In vitro susceptibility of equine Salmonella strains to trimethoprim and sulfonamide alone or in combination.
The in vitro activity of trimethoprim (TMP) and 9 sulfonamides and their combinations in 6 concentration ratios was tested against 62 Salmonella strains isolated from horses over a 3-year period in the Netherlands, using the agar-dilution method. Most of the isolates were S typhimurium strains (n = 52); the others were S heidelberg (n = 3), S hadar (n = 2), S thompson (n = 2), S enteritidis (n = 1), S infantis (n = 1), and S derby (n = 1). The minimal TMP concentration at which 50% of the Salmonella strains were inhibited (MIC50) was 0.12 micrograms/ml. Sulfachlorpyridazine (SCP; MIC50, 16 micrograms/ml), sulfamethoxazole (SMX; MIC50, 32 micrograms/ml), and sulfadiazine (SDZ; MIC50; 32 micrograms/ml) were the most potent of the sulfonamides tested. The antimicrobial effect of the sulfonamides, in combination with TMP (additive, synergistic, or antagonistic), was expressed by the fractional inhibitory concentration (FIC) index. Concentrations of SDZ and SCP with TMP had marked synergism at all tested TMP-to-sulfonamide concentration ratios (1:1 to 1:160; FIC index, 0.10 to 0.50); SMX had synergy with TMP at all ratios, except 1:1 (FIC index, 0.10 to 0.27). Sulfamethazine, sulfamerazine, sulfadoxine (SDX), sulfatroxazole, sulfadimethoxine, and sulfacetamide had MIC50 greater than their breakpoint MIC value and are, therefore, less potent drugs. However, synergy with TMP was found for these less potent sulfonamides at certain concentration ratios, depending on the sulfonamide used. Sixteen Salmonella strains were resistant to TMP, all sulfonamides, and TMP-sulfonamide combinations; 14 of these strains were S typhimurium phage type 200, 1 was S typhimurium phage type 61, and 1 was S typhimurium phage type 10.(ABSTRACT TRUNCATED AT 250 WORDS)
van Duijkeren
E
E
Department of Large Animal Medicine and Nutrition, Faculty of Veterinary Medicine, Utrecht University, The Netherlands.
van Klingeren
B
B
Vulto
A G
AG
Sloet van Oldruitenborgh-Oosterbaan
M M
MM
Breukink
H J
HJ
van Miert
A S
AS
D000818:Animals; D004338:Drug Combinations; D004357:Drug Synergism; D006736:Horses / Q000382:microbiology*; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D012475:Salmonella / Q000187:drug effects*; D013449:Sulfonamides / Q000008:administration & dosage / Q000494:pharmacology*; D014295:Trimethoprim / Q000008:administration & dosage / Q000494:pharmacology*
D004338:Drug Combinations; D013449:Sulfonamides; D014295:Trimethoprim