Kinetic evaluation of claimed synergistic paraben combinations using a factorial design

The antimicrobial effects of methyl and propyl parabens are investigated, with Escherichia coli as test organism, with a view to determining whether the parabens act synergistically. At appropriate concentrations, the parabens killed E. coli cells according to first order kinetics and the bactericidal effects were quantified by the first order kill rate constants. Combinations of methyl or propyl parabens, at concentrations which slow down or inhibit bacterial growth when used singly, produced definite kill. In this sense, the parabens are therefore synergistic since in combination they produce an effect which is not observed when they are used singly. This effect is not true synergism as shown by the results of our experiments with a factorial design. Analysis of variance indicated no significant interaction between the two parabens.