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

436 SCHLEIFER AND KANDLER
antigens. These immunological tests are based
on the presence of specific chemical structures
on or in the outermost surface layers of the cell
(207). Thus, the traditional classification of the
streptococci employs indirectly the differences
in the chemical structure of the cell wall as a
distinguishing feature. Studies on the amino
acid composition and amino acid sequence of
streptococcal peptidoglycans have revealed 12
different types. The distribution of these peptidoglycan
types in the genus Streptococcus is
compiled in Table 20. Most of the peptidoglycans
found in streptococci belong to variation
A&3 with L-Lys in position 3 of the peptide
subunit and interpeptide bridges consisting of
monocarboxylic L-amino acids or glycine, or
both. A few of the streptococci contain a
peptidoglycan cross-linked by a dicarboxylic
amino acid (variation A4a) and that of others is
directly cross-linked (variation Ala).
A large number of different streptococci contain
a peptidoglycan cross-linked by interpeptide
bridges consisting of L-alanyl oligopeptides.
The length of these chains varies from one
to four alanine residues. These types of peptidoglycan
have been found in streptococci
belonging to groups A, A-variant, C, C-variant,
D (S. faecalis), E, F, G, H, K, L, M, P, U, and S.
thermophilus.
In a few other species one of the Ala residues
of the interpeptide bridge is replaced by L-Ser
or Gly. The peptidoglycans of S. agalacticae
(group B), of one strain of group K, and of one
strain of group C contain in part L-Ser residues
instead of L-Ala residues in the interpeptide
bridge. In Streptococcus sp. of group L and S.
viridans IV, one L-Ala residue is replaced by a
Gly residue. Several types of peptidoglycans
occurring among streptococci reveal as an additional
amino acid L-Thr which is bound to the
e-amino group of L-Lys. The complete interpeptide
bridges consist of dipeptides which
contain, besides L-Thr, the amino acids Gly,
L-Ala, or L-Ala/L-Ser. The strains of S. mutans
and some strains of S. equinus show the peptidoglycan
type Lys-L-Thr-L-Ala (Fig. 19a).
This type was first described by Schleifer and
Kandler (336) for a Streptococcus which was
most probably wrongly delineated as S. cremoris
(Hladny, Ph.D. thesis, Technical University,
Munich, 1971). Almost the same type was found
in S. bovis (179), in some strains of S. equinus,
and in a strain of S. milleri (14a); the only
difference is that some of the Ala residues in the
interpeptide bridges are replaced by Ser. In one
strain of S. equinus, in two strains of S. milleri,
and in S. salivarius II (not reacting w^ith an-
BACTERIOL REV.
tiserum against group K), the interpeptide
bridges consist of Gly-L-Thr peptides (Fig. 19b;
141a).
The L-Lys-D-Asp type is found in S. faecium,
Streptococcus sp. (group Q), S. lactis, and
S. cremoris (group N).
The directly cross-linked peptidoglycan
(Ala) occurs among strains of groups K, 0, R,
and S. viridans I, II, and IV.
From a comparison of the serological grouping
and the occurrence of peptidoglycan types,
it is evident that most of the serological groups
belong to one type of peptidoglycan, the only
exceptions being groups D and K. Group D is a
very heterogeneous group and the analysis of
the peptidoglycan confirmed the separation of
this group, on the basis of physiological tests
(87), into five different species. By determining
the qualitative amino acid composition of the
cell walls, it is easily possible to distinguish
between S. faecalis and S. faecium (175) and
also between these two species on the one hand
and S. bovis and S. equinus on the other hand.
Strains of S. salivarius and other Streptococcus
spp. belonging to group K can be distinguished
not only by their peptidoglycan types
but also by their ability to form levan as seen in
Table 21. It is likely that group K will have to be
subdivided into several species like group D,
but many more strains need to be studied.
S. sanguis I and S. sanguis II can be separated
according to their reactions with antiserum to
group H and their physiological and different
cell wall compositions. It was suggested that S.
sanguis II should be separated from S. sanguis I
and reclassified as S. pseudosanguis (Hladny,
Ph.D. thesis, Technical University, Munich,
1971). Most of the strains of S. viridans reveal
the directly cross-linked, L-Lys-containing
peptidoglycan type. Only the strain of S. viridans
IV is different. This suggests that it may
be necessary to study more strains of S. viridans
to establish this distinction between S. viridans
IV and the other streptococci of the viridans
group.
The grouping of streptococci according to
peptidoglycan types can shed some light on the
relationships among various species or serological
groups. A closely related cluster may be
formed by S. pyogenes (group A), S. equisimilis
(group C), and Streptococcus sp. (group G).
They behave almost identically in their physiological
properties and in their human pathogenicity,
and they also contain quite similar
peptidoglycan types (Table 22). A similar relationship
can be observed between S. uberis
(group E), Streptococcus sp. (group P), and
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