Characterising the mechanism of action of an ancient antimicrobial, honey, using modern transcriptomics

Abstract

16Manuka honey has broad-spectrum antimicrobial activity and unlike traditional antibiotics, 17 resistance to its killing effects has not been reported. However, its mechanism of action 18 remains unclear. Here we investigated the mechanism of action of manuka honey and its key 19 antibacterial components using a transcriptomic approach in a model organism, Pseudomonas 20 aeruginosa. We show that no single component of honey can account for its total 21 antimicrobial action, and that honey affects the expression of genes in the SOS response, 22 current array of wound treatments due to its effective antibacterial activity that does not 40 promote resistance in bacteria. 41While MGO is a key antibacterial component of manuka honey, it alone cannot account for 65 its total antimicrobial activity 21-23 , as manuka honey inhibits the growth of pathogenic 66 bacteria (including Pseudomonas aeruginosa, Escherichia coli and Staphylococcus aureus) at 67 concentrations well below the minimum inhibitory concentration (MIC) of MGO alone 21-24 . 68 Additionally, many bacteria are innately equipped to detoxify MGO 25-27 , so additional 69 components in honey must also modulate its activity. From this, we hypothesise that the 70 antibacterial activity of manuka honey comes from a combination of its various constituents 71 and that its mechanism of action cannot be elucidated based exclusively on investigations of 72 the individual components. Rather, to generate a fundamental understanding of the 73 mechanism of antibacterial activity, the effects of the key components of manuka honey 74 against microorganisms must be studied in isolation and in combination with each other. 75Despite the prominent role of MGO in the antibacterial activity of manuka honey, to what 76 degree it contributes to the effect manuka honey has on bacterial gene expression and 77 physiology has not been thoroughly investigated [28][29][30][31][32][33][34] . Currently, the antimicrobial activity of 78 manuka honey is reported and marketed based on its NPA, which can be directly tested via 79 bioassays or derived from the MGO concentrations of manuka honey since MGO and NPA 80 are well correlated 18 . This is problematic since NPA is only a measure of anti-staphylococcal 81 activity and not representative of activity against other bacterial species 35 . Therefore, it is 82 important to understand how MGO alone and in combination with sugars works against 83Gram-negative microorganisms like P. aeruginosa, in order to better understand the 84 mechanism of whole manuka honey. This is critical for its use in infection control, which 85 requires killing of multiple species of bacteria present in wounds. 86Previous studies have identified a number of biological processes in bacteria that may be 87 affected by the action of honey, including cell division 22,29,32,33 , motility 28 , quorum sensing 88 (QS) 36-40 , protein synthesis 29,32,41 and responses to oxidative stress 7,41 . With the increased 89 128 MICs of all treatments were determined using the brot...