Colistin, sulphamethoxazole, and trimethoprim in synergy against Gram-negative bacteria.

The antibacterial activity of the four possible combinations of the three drugs, colistin, sulphamethoxazole, and trimethoprim, has been investigated with Gram-negative bacteria. All of the strains examined, with the exception of the strains of Proteus, were sensitive to colistin. The combination of colistin and sulphamethoxazole was synergic against all 141 sulphamethoxazole-sensitive bacteria out of a total of 164 organisms against which it was tested. The sensitive strains comprised 27 of the 37 Esch. coli, 51 of the 54 Ps. aeruginosa, 24 of the 30 Kl. aerogenes, eight of the 12 shigellae, and all 21 Proteus and 10 salmonellae tested. The combined effect was indifference against the remaining 23 organisms which were resistant to sulphamethoxazole. The combination of colistin and trimethoprim was synergic against all 72 organisms against which it was tested, which comprised 10 Esch. coli, 14 Ps. aeruginosa, 14 Kl. aerogenes, 12 Proteus spp, 10 salmonellae, and 12 shigellae. The combination of sulphamethoxazole and trimethoprim was synergic against 61 of the same 72 organisms; the exceptions were three Esch. coli, four Kl. aerogenes, and four shigellae, all of which were sulphamethoxazole resistant. The combination of all three drugs-colistin, sulphamethoxazole, and trimethoprim-was more active than combinations of any two against 66 of the 72 organisms. The exceptions were three strains of Esch. coli, two of Kl. aerogenes, and one shigella, all of which were sulphamethoxazole resistant.