Comparison of the Selection of Resistant Bacterial Mutants and Stability of Resistance Profiles in Escherichia coli and Staphylococcus aureus Exposed to Essential Oils and Antibiotics at Subinhibitory Concentrations

The excessive and inappropriate use of antibiotics promotes the emergence of bacterial resistance, prompting research to explore therapeutic alternatives, such as essential oils, due to their bioactive substances. However, the potential for future resistance to essential oils concerns the scientific community. In this context, the present study was designed to compare the responses of bacteria exposed to sub-inhibitory concentrations of essential oils (EOs) and antibiotics. We assessed the antibacterial activity of EOs and reference antibiotics using the liquid microdilution method. We then exposed strains of Escherichia coli and Staphylococcus aureus to fixed and increasing sub-inhibitory concentrations of these antimicrobials, selected resistant mutants, and transferred them to a medium without antimicrobials to determine new inhibition parameters and evaluate the stability of resistance. The results show that Lippia multiflora EO exhibited the best activity, with an MIC ranging from 0.0976 mg/mL to 12.5 mg/mL. Levofloxacin displayed bactericidal activity against Staphylococcus aureus and Pseudomonas aeruginosa and bacteriostatic activity against Streptococcus pyogenes. Ciprofloxacin, on the other hand, showed tolerance against Escherichia coli. After discontinuous exposure to EOs and antibiotics, the Staphylococcus aureus and Escherichia coli strains modified their sensitivity to EOs, with MICs varying from 2 to 16 times the initial MIC, while Ciprofloxacin increased 8-fold and Levofloxacin increased 256-fold. Lippia multiflora EO showed no variation in the stability of resistance, whereas strains exposed to Chromolaena odorata EO maintained their resistant character. The strains also maintained a resistance to antibiotics. This study demonstrates that EOs play an important role in the selection of resistant mutants, and their rational use is essential. Although Lippia multiflora and Chromolaena odorata EOs exhibited antibacterial activity against some Gram-positive bacteria, long-term and inappropriate exposure could contribute to the emergence of resistance.