Synergistic and Additive Effect of Oregano Essential Oil and Biological Silver Nanoparticles against Multidrug-Resistant Bacterial Strains

Abstract

Bacterial resistance to conventional antibiotics has become a clinical and public health problem, making therapeutic decisions more challenging. Plant compounds and nanodrugs have been proposed as potential antimicrobial alternatives. Studies have shown that oregano (Origanum vulgare) essential oil (OEO) and silver nanoparticles have potent antibacterial activity, also against multidrug-resistant strains; however, the strong organoleptic characteristics of OEO and the development of resistance to these metal nanoparticles can limit their use. This study evaluated the antibacterial effect of a two-drug combination of biologically synthesized silver nanoparticles (bio-AgNP), produced by Fusarium oxysporum, and OEO against Gram-positive and Gram-negative bacteria, including multidrug-resistant strains. OEO and bio-AgNP showed bactericidal effects against all 17 strains tested, with minimal inhibitory concentrations (MIC) ranging from 0.298 to 1.193 mg/mL and 62.5 to 250 μM, respectively. Time-kill curves indicated that OEO acted rapidly (within 10 min), while the metallic nanoparticles took 4 h to kill Gram-negative bacteria and 24 h to kill Gram-positive bacteria. The combination of the two compounds resulted in a synergistic or additive effect, reducing their MIC values and reducing the time of action compared to bio-AgNP used alone, i.e., 20 min for Gram-negative bacteria and 7 h for Gram-positive bacteria. Scanning electron microscopy (SEM) revealed similar morphological alterations in Staphylococcus aureus (non-methicillin-resistant S. aureus, non-MRSA) cells exposed to three different treatments (OEO, bio-AgNP and combination of the two), which appeared cell surface blebbing. Individual and combined treatments showed reduction in cell density and decrease in exopolysaccharide matrix compared to untreated bacterial cells. It indicated that this composition have an antimicrobial activity against S. aureus by disrupting cells. Both compounds showed very low hemolytic activity, especially at MIC levels. This study describes for the first time the synergistic and additive interaction between OEO and bio-AgNP produced by F. oxysporum against multidrug-resistant bacteria, such as MRSA, and β-lactamase- and carbapenemase-producing Escherichia coli and Acinetobacter baumannii strains. These results indicated that this combination can be an alternative in the control of infections with few or no treatment options.

Antibiotic‐resistantStaphylococcus aureusstrains of swine origin: molecular typing and susceptibility to oregano (Origanum vulgareL.) essential oil and maqui (Aristotelia chilensis(Molina) Stuntz) extract

“…Scandorieiro et al . () showed previously a MIC and MBC of oregano EO of 0·596 and 1·193 mg ml −1 respectively against S. aureus . The efflux of potassium ions from S. aureus cells occurred immediately after the addition of the oregano EO following a steady loss along the evaluated intervals.…”

The Synergic Activity of Eucalyptus Leaf Oil and Silver Nanoparticles Against Some Pathogenic Bacteria

“…Scandorieiro et al showed that oregano essential oil plus Ag NPs had synergic effect against all 3 ESBL-producing E. coli and this combination also showed bactericidal activity (21).…”

“…The combination of Ag NP with organic antibacterial agents has effective antibacterial activity (20). In other studies, the NPs offered improved properties to essential oils; Scandorieiro et al showed oregano essential oil plus Ag NPs had synergic effect against all 3 extended spectrum beta-lactamase (ESBL)-producing E. coli and this combination had bactericidal activity (21). The antibacterial activity of the eucalyptus oil was shown in different studies (22)(23)(24)(25)(26).…”

Nanomaterials for alternative antibacterial therapy

“…120 The combination of biologically synthesized Ag NPs (produced by Fusarium oxysporum) and oregano (Origanum vulgare) essential oil showed bactericidal effects against non-methicillin-resistant S. aureus (non-MRSA) and β-lactamase-and carbapenemaseproducing E. coli and A. baumannii strains. 121 Nigella sativa essential oil-synthesized Au NPs effectively inhibited the biofilm formation of S. aureus and Vibrio harveyi.…”