Checkerboard Assay for MRSA Combination Therapy

The checkerboard assay was utilized to asses if the most potent thiazole compounds (1–3) have potential to be combined with mupirocin for treatment of MRSA infections [22 (link)]. Briefly, a bacterial suspension (1 × 10⁵ CFU/mL) in CAMHB was added to each well of a 96-well microtiter plate. Compounds 1–3 and mupirocin were diluted in CAMHB in order to reach the desired starting concentration (2 × or 4 × MIC). Mupirocin was serially diluted along the horizontal axis of the plate while compound 1, 2, or 3 was diluted along the vertical axis. Plates were incubated for at least 18 hours at 37°C and the MIC of each compound was recorded. The fractional inhibitory concentration index (ƩFIC) was computed for each combination using the following equation:
$\sum FIC\mathrm{ }=\mathrm{ }\left(\frac{MICthiazole\mathrm{ }compound\mathrm{ }in\mathrm{ }combination\mathrm{ }with\mathrm{ }mupirocin}{MICthiazole\mathrm{ }compound\mathrm{ }alone}\right)+\left(\frac{MICmupirocin\mathrm{ }in\mathrm{ }combination\mathrm{ }with\mathrm{ }thiazole\mathrm{ }compound}{MICmupirocin\mathrm{ }alone}\right)$
A FIC index less than or equal to 0.50 was classified as synergism, as described previously [19 (link)]. FIC values above 0.50 but less than 4.00 were classified as indifference, while FIC values greater than 4.00 were indicative of antagonism.