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  • Author or Editor: B. van Klingeren x
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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.

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in American Journal of Veterinary Research


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 (mic 50) was 0.12 |ig/ml. Sulfachlorpyridazine (scp; mic 50, 16 fig/ml), sulfamethoxazole (smx; mic 50, 32 μg/ml), and sulfadiazine (sdz; mic 50; 32 μg/ 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 mic 50 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. Four additional Salmonella strains were resistant to the sulfonamides alone (1 S typhimurium phage type 171 and 3 S typhimurium strains that could not be biotyped). Results of this study indicate that sdz, scp, and smx are the best sulfonamides to combine with tmp for treatment of salmonellosis in equids, because they are the most potent sulfonamides and have strong synergism with tmp at a wide range of tmp-to-sulfonamide concentration ratios.

Free access
in American Journal of Veterinary Research