What did kwang jeon discover when researching amoeba infected by bacteria?

Kwang Jeon observed that Amoeba had been attacked by a bacterial infection, and lots of the Amoeba had died. However, some survived and continued to reproduce. After investigating the remaining Amoeba and their offspring, he noticed they were very healthy. He thought maybe they were able to fight off the bacteria, but instead, he found they were still infected with the bacteria but were not dying. The bacteria were no longer making the Amoeba sick. Then, he killed off the bacteria using antibiotics and was surprised to see that the Amoeba also died. It seemed the Amoeba and bacteria had formed a relationship in which they both needed each other to survive. After researching, Jeon found that the bacteria made a protein that the Amoeba needed to survive.

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Ido not understand.