Comparative in vitro killing activities of Enrofloxacin alone and in combination with Doxycycline against E. coli isolated from dogs and cats

Abstract

Enrofloxacin and doxycycline are two of the most frequently used antibacterial agents in the veterinary practice. Enrofloxacin exhibits bactericidal activity especially against Gram-negative bacteria, while doxycycline demonstrates only bacteriostatic effect and is the drug of choice for treating intracellular bacteria, Ehrlichia canis (E. canis) in particular. On the occasion of Gram-negative bacteria and E. canis co-infection, using enrofloxacin in combination with doxycycline is inevitable. Theoretically, the antagonism between bactericidal and bacteriostatic agents has been well established. However, the drug interaction between enrofloxacin and doxycycline has not been yet identified especially against a common pathogen, Escherichia coli (E. coli). Therefore, the objective of this study was to compare in vitro killing activities and times of enrofloxacin alone to its combination with doxycycline, against ten E. coli isolates derived from dogs and cats using time-kill curves. The combinations were tested into two ways - simultaneous and separated combinations (12-hour interval). The results showed that enrofloxacin alone could kill E. coli best with a log reduction of 3.96±0.49 (97.96-100.00% kill) within two hours, followed by the simultaneous combination with a log reduction of 3.78±0.37 (98.73-100.00% kill) within six hours, and the separated combination with a log reduction of 3.51±0.47 (99.19-100.00% kill) after 12 hours of enrofloxacin exposure. Moreover, when comparing time to 3 log reduction (T3K), the separated combination killed E. coli significantly slower than that of enrofloxacin alone (8.04±0.94 Vs 2.47±0.40 hours, P<0.05), which was consistent with the results of time to elimination (TE) (14.97±1.35 Vs 5.83±0.58 hours, P<0.05). From this study, it can be concluded that the killing activity of enrofloxacin is reduced and delayed especially by the presence of doxycycline 12 hours prior to enrofloxacin. The findings in this study can be applied to predicting clinical outcomes from the use of enrofloxacin and doxycycline combination and to studying more about drug interaction between these two classes of antibacterial drugs in the future.