25 May

The Confirmed Test

The second of three tests to determine the coliform count is the confirmed test.  In this test plates of Levine EMB agar or Endo agar are inoculated from positive (gas producing) tubes to see if the organisms producing the gas are gram-negative.  Both types of media inhibit the growth of gram-positive bacteria and cause colonies of coliforms to be distinguishable from non-coliforms.  Levine EMB agar contains methylene blue which inhibits gram positive agar.  Gram negative lactose fermenters that grow on this medium will form colonies with dark centers. Endo agar contains a fuchsin sulfite indicator that makes it easy to identify lactose fermenters.  The coliform colony and the surrounding media appear red on the Endo agar.  Non-fermenters of lactose are colorless and do not affect the color of the medium.  The confirmed test takes place after the presumptive test and before the completed test.