Mathematical modeling of the combined action of Origanum grosii essential oil and sodium chloride on the growth of methicillin-resistant Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the emerging opportunistic pathogens that can cause food poisoning and is responsible for several infectious diseases, including hospital-acquired infections. The main objective of this work was to assay the antibacterial effect of Origanum grosii essential oil (OGEO) and NaCl, alone and in combination against the MRSA growth. For this purpose, response surface methodology (RSM) was adopted to generate a mathematical model representing this effect. O. grosii was harvested from the north of Morocco. GC-FID and GC/MS techniques were used in order to determine the chemical profile of OGEO. The main compounds of OGEO were carvacrol (49.29%), thymol (13.12%), p-cymene (12.68%), and γ-terpinene (8.78%). The method of micro-dilution was used in order to determine the minimal inhibitory concentration (MIC) and the minimal bactericidal concentration (MBC). The MIC value of OGEO was 0.5% (v/v), while the MBC value was equal to 1%(v/v). The OGEO showed a strong bactericidal effect against MRSA. RSM applied at two different incubation times, 6 h and 11 h, and at the total time average value (TAV), allowed to create a mathematical model representing the kinetics of MRSA growth depending to the concentrations of OGEO and salt. The interaction between these two variables was significant. The quadratic polynomial regression was sufficient to adequately represents the observed results with a high coefficient of determination R2 ranging from 95.8 to 96.4%. A synergy at first order was observed between the two factors, but it was not present at all time points; a slight antagonistic effect was observed at time t = 6 h. These results show that RSM provided a useful and accurate mathematical method by combining low sub-inhibitory concentrations of OGEO with NaCl that can be used as a natural product against MRSA infections or as an additive in the food system.