IN-VITRO ACTIVITY OF CEFTIOFUR AND ITS PRIMARY METABOLITE, DESFUROYLCEFTIOFUR, AGAINST ORGANISMS OF VETERINARY IMPORTANCE

Ceftiofur (XNL) and its primary metabolite, desfuroylceftiofur (DXNL), were evaluated for in vitro activity against 539 isolates from veteri nary sources. Actinobacillus pleuropneumoniae, Pasteurella spp., Haemo philus somnus, Salmonella spp., Escherichia coli, staphylococci, and s treptococci were tested. Overall, XNL and DXNL were equivalent in acti vity against the gram-negative organisms with all minimum inhibitory c oncentrations (MICs) within 1 serial dilution. Against the staphylococ ci, MIC differences of 2-3 serial dilutions were detected with an MIC( 90) for XNL and DXNL of 1.0 and 4.0-8.0 mu g/ml, respectively. Althoug h the MIC(90) obtained for Streptococcus suis for each compound was wi thin 1 dilution, the MIC values against individual strains were 2-3 di lutions greater for DXNL than for XNL. The MICs obtained with the bovi ne and equine streptococci for DXNL (MIC(90) = 0.03 mu g/ml) were 5 se rial dilutions higher than those obtained for XNL (MIC(90) less than o r equal to 0.0019). Although DXNL was less active than XNL against the streptococci, these differences were not clinically important because both XNL and DXNL were highly active for these bacteria. Although the se differences are of little importance with the streptococci, they ma y have important implications for susceptibility testing of the staphy lococci. In conclusion, with the exception of the staphylococci, both XNL and DXNL were highly active against the organisms tested, with MIC s for both compounds several fold lower than plasma levels achieved du ring dosing of XNL.