In situations where the use of MTT is problematic, PI or Sytox Green screening in a fluorescence plate reader can be considered.
The half-maximal Inhibitory Concentration (IC50) can be calculated, which measures the potency of the antibiotic.
The selectivity factor is calculated as follows: SF = IC50R/IC50S whereas SF = selectivity factor; IC50 = half-maximal inhibitory concentration; R= resistant cells; S = sensitive cells The selectivity factor has the following advantages: it is quantitative thus can be reported numerically using a microplate reader, it streamlines the process of generating stable cell lines (assay can be completed in 3 days), it considers both sensitive and resistant cells, and it allows comparison of the consistency and quality of antibiotics from different batches, vendors, and manufacturing methods.
[citation needed] The selectivity factor can be used for the creation of stably transfected cell lines, an important tool in drug discovery, biomedical research, and biological pathway investigation.
Cell line creation involves transfection (transferring the gene into the cell line), and selection (applying selective pressure in the form of an antibiotic.