Fabrication of Low Cost Optical Detection System For E. coli 0157 Based On 4-Methyl Umbelliferyl- ß -D-Galactoside– ß-Galactosidase Reaction. Joseph Michael D. Racho, Erba V. Del Rosario, Karlen C. Dawal, Dan R. Laurente, and Raymond J. Sucgang*1, In cooperation with the Chemists & Educators Support for the Advancement of Relevant Science Education, Inc., CESARSED, Inc. 1R.J. Sucgang Center for Research in the Natural Sciences Napti, Batan, Aklan Keywords: Sensor, Optical detection, E.coli This paper reports the fabrication of a handheld, simple to use sensor which detects contamination by deadly strains of E coli in drinking water. E. coli 0157, predominately 0157:H7, has been implicated as the causative organism of hemolytic uremic syndrome and hemorrhagic colitis. The sensor works based on irreversible color changes that occur subsequent to hydrolysis of a chromogenic substrate by a ß-galactosidase enzyme of E.coli. The recognition material, 4-m collagen cream ethyl umbelliferyl- ß -D-galactoside has two components: a fluorescent dye and a “quencher” which is sensitive to cleavage by ß-galactosidase. In the absence of E.coli, the quencher binds to the dye and prevents fluorescence, while the interaction of ß-galactosidase with the quencher results to the dissociation of the complex ,7-hydroxy-4-methylcoumarin, giving off a blue fluorescence. The sensor uses an LED light source shining on a photodetector. When the colorimetric reaction has taken place, light shines through a beaker containing the sample, and onto a light-sensitive meter. A clear tube of water or other clear solution is the BLANK and has zero absorbance. A fraction of the incident light that is blocked by the sample, can be used for quantitative determination of the bacterial population in the water. The E. coli population is related to the absorbance reading, following Beers law. The amount of incident and transmitted light is expressed in foot candle units, by a LX 104 light meter.