With the development of strictly anaerobic techniques and habitat-simu
lating media, a variety of bacteria were isolated from the rumen in th
e 1940s and 1950s. Based on standard morphological and physiological c
haracteristics, the microbial ecosystem of the rumen contains a very c
omplex population of bacteria. In recent years, ruminal bacteria have
been re-evaluated with newer, more objective, and genetically valid me
thods of classification. Ribosomes are complicated structures, and the
ir DNA-encoding sequences are relatively free from selective pressure.
Because ribosomes have evolved slowly, they provide a long-term natur
al history of evolution. The invariable and hypervariable regions of r
RNA genes can be used to group bacteria into kingdoms, genera, and spe
cies. The 16S rRNA sequences have provided a basis for renaming some r
uminal species (Bacteroides amylophilus is now Ruminobacter amylophilu
s and Bacteroides succinogenes is now Fibrobacter succinogenes) and fo
r classifying at least one recently isolated ruminal bacterium (e.g.,
Clostridium aminophilum). The DNA:DNA hybridization is a more sensitiv
e method of assessing bacterial relatedness than is 16S rRNA. Bacteria
l strains within a species should have a high degree of DNA:DNA homolo
gy, but some species of ruminal bacteria (e.g., Prevotella ruminicola
and Butyrivibrio fibrisolvens) had highly unrelated strains. Studies o
f 16S rRNA and DNA:DNA hybridization indicate that the diversity of ru
minal bacteria has been greatly underestimated. Traditional studies of
phylogeny of ruminal bacteria were stymied by the fastidious growth r
equirements of many ruminal bacteria, and enumeration was tedious and
inaccurate. Modern methods of bacterial classification do not require
in vitro culture and have the potential of detecting even a single cel
l.