Pterostilbene Reduced Post-antibiotic Effect Time of Linezolid against Methicillin-resistant Staphylococcus aureus

Pterostilbene is biologically classified as a phytoalexin, which is part of the plant's defence system in response to pathogen infection. The present study is to identify the type of interaction and post-antibiotic effect time of pterostilbene in combination with linezolid against methicillin-resistant Staphylococcus aureus American Type Culture Collection 43300 and 33591. The minimum inhibitory concentration values of individual antimicrobial agents were determined using microbroth dilution technique whereas the microdilution checkerboard assay was employed to verify the type of interaction of the combined agents from the fractional inhibitory concentration index values. The post-antibiotic effect time of pterostilbene and linezolid and in combination treatment was evaluated using viable plate count method. The minimum inhibitory concentration value of pterostilbene against both strains were the same (31.25 mu g/ml) compared to linezolid at 0.78 mu g/ml and 1.56 mu g/ml, against 43300 and 33591, respectively. Microdilution checker analysis of pterostilbene in combination with linezolid against both methicillin-resistant S. aureus strains displayed fractional inhibitory concentration value of 1.25. Despite the indifference interaction between pterostilbene and linezolid as shown by microdilution checker study, post-antibiotic effect analysis demonstrated antagonism as depicted by a decrease of post-antibiotic effect time of linezolid by pterostilbene against both methicillin-resistant S. aureus strains. The duration of post-antibiotic effect time obtained for pterostilbene were twice (3.6 +/- 0.15 h) that of linezolid (1.8 +/- 0.31 h) against methicillin-resistant S. aureus American type culture collection 43300. However, pterostilbene exhibited shorter post-antibiotic effect time of 0.3 +/- 0.10 h compared to linezolid (0.9 +/- 0.06 h) against 33591 strain. In conclusion, pterostilbene was not able to prolong the post-antibiotic effect duration of linezolid in both strains and each of the antimicrobials demonstrated a more favourable timing of doses when these agents were used singly.