Fifteen bacterial colonies growing on a complete medium are transferred to a minimal medium. Twelve of the colonies grow on minimal medium.

Using terminology from the chapter, characterize the 12 colonies that grow on minimal medium and the 3 colonies that do not.

Key Concepts

Complete Medium vs. Minimal Medium

Complete medium contains all the nutrients necessary for bacterial growth, including amino acids, vitamins, and carbohydrates. In contrast, minimal medium provides only the essential nutrients required for growth, typically a carbon source, nitrogen, and salts. Understanding the difference between these media is crucial for characterizing bacterial colonies based on their nutritional requirements.

Auxotrophs and Prototrophs

Prototrophs are bacteria that can synthesize all the compounds they need for growth from minimal medium, while auxotrophs lack the ability to synthesize one or more essential compounds and require additional nutrients. In this context, the 12 colonies that grow on minimal medium are likely prototrophs, whereas the 3 colonies that do not grow are auxotrophs, indicating a mutation that affects their metabolic capabilities.

Selection Pressure

Selection pressure refers to environmental factors that favor certain traits over others, influencing the survival and reproduction of organisms. In this experiment, transferring colonies from complete to minimal medium creates a selection pressure that favors prototrophic bacteria, as only those capable of synthesizing all necessary nutrients will survive. This concept is essential for understanding the dynamics of bacterial growth under varying nutrient conditions.