Sulfonamides inhibit bacterial synthesis of dihydrofolic acid by preventing the condensation of the pteridine with aminobenzoic acid through competitive inhibition of the enzyme dihydropteroate synthetase. Resistant strains have altered dihydropteroate synthetase with reduced affinity for sulfonamides or produce increased quantities of aminobenzoic acid. Quantitative methods that require measurement of zone diameters give the most precise estimate of the susceptibility of bacteria to antimicrobial agents. A report of ''moderately susceptible'' suggests that the organism would be susceptible if high dosage is used or if the infection is confined to tissues and fluids in which high antimicrobial levels are attained. A report of ''resistant'' indicates that achievable concentrations are unlikely to be inhibitory , and other therapy should be selected.
Standardized procedures require the use of laboratory control organisms.
Resistant As with standard diffusion techniques, dilution methods require the use of laboratory control organisms.