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

424 SCHLEIFER AND KANDLER
BACTERIOL REV.
Variation B1y: L-Glu in position 3 (1 type,
Fig. 11, Table 11)
Variation B16: L-Ala in position 3 (1 type,
Fig. 11, Table 11)
Subgroup B2: interpeptide bridge containing
a D-diamino acid.
Variation B2a: L-Orn in position 3 (1 type,
Fig. 12, Table 12)
Variation B2B,: L-Hsr in position 3 (2
types, Fig. 12, Table 12)
Variation B2Y: L-Dab in position 3 (1 type,
Fig. 12, Table 12)
Stability of Peptidoglycan Structure
Under Different Conditions of Growth
To establish the value of the peptidoglycan
structure as a taxonomic criterion, it is necessary
to demonstrate its phenotypic stability
and to determine whether structural changes
dependent on the growth phase or environmental
factors can be observed. Changes of the cell
wall composition have been observed only under
conditions where different nutrients are
limiting factors for growth (98, 99, 378) or in the
presence of a quite unbalanced growth medium.
Since detailed studies about these subjects will
be published elsewhere (99a; Schliefer and
Kandler, manuscript in preparation), we can
restrict ourselves to a short survey of the phenotypic
variations of the peptidoglycan.
Modifications of the peptidoglycan are not as
dramatic as those of the cell wall polysaccharides
(98, 99a, 378); in particular the type of
peptidoglycan is rather stable, and, up to now,
no drastic alterations of the peptidoglycan
types have been found.
A few rather similar amino acids may be
exchanged. In the case of M. luteus
(lysodeikticus), glycine bound to the a-carboxyl
group of glutamic acid can be replaced by
D-serine (128, 419). In strains of staphylococci,
glycine residues of the interpeptide bridges can
be replaced by L-serine (52, 334, 346, 433) or
L-alanine (334, 346). But these alterations
only occur when these amino acids are added in
rather high concentrations to the growth medium
and when the original amino acids are
present in limiting amounts. The same is true
for the incorporation of hydroxylysine instead
of lysine into the peptidoglycans of S. faecalis
and Leuconostoc (346, 359) or for that of lanthionine,
a monosulfur analogue of Dpm, instead
of Dpm in a Dpm-auxotroph mutant of E. coli
(198). The most pronounced phenotypic variation
of the peptidoglycan was found among
staphylococci (334, 346; G. Rauch, Diplom
thesis, University of Munich, 1970; W.
Hammes, Diplom thesis, University of Munich,
1970). These studies have indicated that the
peptidoglycan composition is fixed genetically
but that phenotypic alterations are possible.
Under extreme conditions, two genetically different
strains can even appear to be phenotypically
similar. For example, the peptidoglycan of
S. aureus normally contains little or no L-Ser,
whereas that of S. epidermidis has a rather
high serine content. When S. aureus is grown
in a serine-enriched medium, however, its
peptidoglycan now reveals a similar chemical
composition to that of a S. epidermidis grown
on a serine deficient medium. Despite the
phenotypic resemblance of these two peptidoglycans
under unusual conditions, the
genetic differences are obvious if these organisms
are cultivated in identical balanced
media. The phenotypic variations of the staphylococcal
peptidoglycans are rather an
exception than a rule for the modifiability of
peptidoglycans. Studies in our laboratory and
other laboratories (373, 430) have indicated
that the amino acid composition of the cell
walls of various organisms grown in several different
media did not vary significantly.
Some organisms normally contain two amino
acids alternately at the same position, e.g.,
strains of Bifidobacterium adolescentis contain
both L-Lys and L-Orn, in varying amounts in
the same peptidoglycan. Hereby the ratio
L-Lys/L-Orn varies with different strains but is
not dependent upon the amount of Lys or Orn
in the growth medium. A similar constancy was
found among strains of Leuconostoc gracile
which contain interpeptide bridges alternately
formed by L-Ser-L-Ala and L-Ser-L-Ser (Lauer
and Kandler, manuscript in preparation). The
variation of the serine content is not caused bv
the amino acid composition of the growth
medium since the serine content of the cell wall
changed only very little when the strains were
grown in a serine-enriched medium. These
findings indicate that in both organisms, B.
adolescentis and L. gracile, differences in the
ratio of the amino acids in question are genetic
properties.
In conclusion one can say that the only
significant phenotypic alterations of amino acid
composition in the peptidoglycan from cells
without growth limitations were found among
staphylococci. But even these alterations are
not so extensive that the peptidoglycan type is
considerably changed. In other words, a glycine-rich
staphylococcal peptidoglycan will
never be converted to an alanine-rich micrococcal
peptidoglycan. The only possible modification
of the peptidoglycan is seen in organisms
grown in a serine-enriched and glycine-limited
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