The complex formulation of a cosmetic product usually contains chemical agents such as preservatives and antimicrobials able of arresting or inhibiting the growth and the duplication of microorganisms. The effectiveness of chemical agents is due to the wide range of action sites represented by biological molecules with a structural role present in the microbial cell. The most common mechanisms of action of molecules with antimicrobial activity are: damage to cell walls (in the case of Gram negative bacteria), alteration of cell permeability, denaturation of cytoplasmic or membrane proteins, inhibition of protein and nucleic acids synthesis (Hayat A. et al., 2016). Chemical agents that are able to induce a reversible effect are commonly defined as having bacteriostatic action (growth inhibition), unlike those agents able to induce irreversible damage are classified as compounds with bactericidal action (99.9% reduction of the microbial load).

The great variety, variability and different degree of microorganism’s sensitivity to various antimicrobial formulations, makes it necessary to use appropriate and specific in vitro methods to verify bacteriostatic or bactericidal activity of these chemical formulations. Among the traditional methods used to determine the antimicrobial activity of a substance, the most reliable and widely used techniques are: the dilution method Minimum Inhibitory Concentration (MIC) and the evaluation of the Minimum Bactericidal Concentration (MBC).

MIC is an extremely precise and reproducible technique for testing the bacterial sensitivity of antimicrobials by identifying the lowest concentration that inhibits microbial growth (bacteriostatic action); it can be evaluated using the broth dilution method or by using 96-well microtiter plates. The MBC is represented by the lowest concentration of the product able to induce a 99.9% reduction of the microbial load. Its determination is subsequent to the execution of the MIC and is based on the preparation of subcultures in agar medium starting from the concentration of the product in which growth inhibition has occurred (MIC).

The reference strains to be tested are usually selected based on the field of application of the product. To evaluate the antimicrobial activity of a cosmetic product, the most common cutaneous strains are selected in order to determine the efficacy of the formulation (S. aureus, S. epidermidis, E. coli, P. aeruginosa ,C.albicans and A. brasiliensis).

The evaluation of the antimicrobial activity of a product against these strains, represents a relevant and appropriate method for the study of their effectiveness (Deshmukh SP., 2019).