Study on the Minimum Water Activity Required for Growth of Staphylococcus Aureus and Aspergillus Fumigatus

OBJECTIVE To explore the effects on Staphylococcus aureus and Aspergillus fumigatus growth of different water activities by adding glucose glycerol and sodium chloride. METHODS Three different media were added to control the water activity of TSB medium and SDA medium respectively. The minimum water activity for microbial growth was judged by counting Staphylococcus aureus and measuring the colony growth diameter of Aspergillus fumigatus in successive time. RESULTS The colony number of Staphylococcus aureus decreased under the water activity regulated by glycerol and sodium chloride. When the water activity was adjusted at 0.85 and 0.86 in sucrose medium, the colony number fluctuated up and down at the initial value, and basically remained unchanged. When the water activity was adjusted at 0.87, the colony number increased rapidly after a slight decrease. The growth of Aspergillus fumigatus was stagnant under all water activities regulated by three media. CONCLUSION The level of environmental water activity has a certain impact on the growth of Staphylococcus aureus and Aspergillus fumigatus. The minimum water activity required for the growth of Staphylococcus aureus in sucrose medium is approximately 0.86 and the minimum water activity required for the growth of Aspergillus fumigatus is greater than 0.83. The minimum water activity of Staphylococcus aureus in glycerol and sodium chloride substrates, and Aspergillus fumigatus in the three substrates do not match the minimum water activity values recorded in the US Pharmacopoeia.