Fifteen bacterial colonies growing on a complete medium are transferred to a minimal medium. Twelve of the colonies grow on minimal medium.
The three colonies that do not grow on minimal medium are transferred to minimal medium supplemented with the amino acid serine (min + Ser), and all three colonies grow. Characterize these three colonies.

Verified step by step guidance

1. Identify the key difference between the complete medium and the minimal medium. The complete medium contains all necessary nutrients, while the minimal medium contains only the essential nutrients required for bacterial growth.
2. Analyze the observation that 12 out of 15 colonies grow on the minimal medium. This indicates that these 12 colonies can synthesize all the necessary compounds for growth from the nutrients provided in the minimal medium.
3. Focus on the 3 colonies that do not grow on the minimal medium. This suggests that these colonies are auxotrophic for a specific nutrient, meaning they lack the ability to synthesize a particular compound required for growth.
4. Examine the result of transferring the 3 non-growing colonies to the minimal medium supplemented with serine. Since all three colonies grow in this condition, it indicates that these colonies are specifically auxotrophic for serine. They cannot synthesize serine on their own and require it to be provided externally.
5. Characterize the three colonies as serine auxotrophs. This means they have a genetic mutation or defect in the biosynthetic pathway for serine, preventing them from producing this amino acid independently.

Key Concepts

Minimal Medium

Minimal medium is a growth medium that contains only the essential nutrients required for the growth of microorganisms, such as carbon, nitrogen, and salts. It lacks additional growth factors, such as amino acids or vitamins, which some organisms may need to survive. In this context, the ability of bacterial colonies to grow on minimal medium indicates their metabolic capabilities and nutritional requirements.

Amino Acid Supplementation

Amino acid supplementation involves adding specific amino acids, like serine, to a growth medium to support the growth of organisms that cannot synthesize these compounds. The three colonies that failed to grow on minimal medium likely have mutations affecting their ability to produce serine, making them auxotrophic. By supplementing the medium with serine, these colonies can thrive, demonstrating their dependency on external sources of this amino acid.

Auxotrophy

Auxotrophy refers to the condition of an organism that cannot synthesize a particular compound required for its growth due to a mutation. In this case, the three bacterial colonies are auxotrophic for serine, meaning they lack the genetic capability to produce it. This characteristic is crucial for understanding their growth requirements and metabolic pathways, as it highlights their reliance on external sources of nutrients.