A Novel Mathematical Model to Study Antimicrobial Interactions against Campylobacter spp.

Introduction:  Campylobacter spp. is one of the leading causes of foodborne illness worldwide. In Canada, it is estimated that Campylobacter bacteria cause ~145,350 cases of foodborne illness per year. Thus, there is an urgent need to develop a new generation of antimicrobials to reduce the high numbers of Campylobacter-contaminated food products. Development of synergistic antimicrobial treatment can reduce the use of individual antimicrobial and maximize antimicrobial effect.

Purpose:  In this investigation we searched for antimicrobial synergy against Campylobacter jejuni.

Methods:  We compared the interaction of binary or tertiary combinations of cinnamon oil, encapsulated curcumin, and zinc oxide nanoparticles (ZnO NPs) using three methods; the time-killing assay, fractional inhibitory concentration index (FICI) method, and a mathematical model.

Results:  We identified that each antimicrobial had a unique dose response curve, validating the need for quantitative assessment of an accurate interaction. The time-killing assay overestimated a synergistic effect between antimicrobials, while FICI method could not detect an existing synergistic phenomena. Specifically, encapsulated curcumin showed a sharp sigmoidal dose-response curve and cinnamon oil and ZnO NPs had slightly different hyperbolic dose response curves. In contrast, a mathematical model was constructed to successfully study the interaction in each combination and identify an accurate and reliable additive line. Nonlinear isobologram analysis was performed to evaluate the synergy in combinations, and a median effect equation was applied to identify combinations of synergistic effect at different reduction levels.

We observed an additive effect between cinnamon oil and ZnO NPs. The combination of encapsulated curcumin and ZnO NPs, at the same potency level at MIC₁₀, resulted in 99.42% reduction of Campylobacter with individual antimicrobial reduction of 60.51% and 81.76%, respectively. Combining cinnamon oil and encapsulated curcumin at MIC₁₀ resulted in 92.65% reduction in bacterial cell count. Both antimicrobials were reduced by 93.40% and 60.51%, respectively. Tertiary antimicrobial combinations at MIC₂₅ reduced bacterial population over eight log CFU/mL.

Significance:  This novel mathematical model could accurately study antimicrobial interactions against Campylobacter bacteria and provide an alternative strategy to develop new antimicrobial chemotherapy.