Sessions

The global spread of antimicrobial resistance also limits therapeutic options in veterinary medicine. Potentiated sulfonamides are widely used active substances in the treatment of respiratory disease complex in poultry (CRDC).

The aim of our research was to examine the susceptibility of P. multocida, Staphylococcus spp., O. rhinotracheale, and E. coli strains isolated from poultry against combinations of trimethoprim–sulfamethoxazole (TMP-SMX), trimethoprim–sulfachlorpyridazine (TMP-SCP), and trimethoprim–sulfadiazine (TMP-SD). We determined the minimum inhibitory concentrations (MIC) of the individual active substances and combinations in various ratios (1:5, 1:10, 1:19, 1:40), then evaluated the degree of synergism based on the fractional inhibitory concentration index (FICi).

In the case of the strains examined, the individual sulfonamides mostly showed high MIC values, while trimethoprim alone had a more favourable efficacy. The combinations resulted in a decrease in MIC values in all cases, which was particularly evident in P. multocida and Staphylococcus spp., where MIC90 values of 0.25–0.5 µg/ml were measured in several combinations. TMP-SCP proved to be the most effective against O. rhinotracheale isolates, while E. coli strains showed the highest resistance, with only certain TMP-SCP ratios resulting in moderate improvement.

The synergistic study confirmed that the combination of the two components resulted in a favourable interaction in most cases. In the case of P. multocida and Staphylococcus spp. isolates, synergistic effects were observed in more than half of the combinations, particularly at a ratio of TMP-SMX 1:10 and TMP-SD 1:40, where favourable interactions were detected in more than 90% of the isolates. In the case of Staphylococcus spp., TMP-SCP combinations showed the strongest synergism. In O. rhinotracheale strains, the strongest synergistic effect was observed at a TMP-SD ratio of 1:10 (87.5%). In E. coli isolates, most combinations resulted in indifferent interactions, but the TMP-SCP 1:40 ratio combination proved synergistic in half of the strains. Antagonism occurred only rarely, exclusively in a few TMP-SCP combinations.

Our results confirm that potentiated sulfonamides can be used effectively against several pathogens, but the differences between the individual combinations emphasize the need for susceptibility testing. The data may contribute to more targeted and responsible antibiotic therapy for respiratory infections in poultry, thereby reducing antimicrobial resistance.