Page:Bergey's manual of determinative bacteriology.djvu/34

 Eubacteriales is defined to include not only the bacteria that are peritrichously flagellated but also such non-motile forms as seem by their physiology to be closely related to these peritrichous species.

The placing of non-motile species of bacteria in systems of classification has always caused difficulty. Some students think that lack of motility is a character which should be used as a basis for separation of groups. However, evidence is continually accumulating that indicates that separation of larger groups among the bacteria solely by means of motility or lack of motility leads to a violent disarrangement of natural groupings. Some non-motile bacteria present fundamental physiologies and other characters that show that they are much like certain polar flagellate organisms. Such non-motile species are placed in the classification used here in Order I, Pseudomonadales. However, where non-motile species show fundamental physiologies and other characters more like those of peritrichous species, then they have been placed in Order IV, Eubacteriales. Organisms living in habitats where they are unable to use organs of locomotion are usually found to be non-motile. This is very natural from the standpoint of evolution.

Some bacteria develop into trichomes, which may be defined as chains (filaments) of bacteria where the relationship between the cells in the chain have become so intimate that the cells rarely live a separate, independent existence. Sometimes the cells in the chain show a differentiation into hold-fast cells and/or reproductive cells distinct from the usual vegetative cells. This differentiation resembles that found among the simpler algae. Because the cells in these trichomes sometimes develop flagella that are placed singly or in a tuft near or at the pole of the cell, while others develop cells with peritrichous flagella, it has been felt desirable to recognize two orders among these bacteria that occur in trichomes: Order II, Chlamydobacteriales, for the polar flagellate types and Order VI, Caryophanales, for the peritrichous types. Some non-motile species occur in these orders also.

Little is known about the relationships of certain species of bacteria which show a budding form of reproduction that is different from the simple cell division (fission) that takes place in the four orders previously discussed. Only a few of these species that reproduce by budding are well known, though some of them occur abundantly in suitable habitats. Because the indications are that many species of these organisms exist in nature. Prof. H. C. Douglas has set these apart in a new order, Hyphomicrobiales, p. 276. Where flagellation has been observed among these budding forms, it is of the polar type so that Order III has been associated with Order I, Pseudomonadales, and Order II, Chlamydobacteriales, in the arrangement of the 10 orders as given above.

Until recently everyone has thought of Order V, Actinomycetales, as including species all of which were non-motile. However Couch, in a series of papers, the latest published in 1955 (Jour. Elisha Mitchell Sci. Soc, 71, 1955, 148-155),