One year later, there was another outbreak at the farm involving Salmonella bacteria. Antibiotic B did not have an effect. Suggest why antibiotic B no longer had an effect.

The reason antibiotic B no longer had an effect is due to antibiotic resistance. Within the original population of Salmonella, there would have been natural variation. A few bacteria may have had a random mutation that made them resistant to antibiotic B. When the antibiotic was used, it killed the non-resistant bacteria, but the resistant ones survived and reproduced. Over time, the entire population of bacteria on the farm evolved to be resistant to antibiotic B, making the antibiotic ineffective in the later outbreak.