Page:Bergey's manual of determinative bacteriology.djvu/1007

 ORDER II. VIRALES BREED, MURRAY AND KITCHENS, 1944.

Viruses are etiological agents of disease, typically of small size and capable of passing filters that retain bacteria, increasing only in the presence of living cells, giving rise to new strains by mutation. A considerable number of plant viruses have not been proved filterable; it is nevertheless customary to include these viruses with those known to be filterable because of similarities in other attributes and in the diseases induced. Some not known to be filterable are inoculable only by special techniques, as by grafting or by the use of insect vectors, and suitable methods for testing their filterability have not been developed; moreover, it is not certain that so simple a criterion as size measured in terms of filterability will prove to be an adequate indicator of the limits of the natural group. Viruses cause diseases of bacteria, plants and animals.

Our incomplete knowledge of the entities known as viruses has made their classification, and consequently their nomenclature, a difficult matter. It is difficult to describe viruses adequately because of their small size and because they are not cultivable. Electron microscopy has enabled a determination of the size and morphology of some of the viruses. Likewise, serological methods have been developed which are proving to be useful in distinguishing between different species and types of viruses, but in many cases these methods have not been applied.

The usual characteristic that permits recognition of viruses is their capacity to produce specific diseases. As indicated in the previous edition of the Manual (6th ed., 1948, 1127), three constituent groups of viruses have come to be recognized, and to some extent named and classified, through the largely separate efforts of bacteriologists, animal pathologists and plant pathologists. Taxonomic overlapping of the three groups, viruses affecting bacteria, viruses having human and other animal hosts and viruses invading higher plants, can hardly be justified as yet by available evidence. Nevertheless, it has been shown that a single virus may multiply within, and cause morphological changes in, both a plant host and an insect vector (Littau and Maramorosch, Virology, 2, 1956, 128). This seems to dispose of the thought that adaptation to one environment necessarily precludes the utilization of other sources for the materials needed for growth and multiplication.

Because of the difficulties involved in preparing adequate descriptions of species of viruses, many investigators have felt it undesirable to use binomials according to the Linnean system and therefore have proposed numbering or lettering systems for species and subspecies of viruses (see Johnson, Wis. Agr. Exp. Sta. Res. Bull. 76, 1927; Proc. Sixth Internat. Bot. Cong. (Amsterdam), 2, 1935, 193; and Smith, Textb. of Plant Virus Dis., Philadelphia, 1937, 615 pp). These systems have made it difficult for persons other than