In vitro susceptibility of equine Salmonella strains to trimethoprim and sulfonamide alone or in combination.

The research tests the effectiveness of various combinations of antibiotics against different Salmonella strains obtained from horses. Around 62 strains were tested over a three-year period in the Netherlands. The strains exhibited varying levels of resistance to the antibiotics, helping to identify more effective treatments.

Study Procedure and Tested Antibiotics

• The researchers collected 62 strains of Salmonella from horses over three years in the Netherlands.
• Six concentration ratios of Trimethoprim (TMP) and 9 different sulfonamides were tested, with the majority of the Salmonella strains being S typhimurium (52 out of 62).
• The researchers used the agar-dilution method, a widely accepted method to test antibiotic susceptibility, where different concentrations of antibiotics are added to a substance (agar) in which bacteria can grow.
• The least amount of TMP required to inhibit 50% of Salmonella growth (MIC50) was measured to be 0.12 micrograms/ml.

Antibiotic Potency and Effectiveness

• Sulfachlorpyridazine (SCP; MIC50, 16 micrograms/ml), sulfamethoxazole (SMX; MIC50, 32 micrograms/ml), and sulfadiazine (SDZ; MIC50; 32 micrograms/ml) were the most effective sulfonamides.
• The researchers combined TMP with sulfonamides to check if the combined effect was additive, synergistic (combined effect is greater than the sum of their individual effects), or antagonistic (combined effect is less than the individual effects).
• The concentrations of SDZ and SCP with TMP showed significant synergy across all tested concentration ratios, while SMX showed synergy with TMP at all ratios except 1:1.
• Sulfamethazine, sulfamerazine, sulfadoxine (SDX), sulfatroxazole, sulfadimethoxine, and sulfacetamide were less effective as their MIC50 was greater than their breakpoint MIC value (the value at which bacteria are considered resistant).

Antibiotic Resistance

• Sixteen Salmonella strains were found resistant to all tested antibiotics, including combinations of TMP and sulfonamides.
• Out of these 16, fourteen were of S.typhimurium phage type 200, one was of S.typhimurium phage type 61, and one was of S.typhimurium phage type 10.

In essence, this study helps identify promising combinations of antibiotics to counter Salmonella strains, while also highlighting the emergence of antibiotic resistance among these strains.