Presence-absence tests are a convenient way to detect coliforms and E. Coli in your water system. Presence-Absence Broth, disposable bottles. When coliform bacteria are in the sample, the bacteria ferment the lactose in the broth during incubation and acidify the sample. The acidity causes the color of the indicator to change from purple to yellow. Positive results must be confirmed with confirmation media. Coliform bacteria will cause gas and turbidity to form in the confirmation media.