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Temperatures that are too cold can make it very difficult for bacteria and fungi to carry out any of their life processes.
Bacteria can be frozen or chilled and continue to survive, but they will not be able to feed or carry out respiration to gain energy. Without enough energy, bacteria can not reproduce. Freezing or chilling bacteria is therefore a good way to control the growth of their populations, but it does not kill them.
High temperatures, on the other hand, do kill bacteria. High temperatures will also kill fungi and permanently damage the protein cases of viruses, leaving them unable to function.
Different bacteria have different optimum temperatures. An optimum temperature is the temperature that the organism function best in. Bacteria that are pathogenic to humans function best at 37 degrees Celsius. But bacteria that live in hot pools function best at temperatures closer to 70 degrees Celsius.

Micro-organisms also have an optimum pH. Most bacteria that live in human beings have a preferred pH of about 6-8, but bacteria that live in your stomach function best in acidic conditions. (pH 1 or 2.) If a micro-organism is not in its optimum pH, then it will not be able to carry out its life processes so well. If the pH of its environment is too different to its optimum pH, then the micro-organism may not survive.