Evaluation of the Antibacterial Effects of Phellinus baumii Extract on Methicillin-Resistant Staphylococcus aureus by Using Broth Microdilution Based on a Colorimetric Method

The broth microdilution technique used to measure the minimal inhibitory concentration (MIC) of natural compounds against bacteria is problematic: it is difficult to visualize bacterial growth due to the color of the natural compound. Therefore, the use of a colorimetric method with a redox indicator by broth microdilution can simplify it and increase its objectivity. This study evaluated the usefulness of the colorimetric method in measuring the MIC of Phellinus baumii against methicillin-resistant Staphylococcus aureus (MRSA). The inhibition in disc diffusion method was observed from $8,192{\mu}g/mL$ P. baumii in all 10 MRSA isolates examined; however, the MIC ranges of the 10 MRSA isolates was $512{\sim}2,048{\mu}g/mL$ by broth microdilution using a colorimetric method; with the 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) indicator. In addition, the MIC of P. baumii by broth microdilution using MTT as indicator yielded excellent results. However, the 2, 3, 5-triphenyltetrazolium chloride (TTC) results could not be determined due to the color of the TTC indicator. The MICs of four antibiotics against MRSA using MTT or TTC were equal to those determined by visual interpretation. In conclusion, to evaluate the antibacterial effects of a natural compound, the broth microdilution technique is considered to be better than the disc diffusion method. Moreover, to resolve the problems caused by the colors of natural compounds, a colorimetric method such as that using MTT may be very valuable.