Abstract
Development of non-antibiotic alternatives to treat infections caused by multidrug-resistant (MDR) microbes represents one of the top priorities in healthcare and community settings, especially in the care of combat trauma-associated wound infections. Here, we investigate efficacy of oregano oil against pathogenic bacteria including MDR isolates from the combat casualties in vitro and in a mouse burn model. Oregano oil showed a significant anti-bacterial activity against 11 MDR clinical isolates including four Acinetobacter baumannii, three Pseudomonas aeruginosa, and four methicillin-resistant Staphylococcus aureus (MRSA) obtained from combat casualties and two luminescent strains of PA01 and MRSA USA300, with a MIC ranging from 0.08 mg/ml to 0.64 mg/ml. Oregano oil also effectively eradicated biofilms formed by each of the 13 pathogens above at similar MICs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that oregano oil damaged bacterial cells and altered the morphology of their biofilms. While efficiently inactivating bacteria, there was no evidence of resistance development after up to 20 consecutive passages of representative bacterial strains in the presence of sublethal doses of oregano oil. In vivo study using the third-degree burn wounds infected with PA01 or USA300 demonstrated that oregano oil, topically applied 24 h after bacterial inoculation, sufficiently reduced the bacterial load in the wounds by 3 log10 in 1 h, as measured by drastic reduction of bacterial bioluminescence. This bactericidal activity of oregano oil concurred with no significant side effect on the skin histologically or genotoxicity after three topical applications of oregano oil at 10 mg/ml for three consecutive days. The investigation suggests potentials of oregano oil as an alternative to antibiotics for the treatment of wound-associated infections regardless of antibiotic susceptibility.
Highlights
Skin wound infection is a widespread problem in both civilian and military healthcare settings
We investigate effectiveness of oregano oil in inactivation of MDR bacteria isolated from combat casualties in vitro and bioluminescent strains of P. aeruginosa (PA01) and methicillin-resistant Staphylococcus aureus (MRSA) (USA300) in mouse burn wounds
Chemical ingredients of commercial oregano oil were identified by GC/MS analysis (Table 1)
Summary
Skin wound infection is a widespread problem in both civilian and military healthcare settings. There are about 3,000 wellrecognized EOs, of which 300 are generally recognized as safe (GRAS) to humans by the United States Food and Drug Administration (U.S FDA) and have broad applications in food preservation, additives, and favors, perfume, cosmetic industries, antiseptic oral solutions, toothpastes, cleaner, and air fresheners for centuries (Pandey et al, 2017; Sakkas and Papadopoulou, 2017) These natural products are of particular interest as “green” antimicrobial agents because of their lowcost, biocompatibility, potential antibiofilm properties, and friendly to eukaryote cells and environment (Burt, 2004; Nostro et al, 2007; Kavanaugh and Ribbeck, 2012). We investigate effectiveness of oregano oil in inactivation of MDR bacteria isolated from combat casualties in vitro and bioluminescent strains of P. aeruginosa (PA01) and MRSA (USA300) in mouse burn wounds. The study is the first in vivo attempt on the use of oregano oil for the treatment of burn wounds infected with clinically important MDR bacteria
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