Laboratory Surveillance of Synergy between and Resistance to Trimethoprim and Sulfonamides

Results of susceptibility tests of Enterobacteriaceae isolated at 14 different centers demonstrate synergy between trimethoprim (TMP) and sulfamethoxazole (SMZ) against sulfonamide-susceptible isolates, which account for between 75% of the isolates at different centers. Only 1%-4% of the isolates of Escherichia coli or Proteus mirabilis from the five centers in the United States were found to be resistant when tested with a disk containing both TMP and SMZ, but >8% of such isolates from five of the other centers were resistant to the combination disk. A larger percentage of isolates of Klebsiella pneumoniae or Serratia marcescens were resistant, but the number varied from center to center. In the United States, resistance of human and animal isolates of Salmonella to the TMP-SMZ combination was almost completely absent, although >50% of the animal isolates were resistant to sulfonamides. At a center that tested TMP and SMZ resistance with separate disks, resistance to TMP was found to be 30 times more common in sulfonamide-resistant than in sulfonamide-susceptible E. coli. This ratio may be useful as a monitor as treatment with TMP alone increases.