Abstract
Chronic wound infections are a major burden to both society and the health care industry. Bacterial biofilms are the major cause of chronic wound infections and are notoriously recalcitrant to treatments with antibiotics, making them difficult to eradicate. Thus, new approaches are required to combat biofilms in chronic wounds. One possible approach is to use drug combination therapies. Manuka honey has potent broad-spectrum antibacterial activity and has previously shown synergistic activity in combination with antibiotics against common wound pathogens, including Staphylococcus aureus. In addition, manuka honey exhibits anti-biofilm activity, thereby warranting the investigation of its potential as a combination therapy with antibiotics for the topical treatment of biofilm-related infections. Here we report the first use of MacSynergy II to investigate the response of established S. aureus (strain NCTC 8325) biofilms to treatment by combinations of Medihoney (medical grade manuka honey) and conventional antibiotics that are used for preventing or treating infections: rifampicin, oxacillin, fusidic acid, clindamycin, and gentamicin. Using checkerboard microdilution assays, viability assays and MacSynergy II analysis we show that the Medihoney-rifampicin combination was more effective than combinations using the other antibiotics against established staphylococcal biofilms. Medihoney and rifampicin were strongly synergistic in their ability to reduce both biofilm biomass and the viability of embedded S. aureus cells at a level that is likely to be significant in vivo. Other combinations of Medihoney and antibiotic produced an interesting array of effects: Medihoney-fusidic acid treatment showed minor synergistic activity, and Medihoney-clindamycin, -gentamicin, and -oxacillin combinations showed overall antagonistic effects when the honey was used at sub-inhibitory concentration, due to enhanced biofilm formation at these concentrations which could not be counteracted by the antibiotics. However, these combinations were not antagonistic when honey was used at the inhibitory concentration. Confocal scanning laser microscopy confirmed that different honey-antibiotic combination treatments could eradicate biofilms. Our results suggest that honey has potential as an adjunct treatment with rifampicin for chronic wounds infected with staphylococcal biofilms. We also show that MacSynergy II allows a comprehensive examination of the synergistic effects of honey-antibiotic combinations, and can help to identify doses for clinical use.
Highlights
The incidence, cost, morbidity, and mortality associated with chronic wound infections are a major burden to both society and the health care industry
The results of this study suggest that honey could be used in combination therapy with rifampicin for chronic wounds with established biofilms caused by Staphylococcus, adding weight to the synergistic anti-staphylococcal activity of this combination treatment previously reported against planktonic cells and biofilm prevention (Müller et al, 2013; Liu et al, 2015)
This makes chronic wounds inherently difficult to treat with antibiotics because bacteria present in biofilms are far more tolerant of antibiotics than planktonic bacteria (Percival et al, 2012; Zhao et al, 2013)
Summary
The incidence, cost, morbidity, and mortality associated with chronic wound infections are a major burden to both society and the health care industry. In Australia alone, an estimated 400,000 people suffer from chronic wounds or ulcers at an estimated cost of AU$2.8 billion annually and ∼AU$10, 000 per patient per year (Graves and Zheng, 2014; Whitlock et al, 2014). In the United States, chronic wounds affect 6.5 million people and cost US$25 billion yearly (Sen et al, 2009). Biofilms are up to 1,000 times more recalcitrant to treatment with antibiotics and biocides than planktonic cells (Hoyle and Costerton, 1991; HallStoodley and Stoodley, 2009; Lebeaux et al, 2014), making them difficult to eradicate using antibiotic therapy. One possible approach to eradicate bacterial biofilms is to use drug combination therapy, which can widen the therapeutic window, lower the dosage, and reduce the development of antimicrobial resistance (Bjarnsholt et al, 2013; Wu et al, 2015). A natural product that exhibits potent broad-spectrum antibacterial (bactericidal) activity (Carter et al, 2016) has shown synergistic activity in combination therapy with certain antibiotics for planktonic cells (Jenkins and Cooper, 2012b; Müller et al, 2013; Liu et al, 2015)
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