Bacteriostatic and bactericidal activity of two trimethoprim-sulfonamide combinations.

Sulfamoxole (SDMO) has the same half-life of elimination from human plasma as sulfamethoxazole. Its antibacterial properties, however, are often inferior to those of co-trimoxazole. Its less pronounced antibacterial effect, especially against gram-negative pathogens, also becomes evident in the combination with trimethoprim (TM). The inhibition zones are often smaller around discs containing the same amount of the components SDMO/TM as those with co-trimoxazole and the inhibitory concentrations needed are frequently 2-4 times higher, especially against gram-negative bacteria, such as Escherichia coli and Proteus vulgaris. Accordingly, the curative doses 50% of the new combination are 2-3 times higher than those of co-trimoxazole in experimental infections with E. coli and P. vulgaris in mice. The bactericidal action in human urine, collected after a course of treatment with the combination SDMO/TM in the planned lower dosage, is not only often retarded, but also frequently incomplete in comparison with that in urine after co-trimoxazole in standard dosage. Clinically, this might lead to increased development of resistance or to an increase of recurrent infections.