Resistance to trimethoprim in klebsiellae isolated before its introduction

The sensitivity to trimethoprim (Tm) has been determined of 12 strains of Klebsiella spp. (11 K. aerogenes and 1 K. ozaenae) which were freeze-dried in 1964. Notwith standing the fact that Tm was not available for general clinical use in U.K. until 1968, 6 of these strains (50%) were found to be resistant to Tm (MIC ranged from 3 to 19 £g/ml). Against sensitive strains of K. aerogenes Tm was found to be more active if the organisms were also sensitive to sulphamethoxazole (SMZ): the mean MIC of Tm against 17 SMZ-sensitive strains was 0.24£g/ml, and against 25 SMZ resistant strains its mean MIC was 0 49£g/ml. The reported relatively high incidences of Tm-resistance among klebsiellae may be merely a reflection of a high background rate of primary resistance, rather than caused by selection pressure due to the increasing use of the drug.