After setting up 100 uL of 10000 cells/mL of the HCT-116 colorectal cancer cell line, we notice that when we place 40 uL of water in the pockets between wells, that our cells grow to about half the concentration as cells without the water. We leave these well plates incubated for 4-6 days and then take an MTS-assay reading. For water-cooled cells we read about 1.0-1.2 absorbance. For cells without water-cooling we read about 2.0-2.2 absorbance. Any idea why?
Water helps to maintain humidity inside the plate and prevents from the medium evaporation that affects the drug concentration in it. During evaporation, relative drug concentration in medium increases, also the concentration of salts and other ingredients changes, that also affects the cell viability (absorbance). Thus it is not surprising that without addition of water your tested compound(s) become more toxic.
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