Effects of Tormentic Acid and the Extracts from Callistemon citrinus on the Production of Extracellular Proteases by Staphylococcus aureus

Staphylococcus aureus is among the common nosocomial pathogens. Antibiotics have been used to treat S. aureus infections. However, there has been increased mortality associated with drug-resistant strains of S. aureus. Extracellular proteases have been implicated to be responsible for the transition of S. aureus from an adhesive pathogen to an invasive pathogen. The development of resistant strains has necessitated the search for new sources of drugs. Plants have been traditionally used as sources of therapeutic molecules. The objective of this study was to determine the effect of tormentic acid and the extracts from Callistemon citrinus on the production of extracellular proteases by S. aureus. The broth microdilution antibacterial susceptibility assay was used to determine the antibacterial effects of tormentic acid and the extracts on S. aureus. Both extracts showed a minimum inhibitory concentration (MIC) value of 50 μg/ml. The water : ethanol (50 : 50) and the dichloromethane : methanol (50 : 50) extracts were found to be bactericidal against S. aureus at a concentration of 100 μg/ml and 50 μg/ml, respectively. The effect of tormentic acid and extracts on extracellular protease production was investigated using the protease assay. A zone of proteolytic activity (Pr) was measured as the ratio of the diameter of the colony to the total diameter of colony plus zone of hydrolysis. The extracts reduced the production of extracellular proteases, while tormentic acid completely inhibited the production of extracellular proteases by S. aureus. The Pr value for tormentic acid was found to be 1. The Pr values of the dichloromethane : methanol extract and the water : ethanol extract were 0.92 and 0.84, respectively. In conclusion, tormentic acid was shown to inhibit extracellular protease production; therefore, there is need to explore its use in antivirulence therapy to combat S. aureus infections.