Optimisation of three essential oils against Salmonella spp. and Escherichia coli by mixture design.

The aim of this work is to optimise the antibacterial activity of essential oils (EOs) from Eucalyptus camaldulensis (ECEO), Mentha pulegium (MPEO) and Rosmarinus officinalis (ROEO) plants against Salmonella spp. and Escherichia coli. The qualitative antimicrobial effect was assessed using the disc diffusion method, the broth microdilution method was used to determine the minimum inhibitory concentrations (MIC). Polynomial models were created using an augmented centroid simplex mixture design to highlight the synergy of EOs. The results show a significant antibacterial effect of ECEO and MPEO against both bacterial strains, with inhibition zones (IZs) of 13 and 12 mm respectively against E. coli, and 13 and 11 mm against Salmonella spp. The latter strain showed a MIC of 0.625% (v: v) by the ECEO, while E. coli exhibited a MIC of 0.0781% (v: v). The binary combinations of essential oils display a synergistic effect, the proportions of the optimum EOs in the mixture giving the lowest MICm were of the order of 50.51% ECEO and 49.49% ROEO against Salmonella spp. and around 50% MPEO and 50% ECEO against E. coli. These results indicate the effectiveness of binary combinations EOs against resistant bacterial strains and suggest their importance in bacterial infections treatment.