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 for the inhibition of multidrug-resistant E. coli strains. The influence of pH as an environmental variable on the activity of antimicrobial combinations 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 all combinations were observed at the acidic pH values of 5.0 and 6.0. The results of this study clearly demonstrated that acidic pH of the medium impaired the activity of the antimicrobial combinations. Although ceftriaxone and ceftiofur exert optimal activity at acidic pH values, the synergistic activity of the co-used drugs reached its highest level at pH ≥7.3.

Keywords: pH, antimicrobial combination, E. coli, resistance, animals

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