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Acinetobacter baumannii has emerged as a major cause of nosocomial infections. The ability of A. baumannii to display various resistance mechanisms against antibiotics has transformed it into a successful nosocomial pathogen. The limited number of antibiotics in development and the disengagement of the pharmaceutical industry have prompted the development of innovative strategies. One of these strategies is the use of essential oils, especially aromatic compounds that are potent antibacterial molecules. Among them, the combination of carvacrol and cinnamaldehyde has already demonstrated antibacterial efficacy against A. baumannii. The aim of this study was to determine the biological effects of these two compounds in A. baumannii, describing their effect on the rRNA and gene regulation under environmental stress conditions. Results demonstrated rRNA degradation by the carvacrol/cinnamaldehyde mixture, and this effect was due to carvacrol. Degradation was conserved after encapsulation of the mixture in lipid nanocapsules. Results showed an upregulation of the genes coding for heat shock proteins, such as groES, groEL, dnaK, clpB, and the catalase katE, after exposure to carvacrol/cinnamaldehyde mixture. The catalase was upregulated after carvacrol exposure wich is related to an oxidative stress. The combination of thiourea (hydroxyl radical scavenger) and carvacrol demonstrated a potent bactericidal effect. These results underline the development of defense strategies of the bacteria by synthesis of reactive oxygen species in response to environmental stress conditions, such as carvacrol.

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The recent overuse of antibiotics has led to the emergence of multidrug-resistant bacteria (MRB) responsible for severe infections, which are often difficult or impossible to treat. In the world of infectious diseases, the lack of development of new antibacterial molecules by the pharmaceutical industry is a major problem. Today, the number of new drugs able to treat Gram-negative MRB infections is significantly limited. It is therefore important to find new therapeutic approaches that are effective but limit the emergence of bacterial resistance. Medicinal plants containing essential oils constitute a potentially large source of antibacterial molecules that can be used to treat MRB infections. Essential oils, which primarily consist of phenolic compounds, have been demonstrated to have antibacterial effects against a wide variety of microorganisms. However, these molecules exhibit poor solubility in water and are biologically unstable. In addition, these molecules tend to bind to food constituents, resulting in decreased bioavailability and antimicrobial activity. To overcome these challenges, essential oils can be encapsulated within nanoparticles to enhance their solubility in aqueous media and to increase their antibacterial activity by facilitating contact with bacterial cells. The high antibacterial effectiveness of several delivery systems, including emulsions, liposomes and nanoparticles, indicates the potential of encapsulated phenols. The anti-infective potential of essential oils, combined with various technologies from nanomedicine, provides a new hope in the fight against infectious diseases.