The microbiological rationale for the combination of sulphonamides with trimethoprim

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Abstract
When co-trimoxazole was introduced in 1969 there were a series of microbiological claims made for it: (1) That clinical benefits ensue from the sequential blockade of two steps in a bacterial metabolic pathway; (2) Synergism; (3) Delayed emergence of bacterial resistance to the components; (4) Wide antibacterial spectrum and (5) Bactericidal action. From the discussion it emerges that only claim (4) is unchallenged. The author offers the following microbiological claims for co-trimoxazole now: (1) Continued effectiveness in clinical use (2) Continued sensitivity of bacterial isolates (3) Combined activity extends the spectrum of infections which can be treated by the components alone (e.g. gonorrhoea) (4) Synergism between Su and TMP against Su-resistant bacteria and (5) Reduction of aerobic bowel flora with relative lack of selection of resistant coliform organisms.