3. A CRITIQUE OF BERGEY’S SYSTEM
Admittedly it is a difficult task to frame a definition of the Schizomycetes adequate to include all organisms which belong here but sufficiently specific to exclude other groups of microorganisms. Nevertheless, a more inadequate definition than that given by Bergey would be hard to conceive. Bacterial cells are described as "relatively primitive in organization," but one looks in vain for an explicit statement of the absence of true nuclei, which is perhaps the most important single morphological characteristic of these organisms. In describing cell shape, the word filamentous is used in a most confusing manner; apparently it is applied indiscriminately to the usually non-septate mycelium of the actinomycetes, to chains of individual cells such as occur in the Bacilleae and to the truly filamentous (i.e. multicellular) arrangement found in the Beggiatoaceae. Multiplication is given as being typically by "cell fission," but there is no mention of its wellnigh universal transverse nature. The absence of sexual reproduction is not noted. The formation of endospores and conidia is mentioned but not the formation of myxobacterial spores and cysts. Next comes the astonishing statement,"Chlorophyll is produced by none of the bacteria (with the possible exception of a single genus)." The occurrence of a true chlorophyll-though chemically slightly different from the green plant chlorophylls a and b-in all the purple bacteria, both Thioand Athiorhodaceae, invalidates the inclusion of this character. In the description of motility, the peculiar locomotion so characteristic of the Myxobacteriales goes unmentioned; the motility of the Spirochaetales is described as flexuouss," which certainly does not characterize the mode of locomotion of these forms in any adequate manner.
In order to appreciate the complete inadequacy of this definition of the class Schizomycetes one has only to realize that there is nothing in it which would exclude the fungi and most of the protozoa, whereas the statement about the absence of chlorophyll (clearly put in originally to keep out the algae) now also excludes a whole family of the Thiobacteriales.
The differentiation of the seven orders used in the Manual is no more satisfactory. The first order of the Eubacteriales is carefully segregated from the rest as containing "simple and undifferentiated forms." No mention is made of the flagellar nature of motility or the rigidity of the cell wall, which are really important characters in this order. The statement that "iron (is) not stored as visible particles" applies equally well to all other living organisms.
The remaining orders are described as "specialized or differentiated"; in the absence of a definition of these two terms the characterization becomes entirely meaningless. Even if, from a consideration of the organisms thus grouped together, it would seem possible to sense the implication of these terms, it must be pointed out that forms no more "specialized or differentiated" than members of the Eubacteriales have been incorporated in these orders. To mention an example: in the second order of the Actinomycetales (which is separated from the following orders as being "mold-like") one finds the genera Mycobacterium and Corynebacterium. It is clearly illogical to describe these genera either as "mold-like" or as "specialized and differentiated," an opinion which is substantiated by the fact that in the fifth edition one can find indubitable Corynebacterium species described in no less than three families of the Eubacteriales (Rhizobiaceae, Pseudomonadaceae and Bacteriaceae). The third, fourth and fifth orders Chlamydobacteriales, Caulobacteriales and Thiobacteriales are collectively described as "alga-like." Clearly the recognition of the relationship of organisms such as the Beggiatoaceae, Clonothrix, etc., to representatives of the order Hormogonales of the Myxophyta has prompted the inclusion of this character in the descriptive diagnosis of these orders. But the term "alga-like" is entirely too general since it implies some unspecified resemblance (such as habit of growth?) to some organisms included in some of the seven divisions of the algae. Furthermore, this statement applies only to some of the organisms in each order, certainly not to all; in the Caulobacteriales one can find organisms morphologically very similar to the Eubacteriales except for the possession of a stalk or holdfast (Nevskia pediculata, Caulobacter vibrioides), while the representatives of the entire family Rhodobacteriaceae (Thiobacteriales) are morphologically indistinguishable from their colorless counterparts in the Eubacteriales. The artificiality of the Chlamydobacteriales, Caulobacteriales and Thiobacteriales is clearly shown by the fact that the important differential characteristic for each order is also exhibited by species which have been placed in one of the other two orders. Thus, a sheath, which is the key character of the Chlamydobacteriales, occurs in the genus Thioploca (Thiobacteriales); several Leptothrix (Chlamydobacteriales) and Thiothrix (Thiobacteriales) species are attached to the substratum by a holdfast (Caulobacteriales: "in some species the stalks may be very short or absent, the cells connected directly to the substrate or to each other by holdfasts"); and finally, the species Nevskia ramosa (Caulobacteriales) shows evidence of containing sulfur globules, which might suggest an alternative position in the Thiobacteriales. Even if these species can be placed in the order to which they have been assigned on the basis of other characters, such a situation is apt to cause confusion.
The description of the sixth order Myxobacteriales as "slime mold-like" would hardly appeal to anyone familiar with the organisms belonging to the two groups. The absence of true nuclei, of sexual reproduction, and of amoeboid cell form in the Myxobacteriales shows quite clearly the fundamental lack of similarity to the Myxomycetae. Scientific descriptive keys should not contain such misleading comparisons. On the other hand, the two most important characters of the Myxobacteriales, the type of locomotion and the absence of rigid cell walls, are not mentioned in the key.
After this, it is not surprising to find the seventh order Spirochaetales differentiated from the rest as "protozoan-like." The further characterization is so diffuse that it gives no helpful information concerning this group of organisms. Certainly the determinative significance of the statement "Some forms transmitted by insect vectors" is not apparent.
The stress laid on these points may seem unnecessary. However, the fact that the definitions and segregations of the various orders have remained unchanged through five consecutive editions of Bergey’s Manual shows that its weaknesses (not only from the scientific, but even from the determinative standpoint) have not been generally realized.