Evaluation of toxic effects of several carboxylic acids on bacterial growth by toxicodynamic modelling

Background: Effects of organic acids on microbial fermentation are commonly tested in investigations about metabolic behaviour of bacteria. However, they typically provide only descriptive information without modelling the influence of acid concentrations on bacterial kinetics. Results: We developed and applied a mathematical model (secondary model) to capture the toxicological effects of those chemicals on kinetic parameters that define the growth of bacteria in batch cultures. Thus, dose-response kinetics were performed with different bacteria (Leuconostoc mesenteroides, Carnobacterium pisicola, Escherichia coli, Bacillus subtilis and Listonella anguillarum) exposed at increasing concentrations of individual carboxylic acids (formic, acetic, propionic, butyric and lactic). In all bioassays the acids affected the maximum bacterial load (X-m) and the maximum growth rate (V-m) but only in specific cases the lag phase (lambda) was modified. Significance of the parameters was always high and in all fermentations the toxicodynamic equation was statistically consistent and had good predictability. The differences between D and L-lactic acid effects were significant for the growth of E. coli, L. mesenteroides and C. piscicola. In addition, a global parameter (EC50,tau) was used to compare toxic effects and provided a realistic characterization of antimicrobial agents using a single value. Conclusions: The effect of several organic acids on the growth of different bacteria was accurately studied and perfectly characterized by a bivariate equation which combines the basis of dose-response theory with microbial growth kinetics (secondary model). The toxicity of carboxylic acids was lower with the increase of the molecular weight of these chemicals.