Abstract
The emergence of antibiotic-resistant bacteria has become a major concern worldwide. This trend indicates the need for alternative agents to antibiotics, such as natural compounds of plant origin. Using agar disc diffusion and minimum inhibitory concentration (MIC) assays, we investigated the antimicrobial activity of Citrus aurantium (AEO), Citrus x limon (LEO), Eucalyptus globulus (EEO), Melaleuca alternifolia (TTO), and Cupressus sempervirens (CEO) essential oils (EOs) against three representatives of antibiotic-resistant pathogens and respective biofilms: vancomycin-resistant enterococci (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and extended-spectrum β-lactamase (ESBL)-producing Escherichia coli. Using the checkerboard method, the efficacy of the EOs alone, in an association with each other, or in combination with the reference antibiotics was quantified by calculating fractional inhibitory concentrations (FICs). All the EOs displayed antibacterial activity against all strains to different extents, and TTO was the most effective. The results of the EO–EO associations and EO–antibiotic combinations clearly showed a synergistic outcome in most tests. Lastly, the effectiveness of EOs both alone and in association or combination against biofilm formed by the antibiotic-resistant strains was comparable to, and sometimes better than, that of the reference antibiotics. In conclusion, the combination of EOs and antibiotics represents a promising therapeutic strategy against antibiotic-resistant bacteria, even protected inside biofilms, which can allow decreasing the concentrations of antibiotics used.
Highlights
In recent years, the frequency of bacteria resistant to multiple pharmacological agents has steadily increased [1]
Citrus essential oils (EOs) have been recognized as antibacterial compounds, with limonene as the main component, followed by ß-myrcene, linalool, ß-pinene, and α-pinene [12], the last being the most representative compound found in Cupressus sempervirens EO (CEO) [13]
The emergence of antibiotic-resistant bacteria has become a major problem around the world and has had a significant clinical and economic impact
Summary
The frequency of bacteria resistant to multiple pharmacological agents has steadily increased [1]. The main damage is the cell membrane disruption, linked to the inhibition of efflux pumps responsible for antibiotic resistance in Gram-negative bacteria, but other modes of action have been widely reported, such as the inhibition of the peptidoglycan layer synthesis of bacterial cell walls by binding to PBPs for Gram-positive bacteria [16,17,18,19,20] Their antibacterial activity has been recently studied against pathogenic bacteria frequently responsible for hospital-acquired infections (HAIs) [21,22,23,24,25]. Recent studies have highlighted the synergistic action of EOs with other antimicrobial drugs and how these associations could be used to positively modulate the antimicrobial activity of some antibiotics against antibiotic-resistant bacteria, with a consequent decrease in the minimum effective dose of the drugs [26,27,28,29,30,31,32,33,34]
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