Both studies also found phylotypes from other groups of
bacteria in varying numbers. Gingival plaque, for example,
had considerably more Streptococci and Actinomycetes
than the gut, perhaps because these organisms are relatively
tolerant of oxygen. Overall, representatives of four
distinct ‘divisions’ of bacteria were found among the
clones sequenced from the fecal sample, and five divisions
were found among the plaque-derived clones. ‘Division’
level groupings have been proposed as the penultimate
taxonomic level for bacteria, one level short of the
Bacteria–Archaea–Eucarya split [14]. To get a feeling for
this evolutionary depth, the 16S rRNA sequences of
members of each bacterial division typically differ from
other divisions by > 20%, more for example than the
divergence between yeast and humans.