The molecular biology of the gene and the deployment of the electron microscope (increasing resolving power a thousand-fold over the light microscope) after the Second World War led to refinement in the concept of a bacterium, both biochemically morphologically. That articulation began with a new concept of the virus. The word virus (latin for slimy liquid, poison, offensive odor or taste) had changed its meaning over the centuries. In the sixteenth century, it was used in its original sense to refer to poison or venom, as in the story of Cleopatra pouring the virus of an asp into a wound made in her arm by her own teeth. In the eighteenth century, one referred to virus in the sense of contagious pus. In the nineteenth century, "viruses" were considered as microbes. Bacteria were isolated using filters, permeable to toxins but impermeable to the bacteria, but there were anomalies. Some infectious agents were so small that they could pass through a bacterial filter. These were called "filterable viruses." Other small, obligate parasitic bacteria of the rickettsia type, barely resolvable by the light microscope, were often thought to be transitional between the filterable virus and the typical bacterium. Thus, bacteria were thought to range from the size of some algae to the size of filterable viruses. As late as 1958, Bergey's Manual suggested a new kingdom, "protophytes," which would include both bacteria and viruses.83
A year earlier, however, a major distinction between viruses and bacteria had been articulated, based on molecular structure and physiology, in a now-classic paper by André
Lwoff: "The Concept of Virus." The virus contained either RNA or DNA enclosed in a coat of protein, and it possessed few if any enzymes, except those concerned with attachment to and penetration into the host cell. The virus was not organized like a cell, and it did not reproduce by division like a cell. Its replication occurred only within a susceptible cell, which always contains both DNA and RNA—an array of different proteins endowed with enzymatic functions that are mainly concerned with the generation of ATP and the synthesis of varied organic constituents of the cell from chemical compounds in the environment. "Viruses should be treated as viruses," Lwoff concluded, "because viruses are viruses."84 Only the word "virulence," for poisonousness, was retained for both bacterial and viral diseases. There were no biological entities that could properly be described as transitional between a virus and a cellular organism, and the differences between them were of such a nature that it was indeed difficult to visualize any kind of intermediate organization.
Five years later, Stanier and van Niel wrote a sister paper to Lwoffs, entitled, "The Concept of a Bacterium." Although they were no longer willing to defend the classification scheme they had defended in 1941, Stainer and van Niel were able to offer a definition of bacteria that would distinguish them both from viruses and from other protists. "Any good biologist finds it intellectually distressing to devote his life to the study of a group that cannot be readily and satisfactorily defined in biological terms; and the abiding intellectual scandal of bacteriology has been the absence of a clear concept of a bacterium."85 Certainly many microscopists since the days of Haeckel had recognized a difference between bacteria (and blue-green algae), which lacked a true nucleus, and other protists that contained a nucleus. In the early 1920s, Lwoff's mentor Edouard Chatton (1883-1947), at the University of Strasbourg, referred to Cyano-phyceae, Bacteriacae, and Spirochaetaceae as "procaryotes" (Greek before karyon or nucleus) to distinguish them from the protozoa, which he called "eucaryotes" (Greek: eu-karyon; true nucleus)."86
At Lwoff's suggestion, Stanier and van Niel adopted these terms, asserting that the distinctive property of bacteria and blue-green algae is the procaryotic nature of their cells. "The principle distinguishing features of the procaryotic cell are: 1 absence of internal membranes which separate the resting nucleus from the cytoplasm, and isolate the enzymatic machinery of photosynthesis and of respiration in specific organelles; 2 nuclear division by fission, not by mitosis, a character possibly related to the presence of a single structure which carries all the genetic information of the cell; and 3 the presence of a cell wall which contains a specific mucopeptide as its strengthening element."87 In effect the bacteria were defined negatively in terms of what they lacked. Eukaryotes had a membrane-bound nucleus, a cytoskeleton, an intricate system of internal membranes, mitochondria that perform respiration, and in the case of plants, chloroplasts. Bacteria (prokaryotes) were smaller and lacked all of these structures.
Just as there were no transitional forms between virus and bacteria, Stanier and his collaborators insisted, there were no transitional forms between bacteria and all other organisms. Stanier, Michael Doudoroff, and Edward Adelberg declared in their famed book The Microbial World of 1963 that, "In fact, this basic divergence in cellular structure, which separates the bacteria and blue-green algae from all other cellular organisms, represents the greatest single evolutionary discontinuity to be found in the present-day world."88
During the 1950s, palaeobiologists extended the fossil record back to indicate the presence of eukaryotic fossils in rock that was some 2000 million years old.
Paleontologists who marveled at the great burst in plant and animal diversity of 500 million years ago referred to it as "the Cambrian explosion," but those who studied cellular organization insisted that the real "big bang" of biology occurred some 1500 million years earlier, when the eukaryote arose. With its membrane-bound nucleus and all the associated features, such as mitosis, meiosis, and multiple chromosomes to package up to tens of thousands of genes per cell, the eukaryote provided the organismic conditions for the differentiation of tissues, organs, and organ systems of plants and animals.
Stanier, Doudoroff and Adelberg asserted that there was a common origin for bacteria in the remote evolutionary past, at the same time they insisted that bacteria simply could not be arranged phylogenetically. Only four principle groups were able to be discerned: blue-green algae, myxobacteria, spirochetes, and eubacteria; "Beyond this point, however, any systematic attempt to construct a detailed scheme of natural relationships becomes the purest speculation, completely unsupported by any sort of evidence."89 There was only one possible conclusion: "the ultimate scientific goal of biological classification cannot be achieved in the case of bacteria."90
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