Letter: Co-trimoxazole resistance

Tissues and exudates contain sufficient thymidine for growth of anaerobic bacteria in the presence of inhibitory levels of trimethoprim-sulfamethoxazole

Obligate anaerobes are susceptible in vitro to trimethoprim-sulfonamides. Clinical efficacy of this drug combination for treatment of infectious processes in which anaerobes are involved is uncertain. We hypothesize that this uncertainty is a result of thymidine in tissues and exudates which abrogates the inhibitory effect of trimethoprim-sulfonamides. We shown herein that species of anaerobic bacteria commonly encountered in infectious processes grew on thymidine-containing medium despite the presence of levels of trimethoprim-sulfamethoxazole (S x T) previously shown to be inhibitory. Biologic fluids and tissues, where anaerobic bacteria are commonly encountered in diseased states, were shown to contain thymidine levels that were sufficient to reverse the inhibitory effect of S x T upon these species of bacteria. These observations suggest that the use of trimethoprim-sulfonamides is not a rational choice for treatment of infectious processes in which an obligate anaerobe is a component.

Effect of thymidine on activity of trimethoprim and sulphamethoxazole

Thymidine at levels as low as 0.05 mg/1 reduces the activities of sulphamethoxazole and trimethoprim and their combination in vitro. Using a biological assay procedure, levels of thymidine greater than this were interpreted as being present in urine. The addition of sulphamethoxazole and trimethoprim, singly or in combination, to urine obtained from patients with urinary tract infections showed that all the antibacterial effect towards sensitive organisms was due to the trimethoprim component. It is suggested that trimethoprim should replace the combination co-trimoxazole for the treatment of some lower urinary tract infections, and that laboratory media, if they are to resemble the clinical environment, should contain thymidine.

Thymine-requiring bacteria associated with co-trimoxazole therapy

Since 1971 thymine-requiring (thy-) pathogens have been isolated from the urine of 8 patients with renal calculi, and from the sputum of 1 patient with chronic chest infection. The patients had been treated with co-trimoxazole for several months before the isolation of the mutant pathogens. There was persistent pyuria in the patients with renal calculi, and purulent sputum in the patient with chronic chest infection. The mutants were identified by their inability to grow on diagnostic sensitivity test agar (D.S.T., Oxoid) which is deficient in thymine. It was found that wild-type bacteria can produce growth factors for the metabolism of the mutants in vitro, and the urine of the patients contained by thymine-like compounds. These findings indicate that thy- mutants may develop in renal calculi during co-trimoxazole therapy. Therapy should be changed to a more suitable antimicrobial as soon as possible after diagnosis of mutant infection.