The Classification of Prokaryotes

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Dr. Carl Woese of the University of Illinois is continuing his pioneering studies on the evolution, phylogeny, and classification of bacteria, by using nucleotide sequence data from ribosomal RNA to quantify genetic differences between bacteria. By comparing the sequence of ribosomal RNA from a variety of organisms, a branching pattern of genealogical affinity can be constructed; few nucleotide differences between species indicate close evolutionary relationship (the sharing of similar genomes from a close common ancestor) whereas greater gene differences indicate more ancient divergence between lineages. Dr. Woese is concentrating on clostridial and flavobacterial organisms, and other groups traditionally called Gram-positive types. An added dividend to this phylogenetic analysis of bacteria is the use of ribosomal sequences to design species-diagnostic oligonucleotide probes. These probes have been successfully employed in both medical and ecological studies to identify bacteria that have otherwise proved difficult or impossible to culture in the laboratory. In addition, the phylogenetic relationships revealed through comparative nucleotide sequences provide an evolutionary framework for interpreting and studying other aspects of the physiology, biochemistry, and ecology of microbial organisms.