HOW MANY RUMINAL BACTERIA ARE THERE

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.