= Article Modelling the growth and death of Staphylococcus aureus against Melaleuca armillaris essential oil at different pH condi- tions Daniel Buldain 1,2* , Lihuel Gortari Castillo 1,2 , María Laura Marchetti 1 , Karen Julca Lozano 1 , Arnaldo Bandoni 3,4 and Nora Mestorino 1 1 Laboratorio de Estudios Farmacológicos y Toxicológicos (LEFyT), Facultad de Ciencias Veterinarias, Uni- versidad Nacional de La Plata, La Plata, Argentina. 2 Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), La Plata, Argentina. 3 Universidad de Buenos Aires, Facultad de Farmacia y Bioquímica, Cátedra de Farmacognosia, C.A. de Buenos Aires, Argentina. 4 CONICET-Universidad de Buenos Aires, Instituto de Química y Metabolismo del Fármaco (IQUIMEFA), C.A. de Buenos Aires, Argentina. * Correspondence: dbuldain@fcv.unlp.edu.ar Abstract: Essential oils (EO) are a great antimicrobial resource against bacterial resistance in public health. Math models are useful describing the growth, survival, and inactivation of microorgan- isms against antimicrobials. We evaluated the antimicrobial activity of M. armillaris EO obtained from plants placed in the province of Buenos Aires (Argentina) against Staphylococcus aureus. Minimum Inhibitory and Bactericidal Concentrations were close and decreased slightly acidifying the medium from pH 7.4 to 6.5 and 5.0. This result was also evidenced by applying a sigmoid model, where the time and EO concentration necessaries to achieve 50% of the maximum effect decreased when medium was acidified. Moreover, at pH 7.4, applying the Gompertz model, we found that subinhibitory concentrations of EO decreased the growth rate and the maximum pop- ulation density, and increased the latency period respect to the control. Additionally, we estab- lished physicochemical parameters for quality control and standardization of M. armillaris EO. Mathematical modelling allowed us to estimate key parameters in the behavior of S. aureus and Melaleuca armillaris EO at different pHs. This is interesting in situations where the pH changes are relevant, such as the control of intracellular infections in public health or the development of pre- servatives for food industry. Keywords: Staphylococcus aureus; Melaleuca armillaris; essential oil; Gompertz model; Sigmoid model; antibacterial 1. Introduction Antimicrobial resistance is a critical problem with high prevalence in both human and animal medicine [1]. Essential oils (EO) are a great resource as an alternative therapy, providing a lot of antimicrobials (ATMs) compounds produced by aromatic plants. In addition to their usefulness in medicine, essential oils are of great importance in the food industry to guarantee food preservation and safety [2].These can act as bacteriostatics or bactericides in several ways responding to different action mechanism, having a wide variety of target sites, which generally lead to destabilization of the phospholipid bilayer, destruction of the function and composition of the plasma membrane, loss of vital intra- cellular components and inactivation of enzyme mechanisms [3]. The genus Melaleuca belongs to the Myrtaceae family which contains a lot of species of plants producing EO. Citation: Buldain, D.; Gortari Cas- tillo, L.; Marchetti, M.L.; Julca Lozano, K.; Bandoni, A.; Mestorino, N. Modelling the growth and death of Staphylococcus aureus against Melaleuca armillaris essential oil at different pH conditions. Antibiotics 2021, 10, x. https://doi.org/10.3390/xxxxx Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 1 February 2021 doi:10.20944/preprints202102.0061.v1 © 2021 by the author(s). Distributed under a Creative Commons CC BY license.