Impact of pH on the activity of co-used antimicrobials against resistant Escherichia coli strains of animal origin

Abstract

In this study, combination antimicrobial therapy, due to its higher potential against resistant bacteria, was evaluated forthe inhibition of multidrug-resistant E. coli strains. The influence of pH as an environmental variable on the activity of antimicrobialcombinations was evaluated by calculating the factional concentration indexes at pH values 5.0, 6.0, 7.3, and 8.0.The highest synergistic activity rates of ceftriaxone + colistin, danofloxacin + colistin, danofloxacin + ceftiofur, and ceftiofur +gentamicin combinations were 50%, 33%, 100%, and 50%, respectively measured at ≥7.3 pH. The lowest synergistic activity rates for allcombinations were observed at the acidic pH values of 5.0 and 6.0. The results of this study clearly demonstrated that acidic pH of themedium impaired the activity of the antimicrobial combinations. Although ceftriaxone and ceftiofur exert optimal activity at acidic pHvalues, the synergistic activity of the co-used drugs reached its highest level at pH ≥7.3.