Answer:
Thermophilic bacteria
Step-by-step explanation:
Thermophiles or thermophilic organisms are those that have the ability to survive extremely high temperature usually above 100 degrees. They are a group of extremophiles, a group of organisms that are capable of existing at extreme conditions such as high or low temperatures, pressure, etc.
Normal bacteria would have been killed by the heat of boiling for several hours but thermophilic bacteria will survive and hence, were able to grow back.