Answer:
gene transfer
Explanation:
According to my research on studies conducted by geneticists, I can say that based on the information provided within the question this is likely due to gene transfer between the two bacteria. This term refers to the movement of genetic material (DNA) from "parent" to "offspring". Since one bacteria was resistant to penicillin once it grew and replicated it transfered it's penicillin resistance to the other bacteria through gene transfer.
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