Antibacterial activity of anacardic acid and totarol, alone and in combination with methicillin, against methicillin-resistant Staphylococcus aureus.

The inhibitory and bactericidal activities of anacardic acid and totarol, alone and in combination with methicillin, were investigated against methicillin-resistant Staphylococcus aureus (MRSA). The growth of two MRSA strains was inhibited by 6 x 25 microg ml-1 of anacardic acid and 0 x 78 microg ml-1 of totarol. The time-kill curve study showed that these two compounds were bactericidal against MRSA. Anacardic acid killed MRSA cells more rapidly than totarol, and no viable cells were detected after being exposed to 6 x 25 microg ml-1 of anacardic acid for 6 h. Anacardic acid showed bactericidal activity against MRSA at any stage of growth, and also even when cell division was inhibited by chloramphenicol. In the combination studies, the minimal inhibitory concentration (MIC) of methicillin was lowered from 800 to 1 x 56 microg ml-1 for MRSA ATCC 33591, and from 800 to 6 x 25 microg ml-1 for MRSA ATCC 33592, by combining with 1/2 x MIC of anacardic acid. The time-kill curves demonstrated synergistic bactericidal activities for these combinations.