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

VOL 36, 1972
PEPTIDOGLYCAN TYPES OF BACTERIAL CELL WALIS461461
many different peptidoglycan types are still
conserved. This fulfills, for the gram-positives,
the third condition of a good taxonomic characteristic.
This does not apply to the gram-negatives,
and they do not show the same chemical and
structural variability of the peptidoglycan as
the gram-positives. The peptidoglycan of the
gram-negatives consists only of a monolayer, in
contrast to the multilayered peptidoglycan of
gram-positives. In such a simple monolayer, the
variability of the cross-linkage may be restricted.
Even the insertion of interpeptide
bridges might distort its structure and theref'ore
decrease the vitality and competitiveness of the
organisms. Thus, a change in the peptidoglycan
type of the "sophisticated" monolayer of a
gram-negative bacterium would be most likely
a disadvantage. This may be the reason why
such alterations have not been conserved.
Consequently, the peptidoglycan type is not a
taxonomic characteristic for classification
among the gram-negative bacteria.
(iv) An unequivocal phylogenetic relationship
of the various peptidoglycan structures is not
readily discernible, but rough deductive sequences
are recognizable. A possible phylogenetic
connection of' the different cross-linkage
variations is sketched in Fig. 29. It is based on
the assumption that phylogenetic advancement
is usually connected with a simplification ( = reduction)
and a loss of variability (=fixation).
Thus the most advanced type of peptidoglycan
should be characterized by the greatest possible
simplification (from a chemical point of view),
A1 p AIy
( monolayer )
Gram -
Spiro- negatives,
choetales Myxobacteriales
whereas the more primitive ones should contain
additional trappings in abundant variations.
This trend to an extensive chemical simplification
of the basic structure is also discernible at a
higher stage of evolution and was mentioned
before in the comparison of the peptidoglycan
of procaryotes and the main cell wall structures
of fungi and plants.
The occurrence of multilayered peptidoglycans
containing interpeptide bridges may represent
a primitive stage. Therefore, we have
placed subgroups A3 and A4 at the basis of our
scheme (Fig. 29). These two subgroups contain
the large majority of the peptidoglycan types
and are represented by the majority of grampositive,
non-spore-forming bacteria including
most of the fermenting organisms. Both subgroups
reveal such an enormous ramification of
peptidoglycan types that phylogenetic relationships
among these different variations are not
readily discernible. Thus, the condition expressed
in this point, that the structure should
reveal its primitive or deduced character, is not
fulfilled here by the peptidoglycan. The great
variability within the two subgroups A3 and A4
can be taken as a further indication of their
primitive stage, since this variability discloses
that the evolution of the peptidoglycan structure
is at this point far from reaching a standstill.
A major side branch of these two subgroups
has probably led to the formation of group B.
Group B may have arisen only once, since it is
restricted to a rather unique group of bacteria,
viz, a relatively small number of coryneform
FIG. 29. A possible phylogenetic relationship of peptidoglycan subgroups and variations.
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