Bacterial Systematics on its Way to Become a Scientific Discipline <B>Bacterial Systematics on its Way to Become a Scientific Discipline</B><P> </B><EM><B></B></EM><B>Erko Stackebrandt</B><BR> Deutsche Sammlung von Mikroorganismen und Zelkulturen GmbH, Mascheroder weg 1b, D-38121 Braunschweig, Germany<B></B><P> The ultimate goal of microbial systematics in the eyes of early microbiologists was to relate microbial taxonomy to evolution, thus putting microbiology on a level footing with botany and zoology. Almost as dramatic as the realisation that gross morphology (rods, cocci, spirilla) or physiological groups (phototrophs, chemolithotrophs, anaerobes) did not consistently form coherent evolutionary units, leading to an abandonment of taxonomy incorporating evolutionary principles, has been the realisation that it is, indeed, possible to examine evolutionary relationships within prokaryotes. Just as previous generations of microbiologists were reluctant to abandon the search for the key to evolutionary relationships, so too have the heirs of the determinative system been reluctant to change the course of prokaryotic taxonomy towards the present evolutionary oriented science which it seems destined to become.<BR> Taxonomic units and ranks, arbitrarily defined by microbiologists to facilitate communication, are absent in nature. The construction of a hierarchical system is necessarily a compromise that takes into account our inability to fully understand the mode and tempo at which individual bacterial cells evolve, the restriction to base taxonomic conclusion on a minority of existing organisms, hence on a small fraction of biodiversity only, and the subjectiveness with which taxonomists consider characters to be "important" or less significant. Eventually it may be possible to define taxa at all levels exclusively on the basis of taxon-specific sequence idiosyncrasies alone. From a taxonomic point of view, the restriction of the wealth of information contained in and expressed by the genome, to the mere analysis of the primary structure of macromolecules would not take advantage of the evolutionary process at all levels of information.<BR> In order to respond to the modern developments in microbial taxonomy the concept of a "polyphasic" taxonomy has been developed. The major emphasis of the polyphasic system is to establish a framework of phylogenetic groupings and to subsequently delineate these groupings based on suitable phenotypic characteristics. Decisions about the rank of an organism in a hierarchical system is made on the basis of contributions of geno- and phenotypic properties, that distinguish it from neighbouring groups.<P>