Exposure to atmospheric levels of oxygen is lethal to obligate anaerobes. It is because they lack the enzymes like superoxide dismutase and catalase that would convert the lethal superoxide formed in their cells due to the presence of oxygen.
Obligate anaerobes may use fermentation or anaerobic respiration. Instead of oxygen, they use sulfate, nitrate, iron, manganese, mercury, or carbon monoxide as electron acceptors for respiration. The energy yield is lower than that in aerobic respiration.
Examples of obligate anaerobes are Bacteroides and Clostridium species.
Word origin: obligate » Latin obligātus (ptp. of obligāre), to bind + anaerobe » an- from Gk., "not, without," + Greek āero-, from āēr, air.
Compare: obligate aerobe.