Anaerobic bacteria oxygen tolerance

Section 4.3.2), (3) in the induction of cytochrome P-450 14DHin Saccharomyces cerevisiae (Section 4.4.1.2). The oxygen tolerance among anaerobic bacteria has been discussed in Section 4.3.4. Further comments on other organisms are given here. 

The microflora of the gastrointestinal tract, which develop shortly after birth, are found in the lumen, associated with remnants of food, on the mucosal surface, and in the crypts. These microorganisms, including prokaryotic and eukaryotic species, tend not to use oxygen in their energy metabolism since they exist in an environment that is essentially anaerobic. Over 99.9% of the bacterial cells are anaerobic, meaning they are unable to use oxygen even when it is provided for them. Of these bacteria, 5-20% are unable to tolerate O2 and die in its presence. 

The Archaea are similar to bacteria and until 1977 most known species were classified as bacteria (see review by Makarova and Koonin, 2003). It was the work of Carl Woese (see Woese and Fox, 1977) who discovered that some so-called bacteria were different genetically and separated the Archaea from the bacteria on the basis of particular rRNA sequences present in the cells of the organisms. The Archaea in existence today are thought to have descended from the first forms of life to have evolved on Eardi some 3.5 to 4.0 x 10 years ago. This relates to the fact that many (but not all) members of the existing Archaea are anaerobic, i.e. they do not require oxygen for survival, and thus could inhabit a world with an oxygen-poor atmosphere as was the case during the early history of the Earth. They also survive under extreme habitats today (see Table 3.2) and so could have tolerated the harsh conditions of the newly-formed planet. 

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