IGEM:MIT/2005/Nitrocefin

Nitrocefin is normally excluded by the outer cell membrane, but if able to pass this barrier it can be cleaved by beta-lactabase localized within the periplasmic space. Cleavage results in a color change from yellow to red, and this can be used to monitor outer membrane permeabilization.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=89666