Antistaphylococcal activity and metabolite profiling of manuka honey (Leptospermum scoparium L.) after in vitro simulated digestion

The antistaphylococcal activity against methicillin-susceptible and -resistant Staphylococcus aureus and
the metabolite profiling of manuka honey (MH) were investigated before and after in vitro simulated
gastric (GD) and gastroduodenal (GDD) digestions. Undigested manuka honey showed antibacterial
activity against all the tested strains, the GD sample showed no activity against S. aureus, and the GDD
honey showed an antistaphylococcal activity, which was slightly reduced in comparison with the undigested
sample. To explain these results, methylglyoxal (MGO), to which most of the antibacterial activity
of MH is ascribed, was subjected to in vitro simulated GD and GDD. After digestion, MGO showed antibacterial
activity at concentrations definitively higher than those registered in digested MH samples. These
results showed that the antistaphylococcal activity registered after digestion cannot be ascribed to MGO.
Thus metabolite analysis, carried out using an explorative untargeted NMR-based approach and a targeted
RP-HPLC-PAD-ESI-MSn analysis focused on bio-active substances, was used to highlight the chemical
modifications occurring from digestion. The results showed that (1) the level of MGO decreases and (2)
the content of aromatic compounds, such as leptosin and methyl syringate, markers of manuka honey,
was stable under gastric and gastroduodenal conditions, whereas (3) the levels of acetic and lactic acids
increase in particular after gastroduodenal digestion, being 1.5 and 2.8 times higher in GDD-MH than in
UND-MH, respectively. Overall, the results obtained from chemical analysis provide at least a partial explanation
of the registered antibacterial activity observed after gastroduodenal digestion.