540
Bacterial Growth and Division
ga
d
a
a
agda
a
g
d
a
agda
Fig. 5
Three-dimensional
representation of peptidoglycan
structure. This is an idealized
representation of peptidoglycan
structure as seen from the outer surface.
The thick bars represent glycan chains
at the outside of the peptidoglycan layer.
The thinner bars represent glycan chains
below the outer layer. The stretched
chains of circles represent amino acids
cross-linking the glycan chains. The
chains below the stretched surface of
the cell rise to the outer layer when the
taut layers of the peptidoglycan are
hydrolyzed. The Fgure shows a
cross-sectional view through the glycan
chains illustrating the taut outer layer
and the more loosely inserted inner
material. The letters indicate the amino
acids composing the cross-links;
a
=
alanine, d
=
diaminopimelic acid,
and g
=
glutamic acid, with italicized
letters present in the D-conFguration.
enzymatic cuts, and volume increases,
one gets an increased cell volume that
precisely accommodates the increase in
cell cytoplasm.
An inside-to-outside mode of peptido-
glycan growth, similar to that for
Bacillus
subtilis
, has been proposed for
E. coli
.This
proposal is based on the observed recy-
cling of murein, the calculated amount of
peptidoglycan per cell, and the existence
of trimeric and tetrameric fragments that
are consistent with a multilayered pepti-
doglycan structure. The insertion of new
peptidoglycan strands in an unstressed
conFguration prior to their movement into
the load-bearing layer of the peptidogly-
can can explain all of these observations.
The recycling of peptidoglycan may be
a strain-speciFc result, as there is no
major release or recycling of peptido-
glycan in
Salmonella typhimurium
or in
E. coli
B/r. At this time, the inside-to-
outside mode of surface growth cannot
be excluded.
Although this discussion of the rate of
surface synthesis during the division cycle
has dealt primarily with peptidoglycan,
it applies equally to membranes and
other surface-associated elements. The
cell membrane grows in response to the
increase in peptidoglycan surface and
coats the peptidoglycan without stretching
or buckling. The area of the membrane
increases in the same way in which
bacterial peptidoglycan increases.
Onemustmakedistinctionsamongcell
components with regard to their location
in the cell, rather than with regard to
their chemical properties. Cell proteins
can be divided into two categories – those
associated with the cytoplasm and those
associated with the surface. Proteins asso-
ciated with the membranes are included
in the surface category of synthesis during
the division cycle. If a bacterial cell had a
histone-like protein associated with DNA,
there would be a third category of protein-
synthetic patterns, proteins synthesized
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