Explain the Confirmed Test - Most Probable Number Test?
Confirmed test is carried out to rule out the possibility of any positive presumptive test because of the presence of noncoliform organisms which are not recognized as indicators of faecal pollution. The test is performed by streaking on selective and differential media, i.e., EMB agar and Endo agar from positive lactose broth tube in presumptive test.
EMB contains lactose and dyes eosin and methylene blue. Methylene blue inhibits the growth of Gram positive bacteria. Lactose fermenters produce acid which results in the precipitation of dyes on to coliform colonies producing dark coloured center and a green metallic sheen. These results are characteristics of E.coli, the major indicator of faecal pollution. Enterobacter aerogenes produce pink and mucoidal colonies while nonlactose fermenters produce colourless, transparent colonies. Endo agar is a nutrient medium containing dye fuchsin in decolourized state which produce a pink complex in presence of acid produced by coliforms resulting in pink colonies of E. coli.