Stress Conditions Induced by Carvacrol and Cinnamaldehyde on Acinetobacter baumannii.

Angélique Montagu,Elisabeth Rossines,Jérome Cayon,Marie Kempf,Patrick Saulnier,Marie-Laure Joly-Guillou

doi:10.3389/fmicb.2016.01133

Abstract

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.

Highlights

Multi-drug-resistant (MDR) Acinetobacter baumannii isolates have emerged as a major worldwide cause of nosocomial infections, exhibiting ever-increasing rates of resistance (Perez et al, 2007)
The objective of this work was to study the inactivation of A. baumannii by carvacrol and cinnamaldehyde and to describe their biological effects on ribosomal RNA (rRNA) and genes involved in the response to environmental stress
The therapeutic challenges presented by A. baumannii infections are partially attributable to the fact that these bacteria exhibit significant genetic versatility in the acquisition of drug resistance and have the ability to survive in hospital environments (Bergogne-Bérézin and Towner, 1996; Peleg et al, 2008)

Summary

Introduction

Multi-drug-resistant (MDR) Acinetobacter baumannii isolates have emerged as a major worldwide cause of nosocomial infections, exhibiting ever-increasing rates of resistance (Perez et al, 2007). A. baumannii infections occur in intensive care units, where they are commonly found as the cause of ventilator-associated pneumonia, urinary tract infections, secondary meningitis, and bacteremia (Bergogne-Bérézin and Towner, 1996). Such strains have a remarkable ability to upregulate or acquire resistance determinants (Karageorgopoulos and Falagas, 2008). The hydroxyl functions (OH) of carvacrol facilitate the contact with the bacterial membrane and the penetration through the outer layers of the bacterial cell wall, causing disintegration of outer membrane components (Ultee et al, 2002; Di Pasqua et al, 2007; Palaniappan and Holley, 2010). A study demonstrated the crucial role of hydroxyl functions in the antibacterial activity of carvacrol after substitution of hydroxyl functional groups by fatty acids (Montagu et al, 2016)

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