Peptidoglycan .Types of Bacterial Cell Walls and their Taxonomic ...

458 SCHLEIFER AND KANDLER
BACTERIOL. REV.
around the cell. With regard to the Gram
reaction, the spirochaetes are presumably
gram-negative. Thus, it was expected that the
peptidoglycan would show the directly crosslinked
m-Dpm type. But already qualitative
studies on cell walls of Treponema reiteri have
yielded ornithine instead of m-Dpm as the
major diamino acid (382). Recent studies on the
ultrastructure and chemical composition of the
cell wall of Spirochaeta stenostrepta have confirmed
the presence of L-Orn as constituent of
the peptidoglycan (166). Two layers of the cell
wall of Spirochaeta stenostrepta were isolated
and analyzed. The outermost of these two
layers consists mainly of lipoprotein. The second
layer was characterized as peptidoglycan.
It is a thin structure or monolayer as in typical
gram-negative organisms which retains the cylindrical
and coiled shape of the cell. Studies on
the amino acid sequence of the purified peptidoglycan
revealed that the peptide subunits
consist of tripeptides (L-Ala-y-D-Glu-L-Orn)
and tetrapeptides (L-Ala-'y-D-Glu-L-Orn-D-Ala).
The peptide subunits are directly cross-linked
(Fig. 28). The primary structure of the peptidoglycan
resembles that of other gram-negative
bacteria with the only difference being that
m-Dpm is replaced by L-Orn.
Order Myxobacteriales. Since the myxobacteria
are flexible cells, it has been assumed
that the cell walls are quite different from those
of eubacteria and that the peptidoglycan may
be lacking. But even the first studies on the cell
walls of myxobacterial strains showed that a
peptidoglycan layer is probably present in their
walls (7, 239). More detailed studies on myxobacterial
cell walls were reported by Verma
and Martin (397) and by White et al. (415). The
-G-M-G-
I
L- Ala
4
D-Glu
'Y 6
L-Orn +-D-Ala
at
L-Orn
FIG. 28. Fragment of the primary structure of the
peptidoglycan of Spirochaeta stenostrepta.
results demonstrated that in all species so far
examined (Cytophaga hutchinsonii, Sporocytophaga
myxococcoides, and Myxococcus
xanthus), the directly cross-linked, m-Dpmcontaining
peptidoglycan type occurs (Fig. 6).
In the case of C. hutchinsonii and S. myxococcoides,
a discrete peptidoglycan layer was isolated
(397). The vegetative cell walls of M.
xanthus, on the other hand, completely disaggregated
after treatment with trypsin and detergent.
White et al. (415) suggested, therefore,
that the rigid layer of the vegetative cell wall of
M. xanthus is not a continous peptidoglycan
layer but consists of patches of peptidoglycan
separated by nonpeptidoglycan material. This
patchlike arrangement may be related to the
flexibility of the myxobacterial cell. In the case
of C. hutchinsonii and S. myxococcoides, however,
the flexibility is explained by the occurrence
of "naked tubes of murein (peptidoglycan)
monolayers" (397).
With the exception of the genus Cytophaga,
all myxobacteria can form resting cells. These
resting cells, the so-called microcysts, are produced
from single vegetative cells. The microcysts
are shorter than the vegetative cells
and their cell walls are much thicker and
inflexible. In both Sporocytophaga and Myxococcus,
not only the vegetative cell walls but
also the cell walls of microcysts were studied.
Verma and Martin (397) explained the transition
of the flexible, vegetative cell wall of
Sporocytophaga to the rigid, thick microcyst
cell wall by the superposition of several peptidoglycan
layers. White et al. (415), on the
other hand, found in Myxococcus, during the
change from the vegetative cell to the microcyst,
a temporary decrease in the cross-linkage
of the peptidoglycan. They assume a high
rate of turnover of the peptidoglycan during the
formation of the microcyst and simultaneously
some uncross-linked peptidoglycan appears. A
newly synthesized extracellular layer of nonpeptidoglycan
material serves to strengthen the
microcyst wall. Moreover, the authors stated
that "the patch-like arrangement of the peptidoglycan
may be related to the change in
shape when M. xanthus converts from the
vegetative rod to the spherical cyst." They
think that the nonpeptidoglycan areas perform
a necessary role in the morphogenesis of the
wall.
FINAL REMARKS
Taxonomic Implications of Other Cell
Wall Polymers
Lipopolysaccharides. A good correlation
between the structure of the lipopolysaccha-
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