Morphological and Dose-Dependent Study on the Effect of Methyl, Hexyl, and Dodecyl Rosmarinate on Staphylococcus carnosus LTH1502: Use of the Weibull Model.

The mechanisms of three antimicrobial rosmarinates (methyl-RE1, hexyl-RE6, and dodecyl-RE12) were investigated against Staphylococcus carnosus LTH1502. Scanning electron microscopy was used to determine the morphology of treated cells to gain information on potential changes in the site of action of compounds. The survival data obtained from antimicrobial activity assays were fitted to a nonlinear Weibull model to assess changes in inactivation behavior. Generally, esters became more effective with increasing length of the alkyl chain, resulting in a lower concentration for inhibition and inactivation. Weibull distribution parameters showed a downward concave inactivation pattern for RE1 above a critical concentration, indicative of a delayed log phase of the antimicrobial activity, with few cells being inactivated immediately after treatment and more cells being affected at later times. In contrast, esters having longer alkyl chains (RE6 and RE12) had an upward concave inactivation behavior, with more cells being inactivated immediately after addition of compounds. Cellular morphologies suggest that the antimicrobial mode of action of esters transitions from one that acts intracellularly (RE1) to one that predominately affects bacterial membrane (RE6 and RE12) due to changes in physicochemical properties of esters. Assessment that is based on the parameters of the Weibull model could, thus, be used to evaluate antimicrobial efficiency, in addition to MIC.