Final answer:
Antimicrobial drugs that inhibit folic acid synthesis like sulfonamides and trimethoprim are selectively toxic to bacteria because humans do not synthesize folic acid. These drugs mimic PABA and block the bacterial enzyme responsible for folic acid synthesis, leading to the inhibition of bacterial growth and cell division with minimal side effects on human cells.
Step-by-step explanation:
Antimicrobial drugs such as sulfonamides and trimethoprim act as antimetabolites, which are substances that interfere with the normal metabolic processes of bacteria. These drugs are effective at inhibiting bacterial folic acid synthesis, which is crucial for the production of nucleic acids and cell division in microorganisms.
Bacteria need to synthesize folic acid from the precursor para-aminobenzoic acid (PABA), whereas mammals, like humans, obtain folic acid from their diet. Sulfonamides resemble PABA and compete with it for the active site of the enzyme involved in the bacterial synthesis of folic acid. By occupying these sites, sulfonamides prevent the bacteria from producing folic acid, leading to a cessation of bacterial growth. Drugs like methotrexate inhibit the enzyme folic acid reductase, stalling DNA synthesis.
Because humans do not synthesize their own folic acid, these drugs are selectively toxic to bacteria and have few side effects on human cells. However, there may be allergic reactions to sulfa drugs, and treatments can lead to a decrease in beneficial gut microflora, potentially resulting in a folic acid deficiency.