Checkerboard assays

Antibacterial synergy for combinations of compounds 1–4 with each other and clinical antibiotics from various class of antibacterial agents such as fluoroquinolone, tetracycline, aminoglycosides, macrolides, and glycopeptides (ciprofloxacin, doxycycline, erythromycin, gentamicin, streptomycin, and vancomycin) was tested for using checker board assays. Cultures of MRSA-MW2 were adjusted to OD₆₀₀ = 0.06 and added to compound pairs that had been serially diluted in the same 96-well plates, vertically for one compound and horizontally for the other. Assays were carried out in triplicate as described for antibacterial susceptibility assays. The inhibitory concentration of compound in combination with clinical antibiotic or other compounds was evaluated using the fractional inhibitory concentration index (FICI) that was calculated using the formula: MICA combination/MICA alone + MICB combination/MICB alone.²³