5. A SURVEY OF THE BACTERIA
A. The Eubacteriales and related assemblages The group which naturally presents itself first is that of the Eubacteriales. These organisms always possess rigid cell walls, demonstrable in the larger species by plasmolysis. Motility is by no means universal, but where it does occur, it is always flagellar. Cell division is always by transverse fission. While true nuclei are absent, there have been a number of reports in recent.
MAIN OUTLINES OF BACTERIAL CLASSIFICATION
years, based on the use of reliable cytological techniques, in which claims have been made for he presence of discrete masses of chromatin material which could be interpreted as primitive chromosome-like structures (Badian, 1933, Stille, 1937, Piekarski, 1937. The work of Schaede (1940) has, however, cast some doubt on these previous results). The endospore, found in certain groups in the Eubacteriales, is highly characteristic, occurringnowhere else among the bacteria. A few members (Azotobacter spp.) form cysts. Endospores and cysts are the only two types of resting stages found in the Eubacteriales. On a purely morphological basis one can arrange the representatives of the Eubacteriales into a hypothetical tree with four main ascending branches, as was done by Kluyver and van Niel (1936). Certain facts which have come to light since 1936 have tended to weaken parts of this scheme, but in spite of that it is still useful for illustrating broad trends within the group. Also,some of these trends clarify the relationships between the Eubacteriales and other assemblages. A slightly modified form of the
tree is given in figure 1.Starting from the hypothetical primitive coccus type, the first line leads through the micrococci to the sarcinae, culminating in the spore-forming sarcinae. These forms may be either gram positive or gram negative. Motility occurs infrequently in members of this group, and spore formation has so far been reported only in Sporosarcina ureae. The second line consists of the polarly flagellated rods, starting with the pseudomonas type and leading through the vibrios to the spirilla. The representatives of this line are all gram negative with one possible exception. This is the species Listeria (Listerella) monocytogenes (Pirie 1940) which was originally described as a gram-positive polarly flagellated rod (Pirie 1927) but which has' been more recently reported as peritrichously flagellated (Paterson 1939), and hence perhaps does not belong in this line. Spore formation is rare, being well established for only one species, Sporovibrio dessulfuricans (Starkey 1938).
There have also been reports, so far unsubstantiated, of spore formation in Spirillum species (see Starkey 1938, and Lewis 1940). The third line is morphologically highly diverse. It takes
its origin in the streptococci, and passes through the lactic acid rods, the propionic acid and other corynebacteria, and the mycobacteria to the actinomycetes. All members of this line are gram positive and none form endospores. Until very recently motility was thought to be absent in the higher forms although motile streptococci (see Koblmiller, 1935) had been known for some time. However, the work of Topping (1937) has shown that there are organisms of the Mycobacterium-The names used denote morphological entities, not necessarily genera. For
example, the designation "Bacterium" includes all genera of non-sporeforming,peritrichously flagellated rods: Kurthia, Escherichia, Aerobacter, etc.Proactinomyces type which are flagellated. This fact may ultimately necessitate a radical revision of this line since, if motility
is conceded to occur all along it, there is no valid means of differentiating between gram-positive peritrichously flagellated rods of the Kurthia type (at present placed in the fourth line) and organisms belonging to the genera Mycobacterium and Corynebacterium. The fourth line of the Kluyver-van Niel scheme comprises all peritrichously flagellated rods; both gram-positive and gramnegative forms occur. It is here that the great majority of sporeformers have up till now been placed. This line is also an unsatisfactory one because, particularly in the spore-forming
representatives, a number of morphologically rather distinct types can be recognized. It is probable that future work will show the necessity of drastic revisions, but on the basis of present
knowledge it is difficult to make satisfactory modifications of this and the preceding line.
The above-mentioned four lines were postulated primarily on the basis of cell shape and mode of insertion of flagella. It is of importance to note that the two most satisfactory groups share
an additional character, viz., the homogeneous behavior of the members with respect to the gram stain. In the remaining two lines no such uniformity exists at the present time. This makes
it necessary to determine the relative values of the type of flagellation and of the gram reaction for systematic purposes. The various recent reports on the difficulties encountered in definitely
ascertaining the mode of insertion of flagella (Pijper, 1930, 1931, 1938, 1940; Pietschmann, 1939; Conn, Wolfe and Ford, 1940) lead one to suspect that the gram stain may ultimately appear to be the more valuable. There are two large groups which show obvious morphological
relationships to the Eubacteriales; namely, the Actinomycetales and the photosynthetic bacteria.
The Actinomycetales have already been mentioned in connection with the third line in the Eubacteriales from which they are clearly derived. The delimitation of this order is a difficult and necessarily arbitrary matter, since the morphological series which takes its origin in the lactic acid bacteria runs practically unbroken to the most complex of actinomycetes. From the purely determinative standpoint it is probably most satisfactory to start the Actinomycetales with the Proactinomycetaceae, the group in which a primitive mycelium formation occurs. In this case a clear and simple delimitation of the order is possible. As was mentioned in the critique of Bergey's system, the inclusion of the mycobacteria and corynebacteria leads to confusion, since these forms can so readily be taken for representatives of the Eubacteriales. On the other hand, the dividing line between the genera Mycobacterium and Proactinomyces is a tenuous one. Like the Eubacteriales, the Actinomycetales always possess rigid cell walls. Schaede (1940) has shown that the distribution of chromatin material in them is identical with that in members of the Eubacteriales. The question of motility in this order must remain open for the present, although it seems likely that the motile organisms described by Topping (1937) should be placed here. All representatives of the Actinomycetales are grampositive; for this reason the inclusion here of the polarly flagellated gram-negative genus Mycoplana, tentatively suggested byWaksman (1940), is undesirable. Endospores are never formed in the Actinomycetales; the characteristic reproductive structures in this order are conidia, formed by fragmentation of the aerial hyphae.
tree is given in figure 1.Starting from the hypothetical primitive coccus type, the first line leads through the micrococci to the sarcinae, culminating in the spore-forming sarcinae. These forms may be either gram positive or gram negative. Motility occurs infrequently in members of this group, and spore formation has so far been reported only in Sporosarcina ureae. The second line consists of the polarly flagellated rods, starting with the pseudomonas type and leading through the vibrios to the spirilla. The representatives of this line are all gram negative with one possible exception. This is the species Listeria (Listerella) monocytogenes (Pirie 1940) which was originally described as a gram-positive polarly flagellated rod (Pirie 1927) but which has' been more recently reported as peritrichously flagellated (Paterson 1939), and hence perhaps does not belong in this line. Spore formation is rare, being well established for only one species, Sporovibrio dessulfuricans (Starkey 1938).
There have also been reports, so far unsubstantiated, of spore formation in Spirillum species (see Starkey 1938, and Lewis 1940). The third line is morphologically highly diverse. It takes
its origin in the streptococci, and passes through the lactic acid rods, the propionic acid and other corynebacteria, and the mycobacteria to the actinomycetes. All members of this line are gram positive and none form endospores. Until very recently motility was thought to be absent in the higher forms although motile streptococci (see Koblmiller, 1935) had been known for some time. However, the work of Topping (1937) has shown that there are organisms of the Mycobacterium-The names used denote morphological entities, not necessarily genera. For
example, the designation "Bacterium" includes all genera of non-sporeforming,peritrichously flagellated rods: Kurthia, Escherichia, Aerobacter, etc.Proactinomyces type which are flagellated. This fact may ultimately necessitate a radical revision of this line since, if motility
is conceded to occur all along it, there is no valid means of differentiating between gram-positive peritrichously flagellated rods of the Kurthia type (at present placed in the fourth line) and organisms belonging to the genera Mycobacterium and Corynebacterium. The fourth line of the Kluyver-van Niel scheme comprises all peritrichously flagellated rods; both gram-positive and gramnegative forms occur. It is here that the great majority of sporeformers have up till now been placed. This line is also an unsatisfactory one because, particularly in the spore-forming
representatives, a number of morphologically rather distinct types can be recognized. It is probable that future work will show the necessity of drastic revisions, but on the basis of present
knowledge it is difficult to make satisfactory modifications of this and the preceding line.
The above-mentioned four lines were postulated primarily on the basis of cell shape and mode of insertion of flagella. It is of importance to note that the two most satisfactory groups share
an additional character, viz., the homogeneous behavior of the members with respect to the gram stain. In the remaining two lines no such uniformity exists at the present time. This makes
it necessary to determine the relative values of the type of flagellation and of the gram reaction for systematic purposes. The various recent reports on the difficulties encountered in definitely
ascertaining the mode of insertion of flagella (Pijper, 1930, 1931, 1938, 1940; Pietschmann, 1939; Conn, Wolfe and Ford, 1940) lead one to suspect that the gram stain may ultimately appear to be the more valuable. There are two large groups which show obvious morphological
relationships to the Eubacteriales; namely, the Actinomycetales and the photosynthetic bacteria.
The Actinomycetales have already been mentioned in connection with the third line in the Eubacteriales from which they are clearly derived. The delimitation of this order is a difficult and necessarily arbitrary matter, since the morphological series which takes its origin in the lactic acid bacteria runs practically unbroken to the most complex of actinomycetes. From the purely determinative standpoint it is probably most satisfactory to start the Actinomycetales with the Proactinomycetaceae, the group in which a primitive mycelium formation occurs. In this case a clear and simple delimitation of the order is possible. As was mentioned in the critique of Bergey's system, the inclusion of the mycobacteria and corynebacteria leads to confusion, since these forms can so readily be taken for representatives of the Eubacteriales. On the other hand, the dividing line between the genera Mycobacterium and Proactinomyces is a tenuous one. Like the Eubacteriales, the Actinomycetales always possess rigid cell walls. Schaede (1940) has shown that the distribution of chromatin material in them is identical with that in members of the Eubacteriales. The question of motility in this order must remain open for the present, although it seems likely that the motile organisms described by Topping (1937) should be placed here. All representatives of the Actinomycetales are grampositive; for this reason the inclusion here of the polarly flagellated gram-negative genus Mycoplana, tentatively suggested byWaksman (1940), is undesirable. Endospores are never formed in the Actinomycetales; the characteristic reproductive structures in this order are conidia, formed by fragmentation of the aerial hyphae.
The photosynthetic bacteria were first rationally treated in the system of Pringsheim (1923), who recognized their differences from the colorless sulphur bacteria with which they had so long
been associated, and created for them the order Rhodobacteriale8. Kluyver and van Niel placed them in the Eubacteriales, an action which was entirely justified from the strictly morphological standpoint. These organisms are morphologically indistinguishable from the true bacteria, falling into the Pseudomonas-Vibrio-SpiriUum and Micrococcus-Sarcina lines. All species are Gramnegative and non-spore-forming. Nothing is known about the distribution of chromatin material. Chromatophores are absent, the photosynthetic pigments being evenly distributed throughout the cell, as in the blue-green algae. Physiologically, these organisms differ from green plants in a number of respects. Bacteriochlorophyll is chemically slightly different from chlorophylls a and b. Photosynthesis is accomplished only in the presence of reducing substances, and never accompanied by oxygen production. The photosynthetic bacteria form a homogeneous group whose photosynthetic metabolism sets them off from the Eubacteriales.
For this reason, it seems wise to keep them as a separate order, recognizing nevertheless their close relationship to groups in the true bacteria.
The three assemblages discussed so far-Eubacteriales, Actinomycetales and Rhodobacteriales-are a well-knit, closely related natural group whose relationships to other bacteria and to nonbacterial microorganisms are not very clear. A number of workers (Drechsler, 1919; Vuillemin, 1912, 1925) have postulated a relationship between the Actinomycetales and the Eumycetae, but beyond the superficial resemblances in the mycelial nature of growth and the formation of conidia (oidia) by hyphal fragmentation there is little support for this hypothesis. True nuclei do not occur in the actinomycetes, which, as mentioned previously, show a typical eubacterial arrangement of the chromatin. Furthermore, the width of the individual hyphae is always of bacterial dimensions, never approaching that of the true fungi. Negative evidence is the complete absence of sexual reproduction in the actinomycetes. The superficial similarities between molds and actinomycetes are probably to be regarded as an example of convergence.
The only relationship which has been seriously suggested for the Eubacteriales is one with the Myxophyta. Although the close relationship existing between bacteria and blue-green algae was
stressed by 19th century microbiologists such as Cohn, van Tieghem, and Hansgirg (the two former workers treated them as one group), the importance of this concept has been appreciated less in recent times.
The common features of true bacteria and blue-green algaemay be summarized as follows:
1. Absence of true nuclei.
2. Absence of sexual reproduction.
3. Absence of plastids.
One major difference, however, is the complete absence of flagellar motility in the Myxophyta, whose representatives are either immotile or exhibit creeping motility. It is among the Chroococcales, the most primitive assemblage of the Myxophyta, that we find forms closely resembling the Eubacteriales. A Chroococcus sp., for example, would be indistinguishable
from a Micrococcus sp. if it were to lose its photosynthetic pigments. The genus Eucapsis would be similarly indistinguishable from the genus Sarcina. Thus it seems at least possible that the primitive blue-green algae of the Chroococcus type have developed from the Eubacteriales as a second photosynthetic line, at first paralleling morphologically the purplebacteria, but undergoing in the course of time a far more complex morphological evolution which resulted in the development of the two higher orders, the Hormogonales and the Chamaesiphonales. If this were the case we must assume that the most primitive blue-green algae were non-motile, being derived from a non-motile group in the true bacteria somewhere close to the primitive coccus type, and that the very characteristic creeping motility of the Myxophyta developed at some later time during their evolution, probably in one of the branches of the Chroococcales.
It should be realized that this is a speculative digression, which does not affect the systematic proposals we shall put forward. It is at least certain that morphologically the Myxophyta
resemble the true bacteria far more closely than they do any of the other algal groups.
been associated, and created for them the order Rhodobacteriale8. Kluyver and van Niel placed them in the Eubacteriales, an action which was entirely justified from the strictly morphological standpoint. These organisms are morphologically indistinguishable from the true bacteria, falling into the Pseudomonas-Vibrio-SpiriUum and Micrococcus-Sarcina lines. All species are Gramnegative and non-spore-forming. Nothing is known about the distribution of chromatin material. Chromatophores are absent, the photosynthetic pigments being evenly distributed throughout the cell, as in the blue-green algae. Physiologically, these organisms differ from green plants in a number of respects. Bacteriochlorophyll is chemically slightly different from chlorophylls a and b. Photosynthesis is accomplished only in the presence of reducing substances, and never accompanied by oxygen production. The photosynthetic bacteria form a homogeneous group whose photosynthetic metabolism sets them off from the Eubacteriales.
For this reason, it seems wise to keep them as a separate order, recognizing nevertheless their close relationship to groups in the true bacteria.
The three assemblages discussed so far-Eubacteriales, Actinomycetales and Rhodobacteriales-are a well-knit, closely related natural group whose relationships to other bacteria and to nonbacterial microorganisms are not very clear. A number of workers (Drechsler, 1919; Vuillemin, 1912, 1925) have postulated a relationship between the Actinomycetales and the Eumycetae, but beyond the superficial resemblances in the mycelial nature of growth and the formation of conidia (oidia) by hyphal fragmentation there is little support for this hypothesis. True nuclei do not occur in the actinomycetes, which, as mentioned previously, show a typical eubacterial arrangement of the chromatin. Furthermore, the width of the individual hyphae is always of bacterial dimensions, never approaching that of the true fungi. Negative evidence is the complete absence of sexual reproduction in the actinomycetes. The superficial similarities between molds and actinomycetes are probably to be regarded as an example of convergence.
The only relationship which has been seriously suggested for the Eubacteriales is one with the Myxophyta. Although the close relationship existing between bacteria and blue-green algae was
stressed by 19th century microbiologists such as Cohn, van Tieghem, and Hansgirg (the two former workers treated them as one group), the importance of this concept has been appreciated less in recent times.
The common features of true bacteria and blue-green algaemay be summarized as follows:
1. Absence of true nuclei.
2. Absence of sexual reproduction.
3. Absence of plastids.
One major difference, however, is the complete absence of flagellar motility in the Myxophyta, whose representatives are either immotile or exhibit creeping motility. It is among the Chroococcales, the most primitive assemblage of the Myxophyta, that we find forms closely resembling the Eubacteriales. A Chroococcus sp., for example, would be indistinguishable
from a Micrococcus sp. if it were to lose its photosynthetic pigments. The genus Eucapsis would be similarly indistinguishable from the genus Sarcina. Thus it seems at least possible that the primitive blue-green algae of the Chroococcus type have developed from the Eubacteriales as a second photosynthetic line, at first paralleling morphologically the purplebacteria, but undergoing in the course of time a far more complex morphological evolution which resulted in the development of the two higher orders, the Hormogonales and the Chamaesiphonales. If this were the case we must assume that the most primitive blue-green algae were non-motile, being derived from a non-motile group in the true bacteria somewhere close to the primitive coccus type, and that the very characteristic creeping motility of the Myxophyta developed at some later time during their evolution, probably in one of the branches of the Chroococcales.
It should be realized that this is a speculative digression, which does not affect the systematic proposals we shall put forward. It is at least certain that morphologically the Myxophyta
resemble the true bacteria far more closely than they do any of the other algal groups.
