7n the early 1870s, the German botanist Ferdinand Cohn published several papers on bacterial classification. Cohn grouped microorganisms according to shape: spherical, short rods, elongated rods, and spirals. This classification scheme implied that bacteria are not plastic and flexible but maintain a constant shape. Thus, spherical organisms give rise to spherical organisms; they do not become rods following binary fission. At the same time, Cohn recognized that classification based solely on shapes was not adequate for categorizing all of the different bacteria. There were too many kinds and too few shapes.
The second major attempt at bacterial classification was initiated by Sigurd Orla-Jensen. Orla-Jensen's early training in Copenhagen was in the field of chemical engineering, but he soon became interested in microbiology and fermentations. In 1908, he proposed that bacteria be classified according to their physiological properties rather than their morphological properties. He considered organisms that gained their energy from inorganic sources as the most primitive.
A quarter of a century later, two Dutch microbiologists, Albert Kluyver and C. B. van Niel, proposed two classification systems that were based on presumed evolutionary relationships. They recognized a very serious problem in their classification scheme, however: There was no way to distinguish between "resemblance" and "relatedness." The fact that two prokaryotes look alike does not mean they are genetically related.
In 1970, Roger Stanier, a microbiologist at the University of California, Berkeley, pointed out that relationships could be determined by comparing either gene products, such as proteins and cell walls, or nucleotide sequences. At that time, leading microbi-ologists, including Stanier, assumed that all prokaryotes are basically similar. When the chemical compositions of a wide variety of prokaryotes were examined in detail, however, it was found that many had features that differed from those present in the intensively studied Escherichia coli, considered a "typical" bacterium. These "unusual" features pertained to the chemical nature of the cell wall, cytoplasmic membrane, and ribosomal RNA.
In the late 1970s, Carl Woese and his colleagues at the University of Illinois took advantage of nucleic acid technologies, which allowed investigators to determine the sequence of bases in ribosomal RNA. When they compared such sequences from a wide variety of organisms, it became clear that prokaryotes could be divided into two major groups that differ from one another as much as they differ from the eukaryotic cell. Thus, prokaryotes,
once believed to be a part of the plant kingdom, have now been separated into two domains, the Archaea and the Bacteria. Each of these is on the same level as the Eucarya, which includes the animals, plants, and fungi; these are all eukaryotes.
—A Glimpse of History
INFORMATION THAT IS LOGICALLY ORGANIZED IS easier to both retrieve and understand. Newspapers, for instance, do not scatter various subjects throughout the paper; rather they are divided into sections such as local news, sports, and entertainment. A large library would be extremely difficult to use if the multitude of books were not split into sections by subject matter. Likewise, scientists have divided living organisms into different groups, the better to understand the relationships among the species.
Take a moment and think about how you would group bacteria if you were to arrange a classification system. Would you group them according to shape? Or would it make more sense to group them according to their motility? Perhaps you would group them according to their medical significance. But then, how would you classify two similar bacteria that differ in their disease-causing potential? And how would you classify a newly identified organism of unknown medical significance?
246 Chapter 10 Identification and Classification of Prokaryotes
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