ResearchArticle Effects of Tormentic Acid and the Extracts from Callistemon citrinus on the Production of Extracellular Proteases by Staphylococcus aureus Rumbidzai Mashezha, Molly Mombeshora, and Stanley Mukanganyama Department of Biochemistry, University of Zimbabwe, Harare, Zimbabwe Correspondence should be addressed to Stanley Mukanganyama; smukanganyama@medic.uz.ac.zw Received 16 September 2019; Revised 6 March 2020; Accepted 16 March 2020; Published 21 April 2020 Academic Editor: Saad Tayyab Copyright © 2020 Rumbidzai Mashezha et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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. e development of resistant strains has necessitated the search for new sources of drugs. Plants have been traditionally used as sources of therapeutic molecules. e objective of this study was to determine the effect of tormentic acid and the extracts from Callistemoncitrinus on the production of extracellular proteases by S. aureus. e 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. e 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. e 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. e extracts reduced the production of extracellular proteases, while tormentic acid completely inhibited the production of extracellular proteases by S. aureus. e Pr value for tormentic acid was found to be 1. e 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. 1. Introduction ere has been an increase in severe Staphylococcus aureus infections caused by drug-resistant strains [1]. is increase in infections has been partly attributed to the hypersecretion of proteases by the bacteria [2]. Extracellular proteases are used by the bacteria to resist antibiotics, for survival during interaction with the innate immune system of host and formation of skin abscess and as effectors of virulence-de- terminant stability. Studies have shown that S.aureus strains which lack protease genes have decreased the abscess for- mation and impaired organ invasion [3]. Extracellular proteases are required for growth of bacteria in peptide-rich environments, serum, and blood and in the presence of antimicrobial peptides. Extracellular proteases increase the invasiveness of the bacteria into the body as they cleave proteins such as elastin. e cleaving of elastin allows the bacteria to enter into organs and the bloodstream allowing the bacteria to spread to different parts of the body. Ex- tracellular proteases also assist in resisting phagocytosis by human leukocytes [4]. Proteases such as aureolysin have the ability to cleave the human cathelicin, therefore, conferring the bacteria’s resistance to this protein [1]. e mechanism by which S. aureus cells control the production of virulence factors is through the action of secreted proteases [4]. Extracellular proteases are produced alongside toxins and other exoenzymes to control their stability [5]. e resulting stability facilitates the coordinated Hindawi Biochemistry Research International Volume 2020, Article ID 6926320, 6 pages https://doi.org/10.1155/2020/6926320