A biosensor refers to an engineered organism, usually a bacterium, which is capable of reporting some ambient phenomenon such as the presence of heavy metals or toxins. In this capability, a very widely used system is the Lux operon of Aliivibrio fischeri .  The Lux operon codes for an enzyme which is the source bacterial bioluminescence , and can be placed after a respondent promoter to express the luminescence genes in response to a specific environmental stimulus.  One such sensor created in Oak Ridge National Laboratory , and named "critter on a chip", consisted of a bioluminescent bacterial coating on a photosensitive computer chip to detect certain petroleum pollutants . When the bacteria sense the pollutant, they begin to luminesce.