Inhibitory Effect of Natural Honey on The Growth of Rhizopus stolonifer, Mucor spp., and Aspergillus niger

Background: Honey is frequently used as an antiseptic, prophylactic, and therapeutic agent for many infections. The chemical and physical properties of honey have an inhibitory effect on a lot of microorganisms including fungi. Rhizopus stolonifer, Mucor spp., and Aspergillus niger are common indoor molds that contaminate bread and eatable plants. During growth on bread, they multiply and produce large numbers of spores that can cause different types of infections, especially in immunocompromised patients. Objectives: 1) To evaluate the effect of natural honey on the growth of Rhizopus stolonifer, Mucor spp., and Aspergillus niger fungi. 2) To determine the minimal inhibitory concentration of honey that can suppress the growth of these fungi. 3) To assess the possibility of using honey as a natural replacement for chemical preservatives. Methodology: This experimental study was conducted in the College of Sciences at King Saud University between August and November 2021 in Riyadh, KSA. Multifloral honey was diluted in sterile distilled water volume to volume (%, v/v) in series concentrations: 5%, 15%, 30%, 45%, and 60%. The diluted samples of honey were then added to potato dextrose agar (PDA). Measured parts of the colonies of Rhizopus stolonifer, Mucor, and Aspergillus niger, were cultured on the PDA plates with the diluted honey. Two replicates were used for every concentration of each fungus. The inoculated plates, including the controls, were incubated at room temperature (25±2°C). Fungal growth was monitored daily for three months. Results: The 5%, 15%, and 35% concentrations of honey mixed with PDA did not inhibit the fungal growth, the fungi grew within the first 24h while the 60 % dilute concentration killed the three fungi and completely inhibited their growth. The fungi were monitored for 3 months; during that time, there was no growth. In conclusion: This study confirms that honey strongly inhibits the growth of Rhizopus Stolonifer, Mucor spp., and Aspergillus niger when used in a concentration of 60% or above. Accordingly based on these findings we determine that the minimal inhibitory concentration (MIC) of multi-floral honey on bread fungi is 60%.