Susceptibility of Recently Isolated Pathogenic Bacteria to Trimethoprim and Sulfamethoxazole Separately and Combined

Abstract

A number of media and modifications were used in preliminary tests of susceptibility of common pathogenic aerobic bacteria to trimethoprim (TMP) and sulfamethoxazole (SMZ) separately and combined. The effect of varying the density of the lawn for the disk-susceptibility test was also investigated. Various serial dilution tests in liquid and on solid media were also compared, and the effect of varying the size of the inoculum for these tests was investigated. As a result of these tests, done largely on laboratory-maintained strains, a lawn of overnight cultures diluted 10^-3 was considered most useful and practical for the disk tests, but the standard dilution specified in the Kirby-Bauer method was generally included for comparison. In the serial dilution tests, the one using the inocula replicator and 10^-3 dilution of culture for inoculum on a modified (thymidine-free) Mueller-Hinton agar was selected as optimal, but similar tests with undiluted cultures and other modifications of various media were also used for some species. By use of these methods groups of recently isolated common pathogenic bacteria were tested by the disk and inocula replicator methods for susceptibility to TMP and SMZ separately and combined in ratio of 1:20 in the former and 1:16 in the latter method. Against nearly all strains of almost all species tested TMP alone and combined with the sulfonamide was found to be active in clinically useful concentrations, and against a large proportion of the organisms the two agents were clearly synergistic even for strains that were moderately or highly resistant to SMZ.