In microbiology cells are called viable when they can be shown to proliferate. Due to long lag phases and the cells' sensitivity to the conditions around them, this definition presents problems when investigating stressed or injured cells. Colony counts only show reproduction, whilst direct single-cell investigation can divide the status of cells into four stages: (i) reproductive viable cells, (ii) vital cells, (iii) intact cells and (iv) dead cells.
Multicolour staining flow cytometry has been found to provide a powerful method for investigating these various states of viability. This is illustrated with examples with some emphasis on mixed bacterial populations.