Why did the second agar plate include an antibiotic?

The inclusion of an antibiotic in the second agar plate is primarily to ensure that only transformed bacteria grow on that plate. When performing genetic transformations, such as with plasmids that carry antibiotic resistance genes, the purpose of the antibiotic is to selectively allow only those bacteria that have successfully taken up the plasmid (and thus gained resistance to the antibiotic) to survive and proliferate.

Without the antibiotic, both transformed and non-transformed bacteria would grow, making it difficult to identify which colonies have successfully incorporated the desired genetic material. By having the antibiotic, researchers can isolate and analyze the transformed bacteria that exhibit resistance, while the non-transformed bacteria are inhibited or killed, verifying the effectiveness of the transformation process.

In this context, promoting the growth of all bacteria or testing the antibiotic's effectiveness does not align with the desired goal, and providing nutrients is a general requirement for all bacterial growth which doesn’t specifically relate to the purpose of using the antibiotic in the second plate.