Answer
The bacteria developed a resistance to the antibiotic.
Work Step by Step
An antibiotic will bind to certain bacterial proteins that make proliferation either slow or stop completely. In a bacterial culture where an antibiotic is present the proliferation rate of the cells would slow but return to normal because some of the bacteria could have mutated that specific bacterial protein in a way that makes the antibiotic unable to bind to it. So while the original strain of the bacteria would be slowed down because they are affected, the mutated bacteria would continue on, being generally unaffected.
