In vitro activity of difloxacin against canine bacterial isolates

The in vitro activity of difloxacin against canine bacterial isolates from clinical cases was studied in the United States and The Netherlands. Minima l inhibitory concentrations (MIC), the postantibiotic effect, the effect of pH on antimicrobial activity, and the bacterial killing rate tests were de termined according to standard techniques. The MICs of American and Dutch i solates agreed in general. The MICs of the American gram-negative isolates ranged from 0.06 to 2.0 mu g/ml, and the MICs of the Dutch gram-negative is olates ranged from 0.016 to 8.0 mu g/ml. A few European strains of Protests mirabilis and Klebsiella pneumoniae had relatively high MICs. Bordetella b ronchiseptica also was less susceptible to difloxacin. The MICs of the Amer ican grampositive cocci ranged from 0.125 to 4.0 mu g/ml, and the MICs of D utch isolates ranged from 0.125 to 2.0 mu 8/ml. Difloxacin induced a concen tration-dependent postantibiotic effect that lasted 0.2-3 hours in cultures with Escherichia coli, Staphylococcus intermedius, Streptococcus canis, Pr oteus spp., and Klebsiella pneumoniae. There was no postantibiotic effect o bserved against canine Pseudomonas aeruginosa. Decreasing the pH of the med ium increased the MIC of Proteus mirabilis for difloxacin. The MICs of Esch erichia coli and Klebsiella pneumoniae were lowest at neutral pH and were s lightly increased in acid or alkaline media. At a neutral pH, most tested b acterial species were killed at a difloxacin concentration of 4 times the M IG. Similar results were obtained when these same bacteria were tested agai nst enrofloxacin. A Klebsiella pneumoniae strain in an acidic environment w as readily killed at difloxacin or enrofloxacin MIG, but at neutral pH the drug concentration had to be raised to 4 times the MIC for a bactericidal e ffect. After 24 hours of incubation at pH 7.1, difloxacin and enrofloxacin had similar bactericidal activity for all bacteria tested except Staphyloco ccus intermedius. Against S. intermedius, difloxacin was more bactericidal than enrofloxacin.