Thesis (M.Sc.)--Chulalongkorn University, 2013

The common causes of community and hospital-acquired infections are Staphylococcus aureus and Staphylococcus epidermidis. One of the key factors enables pathogens to survive, colonize and proliferate in body is the ability to form biofilm. In this study investigated the antimicrobial, antibiofilm formation activities and time-kill assay of (+)-usnic acid against S. aureus and S. epidermidis. Antimicrobial activities were determined by broth microdilution method. The MIC values of (+)-usnic acid for S. aureus and S. epidermidis were 250 and 62.50 µg/ml, respectively. In addition, MBC values of (+)-usnic acid for both pathogens were more than 4 mg/ml. Furthermore, in time-kill assay showed that (+)-usnic acid has no bacteriostatic activity against S. aureus in all concentrations used (125, 250, 500 and 1000 µg/ml) whereas it has bacteriostatic activity against S. epidermidis only at 250 µg/ml after 12 h of incubation. (+)-Usnic acid also inhibited biofilm formation against S. aureus and S. epidermidis. The inhibitory effect on biofilm formation was concentration dependent manner at the concentration equal and greater than MIC values of each microorganism with statistical significance comparing with 5% DMSO (p < 0.05). For in vivo study, silkworms were utilized for efficacy testing of (+)-usnic acid. ED50 of (+)-usnic acid for S. aureus and S. epidermidis were more than 4.00 mg/ml (0.57mg/g of larva). (+)-Usnic acid may lose its therapeutic effect because of the influence of pharmacokinetic in silkworm model. In addition, silkworm infectious model can also be utilized as a screening tool for drugs discovery.