Aerobes are organisms that can tolerate or require the presence of (strict aerobe in this case) oxygen. Oxygen is an extremely strong oxidant and produces very reactive radicals, which react with amino acids or nucleic acids inactivating the functional sites of enzymes or producing lethal mutations. Practically, all animals are aerobes; most fungi and many prokaryotes can survive in the presence of oxygen. To do so, aerobic organisms require the presence of detoxification activities, like catalases and peroxidases. Among aerobes there are different kinds of organisms: obligate aerobes, which require oxygen for growth and use oxygen as final electron acceptor in the respiration process; facultative aerobes, which can use oxygen or not, to obtain energy; microaerophiles, which require low levels of oxygen; and aerotolerants, which are not affected by the presence of oxygen.