## 4 Antimicrobial Properties of Honey
Therefore, these observations could confirm the hypothesis of a phytochemical-dependent production of H\({}_{2}\)O\({}_{2}\) in honey as an alternative to the widely accepted GOx biochemical pathway [76]. A synergism between H\({}_{2}\)O\({}_{2}\) and phenolics has been also linked to the antiseptic activity exerted by Corsican honeys through irreversible plasmidic DNA damage on _P. aeruginosa_ cultures [74]. Manuka honey (MH) is currently regarded as the gold standard for medical applications, and a specific unit, the Unique Manuka Factor (UMFTM), has been adopted to indicate its authenticity based on the content of specific markers, such as methylglyoxal (MGO), dihydroxyacetone (DHA), and leptosperin at the time of pre-packing food quality testing [89]. MGO originates from the precursor DHA found in the nectar, and its content depends on the presence of endogenous co-factors, such as phenolic content (mainly phenyllactic acid and methoxyphenyllactic acid) and aminoacidic composition (in particular proline) that could trigger Maillard-like side reactions, and environmental parameters such as storage temperature and length of time [90; 91]. Given the vast attention granted to Manuka honey as the only medical-grade honey recognised so far, its fingerprint compound MGO has recently attracted increasing interest, with research focusing on its identification in other honey varieties. The MGO content in honeys of different floral origins than Manuka is highly variable. [PERSON] et al. conducted a quantification of MGO in Italian honeys, showing a MGO wide range (0.4-24.01 mg/kg) with cherry (18.62 \(\pm\) 3.69 mg/kg) and almond honey (17.88 \(\pm\) 4.18 mg/kg) having the highest concentrations among the analysed varieties,but still significantly lower than the levels measured in Manuka honey. However, this study did not include an evaluation of the antibacterial activity of the analysed honey samples, so further investigations would be needed to assess whether MGO content is relevant to the antibacterial mechanism of action of these Italian honey varieties [91]. MGO was found in high quantities (up to 166 \(\pm\) mg/kg) in Nordic honey samples (mire and polyfloral), but it was totally absent in heather honey and in some samples of other varieties such as caraway, sweet clover, and dandelion. However, MGO content was not found to correlate with the inhibitory capacity of Nordic honeys against _P. aeruginosa or S. aureus_ at 15% honey dilution [42]. Eleven honey samples of varieties produced from Danish flora showed a significantly lower MGO content (less than 5 ug/mL) than commercial Manuka honey (54.33 \(\upmu\)g/mL) Nonetheless, Water mint, Linden, and mixed organic flora honey were shown to cause the greatest degree of bacterial growth inhibition of all the tested honey varieties, even on the Gram-negative bacteria (_P. aeruginosa_ and _E. coli_) against which Manuka was instead ineffective. Such activity was significantly affected by treatment with catalase, suggesting that it is ascribable mainly to the peroxide-mediated mechanism [80]. The UMF indicator is described as linked to the antibacterial power by comparison with a reference compound, e.g., 5 UMF is assigned to a honey inducing a microbial inhibition equal to that obtained by a 5% phenol positive control [92]. However, the MGO content in MH can increase during honey storage due to DHA conversion [90], generating incongruence with the claims reported on the labelling. Interestingly, [PERSON] et al. recorded the performance in the bacterial clearance test to be significantly greater in samples with lower UMF compared to MH of up to 20 UMF [92]. Similar results were also observed by [PERSON] et al. [93], which compared MIC in vitro against different bacteria for MH of various UMF. These results highlight the urgency for a univocal criterion applicable to all honey varieties, which would be crucial in allowing a standardised comparison of antibacterial potency of honey across different botanical species.