Methicillin-resistant Staphylococcus aureus (MRSA) is a common human pathogen that causes several difficult-to-treat infections, including biofilm-associated infections. The biofilm-forming ability of S. aureus plays a pivotal role in its resistance to most currently available antibiotics, including vancomycin, which is the first-choice drug for treating MRSA infections. In this study, the ability of thymol (a monoterpenoid phenol isolated from plants) to inhibit biofilm formation and to eliminate mature biofilms, was assessed. We found that thymol could inhibit biofilm formation and remove mature biofilms by inhibiting the production of polysaccharide intracellular adhesin (PIA) and the release of extracellular DNA (eDNA). However, cotreatment with thymol and vancomycin was more effective at eliminating MRSA biofilms, in a mouse infection model, than monotherapy with vancomycin. Comparative histopathological analyses revealed that thymol reduced the pathological changes and inflammatory responses in the wounds. Assessments of white blood cell counts and serum TNF-α and IL-6 levels showed reduced inflammation and an increased immune response following treatment with thymol and vancomycin. These results indicate that combinatorial treatment with thymol and vancomycin has the potential to serve as a more effective therapy for MRSA biofilm-associated infections than vancomycin monotherapy.

With continuous emergence and widespread of multidrug-resistant Staphylococcus aureus infections, common antibiotics have become ineffective in treating these infections in the clinical setting. Anti-virulence strategies could be novel, effective therapeutic strategies against drug-resistant bacterial infections. Sortase A (srtA), a transpeptidase in gram-positive bacteria, can anchor surface proteins that play a vital role in pathogenesis of these bacteria. SrtA is known as a potential antivirulent drug target to treat bacterial infections. In this study, we found that erianin, a natural bibenzyl compound, could inhibit the activity of srtA in vitro (half maximal inhibitory concentration—IC₅₀ = 20.91 ± 2.31 μg/mL, 65.7 ± 7.2 μM) at subminimum inhibitory concentrations (minimum inhibitory concentrations—MIC = 512 μg/mL against S. aureus). The molecular mechanism underlying the inhibition of srtA by erianin was identified using molecular dynamics simulation: erianin binds to srtA residues Ile182, Val193, Trp194, Arg197, and Ile199, forming a stable bond via hydrophobic interactions. In addition, the activities of S. aureus binding to fibronectin and biofilm formation were inhibited by erianin, when co-culture with S. aureus. In vivo, erianin could improve the survival in mice that infected with S. aureus by tail vein injection. Experimental results showed that erianin is a potential novel therapeutic compound against S. aureus infections via affecting srtA.