# Anthraquinone Rhein Exhibits Antibacterialativity against _Staphylococcus aureus_
## 4 Discussion  
_S. aureus_ is one of the most common pathogens responsible for community and hospital-acquired infections [28; 29]. Currently, the increase in _S. aureus_ antibiotic resistance is threatening the effective use of the latest-generation antibiotics such as linezolid, daptomycin and glycopeptides [30]. Therefore, research and development of new antibacterial agents to limit the spread of _S. aureus_ antibiotic resistance has become a necessity [31]. Natural products are a rich source of antimicrobial compounds. They have an important role in the discovery of antimicrobial leads and have contributed to the development of most drugs currently in use [32]. Our study assessed the antimicrobial potential of the natural compound, Rhein, against _S. aureus_ strains. The antibacterial assays through the Kirby-Bauer disk diffusion test allowed to obtain preliminary data on the antibacterial efficiency of the compound. In our study, Rhein exhibited remarkable bacteriostatic activity against standard and clinical isolated _S. aureus_ strains with MIC\({}_{90}\) values ranging from 8.13 to 9.31 \(\upmu\)g/mL. Based on antibacterial data and light cytotoxic activity on human erythrocytes at the highest tested concentration (16.4%), the calculated SI values were greater than 10, indicating this compound is suitable for in vivo studies. A significant amount of evidence has reported the antibacterial activity of Rhein on different bacterial strains. Our previous study showed that this compound altered the growth of _S. mutans_, showing MIC\({}_{90}\) values of 5.69 \(\upmu\)g/mL. Moreover, SEM data revealed that the natural product affected the integrity of the wall, inducing the formation of irregular, rough and non-uniform surfaces [23]. [PERSON] et al. proved that Rhein exhibited a bacteriostatic action on _S. aureus_ at a concentration of 8 \(\upmu\)g/mL, impairing the gene expression involved in anaerobic respiration, fermentation, iron-regulated surface determinants and ribonucleotide reductase systems [33]. Additionally, [PERSON] et al. documented the antibacterial efficiency of Rhein against _Salmonella enterica_, detecting MIC values of 250 \(\upmu\)g/mL [34]. The cited studies show a varied repertory of Rhein antibacterial features, presumably due to the different bacterial strains tested, growth conditions and assays used. Figure 9: Impact of 20-h treatment with Rhein on mature biofilms of _S. aureus_ ATCC 1167, ***: _p_-value < 0.0001, ***: _p_-value 0.0003, *: _p_-value 0.0323; ns: non-significant (**A**); multisensitive, ***: _p_-value < 0.0001, **: _p_-value 0.0012, ns: not significant (**B**); resistant to beta-lactams, ***: _p_-value < 0.0001, ***: _p_-value 0.0002, *: _p_-value 0.0036, ns: not significant (**C**); resistant to quinolones, ***: _p_-value < 0.0001, ***: _p_-value 0.0003, **: _p_-value 0.0046, ns: not significant (**D**); constitutively resistant to macrolides, ***: _p_-value < 0.0001, ***: _p_-value 0.0001, **: _p_-value 0.0076, ns: no significant (**E**); resistant to methicillin strains, ***: _p_-value < 0.0001, ***: _p_-value 0.0003, *: _p_-value 0.0392, ns: not significant (**F**). Staphylococcal hemolysins and catalases represent important virulence factors that participate in and promote the bacterial invasion process [35]. Alpha-toxin is a monomer released during exponential growth and is recognized as the main virulence factor of _S. aureus_. It oligomerizes in the host erythrocyte membranes, causing the formation of pores and subsequent osmotic lysis and cell death [36]. On the other side, catalase contributes to the intracellular persistence of _S. aureus_ within macrophages during infection by inactivating H\({}_{2}\)O\({}_{2}\) in the phagocytosis process [37]. During the infectious process, host tissue damage is induced by virulence factors secreted during bacterial replication [38; 39]. Moreover, numerous bacterial toxins stimulate the immune system, resulting in the release of cytokines that cause further tissue damage [40].