Improvement of 96-well microplate assay for estimation of cell growth and inhibition of Leishmania with Alamar Blue

The value of resazurin-based Alamar Blue redox indicator to determine multiplication of Leishmania promastigotes in 96-well microtiter plates was examined. In addition, assay was validated with amphotericin B (AmB) and allicin. The method was tested on L.donovani and L.infantum promastigotes under different culture conditions (variable air-phase, presence of phenol red, initial cell density, incubation time, use of Hepes buffer). Results showed that the gas-phase of promastigote cultures was critical. The method yielded consistent results with initial plating cell densities of 2.5 × 10⁵ promastigotes/well, up to 72 h incubation and 5% CO₂ atmosphere or reduced air availability (sealed plastic bags, film-sealed microplates). Detection of low numbers of promastigotes and earlier results could be obtained using fluorimetry instead of spectrophotometry. The addition of 20 mM Hepes improved the results. Fluorescence intensity correlated to promastigotes number in both Leishmania spp. Inhibitory concentration (IC₅₀) values for AmB and allicin using cell counting and fluorimetry were comparable. Under these conditions this one-step, low-cost redox indicator can be used in drug sensitivity assays and studies of differential proliferation rates of Leishmania isolates or strains in a 96-well format.