COMPARISON OF THE MODES OF ACTION, ON LAB, OF TWO MYRTACEAE ESSENTIAL OILS BY FLOW CYTOMETRY AND PREDICTIVE MICROBIOLOGY

The growing interest in the substitution of traditional food preservatives, both antimicrobials and antioxidants, by natural ones has fostered the screening of plant materials. In this context, essential oils (EOs) and their components are known to exhibit varying degrees of antimicrobial activity namely against pathogens. Nevertheless, there is no data concerning the examination of their effects on beneficial normal microbiota or on the starter cultures largely used in many industries. This work describes the use of a multiparametric flow cytometry technique to assess the mode of action, at the single cell level, of Melaleuca armillaris Sol. ex Gaertn. Sm. and Eucalyptus sideroxylon A. Cunn. ex Woolls EOs against two LAB (Lactic Acid Bacteria) strains. Using an automated microtiter assay, bacteriostatic or bactericidal effects were noticed depending on the studied strain and on the applied concentration of each of the EOs. The mathematical modelling of the kinetics showed that in presence of increasing concentrations of a given EO, the lag phases of growth were extended (0.7% to 80.56%) and both the growth rate and final cell density were reduced. Variations depending on the strain were noticed. Live/dead assays of the multiparametric flow cytometry technique were done by dual staining of each sample culture to differentiate viable, dead and stressed cells. The behaviour of each strain, in presence of increasing concentrations of the EOs, was evaluated by quantifying the relative percentages of each subpopulation throughout 3 days of culture. Results displayed disparate patterns of subpopulations which reveal dynamic change in cells behaviour. Since both EOs contain similar amounts of major components, their disparate behaviour is probably due to the different influences essentially of their minor components on cellular physiological properties during the incubation period. This technique can serve as powerful tool in optimally combining different preservative factors in order to design an effective antimicrobial system for selected foods. Furthermore, such study could be useful to understand how to fully take advantage of LAB as probiotics or as potential candidates to improve food hygiene and to assure food quality; namely when they are associated with natural preservatives such as EOs.