What is the most important criteria for distinguishing between viable and irreversibly damaged cells?

The most important criterion for distinguishing between viable and irreversibly damaged or non-viable cells is membrane integrity. 

Viable Cells

Viable cells have intact cell membranes that effectively regulate the passage of substances in and out of the cell. These cells are capable of growth, division, and reproduction. 

Viable cells do not emit light when passed through a cytometer. This is because the stain cannot penetrate the intact membrane. 

Methylene blue stain is used to detect viable cells. 

Irreversibly damaged cells 

Irreversibly damaged cells have ruptured or compromised cell membranes, leading to loss of selective permeability. This results in uncontrolled substance movement in and out of the cell, which is indicative of cell death. These cells are incapable of growth, division, or reproduction. 

Nonviable cells emit light when passed through a cytometer. This is because the stain easily passes through the compromised membrane. 

Trypan blue, Erythrosin B, and Nigrosin stains are used to detect nonviable cells.