<P> Obligate anaerobes are microorganisms killed by normal atmospheric concentrations of oxygen (20.95% O). Oxygen tolerance varies between species, some capable of surviving in up to 8% oxygen, others losing viability unless the oxygen concentration is less than 0.5% . An important distinction needs to be made here between the obligate anaerobes and the microaerophiles . Microaerophiles, like the obligate anaerobes, are damaged by normal atmospheric concentrations of oxygen . However, microaerophiles metabolise energy aerobically, and obligate anaerobes metabolise energy anaerobically . Microaerophiles therefore require oxygen (typically 2--10% O) for growth . Obligate anaerobes do not . </P> <P> The oxygen sensitivity of obligate anaerobes has been attributed to a combination of factors: </P>

Obligate anaerobic bacteria may be killed by high doses of