We assessed the in vitro activity of Manuka honey against biofilm bacteria in combination with antibiotics and visualized the effect of Manuka honey on bacterial biofilms using scanning electron microscopy. The fractional biofilm eradication concentration ( ) index for vancomycin plus Manuka honey against S. aureus IDRL-4284 biofilms was 0.34, indicating a synergistic interaction. The index for gentamicin plus Manuka honey against P. aeruginosa PAO1 biofilms was 0.78–0.82, indicating an additive interaction. Scanning electron microscopy of S. aureus IDRL-4284 and P. aeruginosa PAO1 biofilms exposed to Manuka honey and artificial honey containing the same sugar composition as Manuka honey showed that the former had more pronounced effects than the latter on both S. aureus and P. aeruginosa biofilms. Visualized effects included distorted cell morphologies for both bacteria and a decrease in P. aeruginosa extracellular matrix. In conclusion, Manuka honey has a synergistic interaction with vancomycin against S. aureus biofilms and an additive interaction with gentamicin against P. aeruginosa biofilms. 1. Introduction Medicinal honeys are registered for topical application in several countries and are formulated in tubes, into ointments and gels, and made into wound dressings. A common medicinal honey is Manuka honey, from the Manuka bush (Leptospermum scoparium), indigenous to New Zealand and Australia. Manuka honey has a high sugar content (high osmolarity) and low pH. Nonperoxide components are predominantly responsible for its antimicrobial activity [1, 2]. It contains the antibacterial methylglyoxal [3, 4]; although it also contains bee defensin-1, bee defensin-1 is modified by methylglyoxal, abrogating its antibacterial activity [5]. Manuka honey has in vitro activity against planktonic bacteria. It also possesses in vitro antibiofilm activity [6–13]. Given the topical uses of Manuka honey, its antibiofilm activity is clinically important, especially since most traditional antibiotics lack activity against biofilms. Preliminary studies demonstrate activity of Manuka honey against biofilms in humans [6, 14]. In medical practice, Manuka honey might be delivered in combination with traditional antibiotics, with both being administered topically or with Manuka honey being administered topically and the traditional antibiotic(s) being administered systemically. Therefore, it is useful to understand the interaction between Manuka honey and conventional antibiotics. Jenkins and Cooper reported a synergistic interaction between oxacillin and Manuka honey
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