Bacteria have the ability to reproduce rapidly, allowing them to evolve and adapt to their environment quickly. Because of the speed with which they evolve, strains of bacteria can grow in extreme heat, extreme cold, acidic or alkaline environments, radioactive surroundings or in the presence of human toxins. Whatever environment they grow in, however, they need specific nutrients to support their growth. Milk contains these nutrients and provides a good medium for bacterial growth.

The most fundamental nutrient bacteria require is water. Without water, no life form can survive, even if other nutrients are in abundance -- and this fact provides the basis for dehydration as a means to preserve food. Although bacteria can survive the resting spore stage of their life cycle with no water, they cannot metabolize nutrients or grow during this waterless phase of life. Fluid milk contains nearly 90 percent water by weight, according to the U.S. Department of Agriculture National Nutrient Database, providing bacteria the moisture they need for growth.

In addition to water, bacteria require a fuel source to supply energy to power the growth process. Depending on the strain of bacteria, they can derive energy from sunlight, from organic molecules such as carbohydrates or from inorganic molecules, including carbon monoxide. Milk is rich in a sugar called lactose, a simple carbohydrate composed of a glucose molecule joined to a galactose molecule. Bacteria can metabolize milk sugar to produce the energy necessary for reproduction.

Milk contains a significant amount of protein, a nutrient made of nitrogen-rich amino acids. Bacteria require nitrogen to synthesize new proteins as they increase their population during growth. The protein in milk also provides a source of carbon the bacteria can use, in addition to lactose, as a fuel source.

Bacteria need a variety of minerals for optimal growth, and milk supplies them. Milk, for example, contains ample calcium, a mineral required for bacterial function. Milk also offers phosphorous, potassium and sodium to help bacteria increase in number.

Pasteurized milk in an unopened container has few or no bacteria growing in it. The milk may become contaminated once it is open, however, particularly if you allow a part of your body to touch the inside of the container. For this reason, avoid drinking milk directly from the carton or jug to prolong its shelf life. Keeping the milk cold also helps retard bacterial growth. Raw milk does not undergo the pasteurization process to kill off potentially pathogenic bacteria, and therefore may pose the risk of food poisoning. Because milk provides such a nutritious environment for bacteria to grow, pathogenic organisms can multiply rapidly in this medium.