What are the commonly used methods for determining cell viability?

Cell viability is a measure of the proportion of living cells in a sample population.  Some of the more commonly used methods for determining cell viability include: 

• Dilution is a simple method that involves serially diluting a cell suspension. The technique is based on the principle that viable cells will continue growing and dividing in progressive dilutions, while non-viable cells will not proliferate. Researchers estimate the percentage of cells in the original sample by comparing cell growth in each dilution. 
• Trypan Blue cell viability assay is a straightforward and widely used cell viability assay that involves the use of the Trypan Blue dye. Trypan Blue selectively stains dead cells with compromised membrane integrity, while live cells with intact membranes remain unstained. After incubating the cells with the stain, researchers use a hemocytometer or automated cell counter to count the number of dead stained cells and viable unstained cells. 
• The roll tube technique involves embedding viable cells in a semi-solid agar medium and pouring the mixture into a glass tube. The tube is rolled to create a thin layer of agar containing the cells and then incubated for a specified period, allowing viable cells to proliferate and create visible colonies within the agar. Researchers count the colonies to determine the number of viable cells in the sample. 
• Surface viable count is based on the ability of viable cells to proliferate and form colonies on a solid surface after exposure to the test substance. In this technique, the cells are first incubated with the test substance. The viable cells are then plated onto a solid surface such as an agar plate or culture dish for a specified period. The number of colonies formed is proportional to the number of viable cells in the sample. 
• Nalidixic acid cell viability assay is based on nalidixic acid’s ability to selectivity inhibit the growth of nonviable cells. In this method, nalidixic acid, an antibiotic, is incorporated into the culture medium. This stops the proliferation of nonviable cells, while viable cells continue to proliferate. Researchers estimate the percentage of viable cells in the sample by comparing cell growth in the presence and absence of nalidixic acid. 
• Fluorogenic dye method involves using two different dyes. The first dye, such as calcein-AM, selectively stains live cells. The second dye, such as propidium iodide, selectively stains dead cells. Researchers determine the proportion of live and dead cells by measuring the fluorescence intensity of each dye in the sample.