To investigate the antimethicillin-resistant Staphylococcus aureus (MRSA) mechanism of Quercus infectoria (nutgalls) extract and its components. Ethanol extract, an ethyl acetate fraction I, gallic acid and tannic acid could inhibit the growth of clinically isolated MRSA strains with minimum inhibitory concentration values between 63 and 250 mu g ml(-1). Clumps of partly divided cocci with thickened cell wall were observed by transmission electron microscopy in the cultures of MRSA incubated in the presence of the ethanol extract, the ethyl acetate fraction I and tannic acid. Because cell wall structure of the organism structures seemed to be a possible site for antibacterial mechanisms, their effect with representative beta-lactam antibiotics were determined. Synergistic effects with fractional inhibitory concentration index ranged from 0.24 to 0.37 were observed with 76% and 53% of the tested strains for the combination of the ethanol extract with amoxicillin and penicillin G, respectively. The appearance of pseudomulticellular bacteria in the treated cells and the synergistic effect of the plant extract with beta-lactamase-susceptible penicillins suggest that the extract may interfere with staphylococcal enzymes including autolysins and beta-lactamase. Our results provide scientific data on the use of the nutgalls, which contain mainly tannin contents up to 70% for the treatment of staphylococcal infections.