymidine. These are constituents of DNA and RNA and are required for cell growth and replication. Lack of THF leads to inhibition of cell proliferation.
Formation of THF from dihydrofolate (DHF) is catalyzed by the enzyme dihydrofolate reductase. DHF is made from folic acid, a vitamin that cannot be synthesized in the body but must be taken up from exogenous sources. Most bacteria do not have a requirement for folate, because they are capable of synthesizing it - more precisely DHF—from precursors. Selective interference with bacterial biosynthesis of THF can be achieved with sulfonamides and trimethoprim.

Sulfonamides possess bacteriostatic activity against a broad spectrum of pathogens. Sulfonamides are produced by chemical synthesis.

Sulfonamides structurally resemble p-aminobenzoic acid (PABA), a precursor in bacterial DHF synthesis. As false substrates, sulfonamides competitively inhibit utilization of PABA, and hence DHF synthesis. Because most bacteria cannot take up exogenous folate, they are depleted of DHF.

Trimethoprim:

Trimethoprim is a pyrimidine derivative used in combination with sulfonamides for enhanced antibacterial activity. Inhibits bacterial DHF reductase. Trimethoprim has bacteriostatic activity against a broad spectrum of pathogens. The antibacterial spectrum of trimethoprim is similar to that of sulfonamides, however, trimethoprim is 20- to 50-fold more potent.

Gram-positive and gram-negative organisms as: Staphylococcus spp., Streptococcus spp., Pasteurella, Salmonella and E.coli. 

The sulfa drugs are acetylated, primarily in the liver. The product is devoid of antimicrobial activity but retains the toxic potential to precipitate at neutral or acidic pH. This causes crystalluria and, therefore, potential damage to the kidney.             

Suppression of bone marrow activity with resultant anaemia. Sulfonamides can cause decrease in egg shell thickness, soft-shelled eggs as well as rough surface (inhibition of the enzyme carbonic anhydrase). 

 Urine alkalinization increases the urinary excretion sulfonamides.