As nitrogen source, most bacteria require protein, peptides, or amino acids, but many can use ammonia, nitrates or nitrogen molecules. Some organisms can make almost everything them need, but others require a long list of vitamins and required factors. Some metabolize their food down to carbon dioxide and water, others have very limited in disimilatory metabolism and leave their food practically untouched. Bacteria used in food processing typically are able to use only a few of the components of the food they used to process. Knowledge of what a bacterium needs helps you design a medium for growing it.
All bacteria require some amount of water.
Some can live in evaporating salt water or moist pastes, but completely dry foods such as a bag of sugar are free from bacterial attack. Dried meat or fruit are free from attack by most bacteria, but those foods may be attacked when the humidity increases. However, attack of damp dried foods is usually by fungi rather than bacteria. While scientists use distilled water in vitamin studies, in instances where hard water might affect pH of the medium, or in situations where the water might contain interferring substances, many bacteriologists prefer tap water for making medium to save money or supply trace minerals. Water is the main ingredient in any medium.