In vitro and ex vivo antibacterial effects of levofloxacin against Pasteurella multocida and Escherichia coli  isolated from rabbits

Introduction: A fluoroquinolone drug for humans, levofloxacin, is not labeled for veterinary use in the EU. However, veterinary formulations exist in Argentina, China, and India (1). The effect of levofloxacin against common rabbit pathogens P. multocida, associated with respiratory disease, and E. coli, associated with gastrointestinal disease, was evaluated using bacterial killing curves and pharmacodynamic analysis.

Materials and Methods: Levofloxacin minimal inhibitory (MIC) and minimal bactericidal concentrations (MBC) were detected in rabbit serum for 10 isolates of P. multocida from diseased rabbits and 5 isolates of E. coli from clinically healthy rabbits. One sensitive isolate of each bacteria species was used for the bacterial killing study. For in vitro killing curves, bacterial inocula were incubated for 24 h in drug-free rabbit serum with the addition of levofloxacin (concentrations 0.25–64 MIC). For ex vivo bacterial killing, the same bacterial isolates were added to pooled levofloxacin-containing rabbit serum samples. These serums were collected at 0, 1, 2, 4, 8, and 10 h from rabbits, receiving 5 mg/kg of levofloxacin intramuscularly and subcutaneously. Levofloxacin concentrations were detected with a validated HPLC method. In vitro AUC24/MIC values were used for building the inhibitory pharmacodynamic Imax model in Phoenix WinNonlin. This model was applied to calculate levofloxacin AUC24/MIC values corresponding to the bacteriostatic, bactericidal, and bacterial elimination effects (2).

Results: P. multocida MIC values were 0.008–0.5 μg/mL, MBC – 0.015–0.5 μg/mL. E. coli MIC values were 0.008–0.03 μg/mL, MBC – 0.03–0.25 μg/mL. Both P. multocida isolate (MIC = 0.015 μg/mL) and E. coli isolate (MIC = 0.03 μg/mL) were growing in the presence of levofloxacin concentrations lower and equal to 1 MIC. For both isolates, levofloxacin concentrations of 2 MIC and higher, resulted in bacterial count reduction to the limit of detection after 24-h incubation. All serum samples from rabbits treated parenterally with levofloxacin (mean concentrations for intramuscular administration – 3.26–0.07 μg/mL, mean concentrations for subcutaneous administration – 2.70–0.08 μg/mL) were able to eradicate sensitive bacterial isolate strains within 24 h of incubation. The bacteriostatic effect, bactericidal and bacterial elimination for P. multocida and E. coli isolates were 9.84, 10.61 and 10.71 h and 14.30, 14.97 and 15.45 h, respectively.

Conclusion: Levofloxacin showed a potent antibacterial effect against selected bacterial isolates. More data on MIC value distribution among rabbit bacterial clinical isolates are needed to calculate optimal doses in rabbits.

References: 1. Sitovs et al. Res Vet Sci 2021; 137,111–126.
2. Lee et al. Res Vet Sci. 2017; 112 26–33.