In vitro susceptibility of some porcine respiratory tract pathogens to aditoprim, trimethoprim, sulfadimethoxine, sulfamethoxazole, and combinations of these agents

SUMMARY The in vitro antimicrobial activities of aditoprim ( ap ), a new dihydrofolate reductase ( dhfr ) inhibitor, trimethoprim ( tmp ), sulfadimethoxine ( sdm ), sulfamethoxazole ( smx ), and combinations of these drugs against some porcine respiratory tract pathogens were determined by use of an agar dilution method. The minimal inhibitory concentrations ( mic ) of these agents were determined twice against Bordetella bronchiseptica (n = 10), Pasteurella multocida (n = 10), and Actinobacillus pleuropneumoniae (n = 20) strains isolated from pigs suffering from atrophic rhinitis or pleuropneumonia. All B bronchiseptica strains were resistant to ap and tmp . The mic 50 values of ap and tmp for P multocida were 0.25 and 0.06 μg/ml, respectively, and for A pleuropneumoniae, 1 and 0.25 μg/ml, respectively. The mic 50 values of sdm and smx for B bronchiseptica were 4 and 1 μg/ml, respectively; for P multocida , 16 and 8 μg/ml, respectively; and for A pleuropneumoniae , 16 and 8 μg/ml, respectively. The investigated combinations of the dhfr inhibitors and the selected sulfonamides had synergism for the A pleuropneumoniae strains; the mic 90 values of the combinations were ≤ 0.06 μg/ml. Potentiation was not observed for the B bronchiseptica and the P multocida isolates. The mic of the combinations against B bronchiseptica and P multocida corresponded respectively to the concentrations of the sulfonamides and the dhfr inhibitors in the combinations. For A pleuropneumoniae , 2 types of strains were used (25% of serotype 2 and 75% of serotype 9). Type-2 strains had lower susceptibility than type - 9 strains to ap and tmp as well as to sdm and smx (at least a fourfold difference in mic between the 2 types of strains). The mic of the combinations were similar for the 2 types of strains.