A chef sprays antimicrobial cleaner on her counter top. At first the bacterial population declines significantly. However, even though she continues to spray in the following weeks, the number of bacteria begins to increase again. Why did this happen?

Bacteria resistance

Explanation:

The antimicrobial cleaner contained certain product which most bacteria living on the counter top were sensitive to. This means, the product was effective and killed most of them. This is why the population declines significantly.

Yet, a few bacteria were already resistant to this product. This happens because of variability in their genome. Some bacteria have mutations in their DNA that confers them this advantage, by chance.

This few resistant bacteria will slowly start to grow again, while the chef continues to spray them, and therefore keeps selecting the resistant ones allowing their population to increase again.