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Some general causes of cell death include contamination from infections (yeast, bacteria, mycoplasma, viruses), chemicals (sterilizing solutions like ethanol or bleach), or issues with the cell line or methods of culturing.
To establish and successfully maintain cell cultures requires standardized approaches for media preparation, feeding, and passaging of the cells. Cultures should be examined regularly to check for signs of contamination and to determine if the culture needs feeding or passaging.
A quick look at the cell culture with basic brightfield microscopy can provide general information on gross contamination with microorganisms or if there is cross-contamination with cells from another culture.
pH drops with most infections of bacteria and yeast, and sometimes with fungal infections. Changes in pH can be measured with pHrodo AM indicators to look at cytoplasmic pH.
Many adherent cell cultures will stop proliferating once they become confluent, and some will die if they are left in a confluent state for too long. Suspension cells will exhaust their culture medium very quickly once the cell density becomes too high. Visualization of the cells with basic brightfield microscopy can help monitor confluency, however, testing of the cells to understand proliferation rates and changes in proliferation can provide very useful information on when it is best to passage a particular cell line. The Click-iT EdU assay can be sued, where the fluorescent signal accumulates in the nuclei of cells where DNA has been synthesized.
The growth rate of cells that have been subcultured repeatedly may sometimes decrease unexpectedly, and there are several options for measuring viability and vitality of your cells. Dye Exclusion using an impermeant dye like DAPI can identify dead cells, while vitality can be measured with assays that include Calcein AM or C12-resazurin.
You may want to also investigate if your cells are undergoing oxidative stress, or apoptosis, and there are easy assays available for these with the CellROX reagents and CellEvent Caspase 3/7 reagent.