Antibacterial activity of honey is due to the presence of methylglyoxal (MGO), H2O2, bee
defensin as well as polyphenols. High MGO levels in manuka honey are the main source of
antibacterial activity. Manuka honey has been reported to reduce the swarming and swimming
motility of Pseudomonas aeruginosa due to de-flagellation. Due to the complexity of honey it is
unknown if this effect is directly due to MGO. In this ultrastructural investigation the effects of
MGO on the morphology of bacteria and specifically the structure of fimbriae and flagella were
MGO effectively inhibited Gram positive (Bacillus subtilis; MIC 0.8 mM and Staphylococcus
aureus; MIC 1.2 mM) and Gram negative (P. aeruginosa; MIC 1.0 mM and Escherichia coli; MIC
1.2 mM) bacteria growth. The ultrastructural effects of 0.5, 1.0 and 2 mM MGO on B. substilis
and E. coli morphology was then evaluated. At 0.5 mM MGO, bacteria structure was unaltered.
For both bacteria at 1 mM MGO fewer fimbriae were present and the flagella were less or
absent. Identified structures appeared stunted and fragile. At 2 mM MGO fimbriae and flagella
were absent while the bacteria were rounded with shrinkage and loss of membrane integrity.
Antibacterial MGO causes alterations in the structure of bacterial fimbriae and flagella which
would limit bacteria adherence and motility.