APTER 20 Physiological Characteristics and the Species Concept in Actinomycetales

Physiological characteristics play a dominant role in characterization, classification, and speciation of procaryotes. Because of limited morphological diversity, conventional systematics of these microorganisms still is in a disappointing state. Fortunately, with many Actinomycetales morphological diversity does occur. This diversity can be used to advantage in systematics because it represents the result of a complex multitude of "physiological reactions" expressed in the phenotypic criterion of morphology. Morphological information coupled with that from more sophisticated approaches to characterization and classification, e.g., cell-wall analyses, determination of guanine and cytosine ratios in nucleic acids, nucleic acid hybridizations, and genetic studies, would seem to offer the best contemporary approach to assessing the significance of "physiological characteristics" as they now are used. Physiological characteristics represent responses of the organism to various aspects of the environment, are based on enzymatic activities, and are subject to natural variation and induced mutation. Many physiological reactions, as expressed in the literature on systematics of procaryotes, represent multiple enzymatic activities. There is need to clarify this aspect of conventional procaryote systematics to determine the real significance of such reactions to systematics and speciation.