Propidium iodide (PI) has been used to monitor membrane integrity due to the fact that it cannot cross the cell membrane. My question is: is it possible for PI to cross the outer membrane of Gram negative bacteria without crossing the inner membrane?
In other words: can I assume that the outer membrane is intact in Gram-negative bacteria that do not stain with PI?
Many factors contribute to ability of bacteria to strain with PI. Outer membrane provides a partial resistance to PI staining. When the outer membrane is compromised, the PI staining increase even in viable cells. Here is an interesting read: http://aem.asm.org/content/73/10/3283.full
If you need to assess the outer membrane permeability, the more direct and commonly used assay is the uptake of the fluorescent probe 1-N-phenylnaphthylamine (NPN).
Good luck!
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