Maybe it was caused by the plastic in the Petri dish. Maybe the air in the Petri dish was too cold or went bad, or maybe it was handled in a weird way, or maybe that bacteria was just a bad culture and it somehow just spontaneously died on its own. She can't say with confidence that it was definitely the antibiotic. In order for her to make that statement, she would have to run a proper experiment. She would have to have a control and a treatment group where everything is equal except for the treatment group has the treatment. So if she had two Petri dishes that were kept in the same conditions with the same lighting, the same air, the same material that the bacteria is growing on, everything the same, except for the treatment group has the antibiotic applied to it, and then she saw that in the treatment group that most of the bacteria died while in the control group most of the bacteria didn't die, then she could say, okay, it looks like the antibiotic caused the bacteria to die, that there was actual causality here. So she would have had to run an experiment.