For example, if there were only one bacterial cell in 100 ml of water filtered through the membrane filter, it would become entrapped into the top of the filter as the water passed through. What is better is to grow the single bacterial cells into large colonies containing millions of its offspring in a single heap of growth called a colony. Bacteria, like human beings, prefer particular foods and are repelled by other foods. Even when a colony grows into either a dome, pancake, spreading slime, a feathery rhizoid or some other pattern, those characteristics are typical. If you know how much water you filtered and count the number of typical colonies, then you can calculate the numbers present in 100 ml.
All bacteria are like bags of enzymes and they selectively use different foods, particularly sugars.
One of the favorite sugars for the coliform group is lactose. Strangely, that is the main sugar in mother's milk and, therefore, the first one that the bacteria in the baby's intestine come across after the baby has been born and is suckling the milk. Bile is pretty lethal to bacteria, but the coliform bacteria can tolerate it. Not only that, but the coliform. It was Durham who first trapped this gas in an upside down test which was then named after him.