Overview: A Phylogenetic Backbone and Taxonomic Framework for Procaryotic Systematics

Abstract

Despite its relatively short history, microbial systematics has never been static but rather constantly subject to change. The evidence of this change is provided by many reclassifications in which bacterial taxa have been created, emended, or dissected, and organisms renamed or transferred. The development of a procaryotic systematics that reflects the natural relationships between microorganisms has always been a fundamental goal of taxonomists. However, the task of elucidating these relationships could not be addressed until the development of molecular methods (the analysis of macromolecules) that could be applied to bacterial identification and classification. Determination of genomic DNA G + C content, and chemotaxonomic methods such as analysis of cell wall and lipid composition, in many cases proved superior to classical methods based upon morphological and physiological traits. These tools provide information that can be used to differentiate taxa, but do not allow a comprehensive insight into the genetic and phylogenetic relationships of the organisms. DNA—DNA reassociation techniques provide data on genomic similarity and hence indirect phylogenetic information, but the resolution of this approach is limited to closely related strains. DNA—DNA hybridization is the method of choice for delimiting procaryotic species and estimating phylogeny at and below the species level. The current species concept is based on two organisms sharing a DNA—DNA hybridization value of greater than 70% (Wayne et al., 1987).