What characteristic defines a coliform bacteria colony on agar plates?

Coliform bacteria are a group of microorganisms commonly used as indicators of water quality and potential contamination. When cultured on specific types of agar plates, particularly MacConkey agar, coliform colonies exhibit distinct visual characteristics that help in their identification.

The characteristic that defines a coliform bacteria colony on agar plates is its ability to ferment lactose. When this fermentation occurs, it leads to the production of acid and gas, which results in specific visible changes in the agar surrounding the colony. The presence of acid changes the color of the pH indicator in the agar, typically leading to colonies that can be pink in appearance. Additionally, these colonies often appear shiny due to the presence of bile salts in the medium, which inhibit the growth of non-lactose fermenters.

Cultures that develop as pink to dark red and shiny on the agar indicate that the bacteria are lactose fermenters, and thus are likely to include coliforms. This visual characteristic serves as a good preliminary identification of potentially harmful organisms present in water, making it essential for water treatment and quality assurance processes.