OOCH 2 OHHNHCOCH 3HH CH 2 OHHO
ONHHNHCOCH 3HHH
OCOCH 3 HCOCHCH 3COO−
CONHCHCH 2 CH 2 CONHCHCONHCHCOO−(CH 2 ) 4 CH 3NH 3OHnL-alanine D-glutamic L-lysine
acidTetrapeptide chain D-alanine
+Lactic acid
residueNAM NAG(^62)
5
4
32
1
4
3
5
6
1
Figure A2.2 The structure of the monomer unit of the polyglycan ofS. aureus. The cross links
between chains are from theg-amino group of the lysine to the carboxylate of the C-terminal D-
alanine. The tetrapeptide can contain other amino acid residues
contains transport sugars, enzymes and other substances. The complete struc-
ture separating the cytoplasm of the bacteria from its surroundings is known as
thecell envelope.
The peptidoglycans found in both Gram-positive and Gram-negative bacteria
are commonly known as mureins. They are polymers composed of polysacchar-
ide and peptide chains, which form a single, netlike molecule that completely
surrounds the cell. The polysaccharide chains consist of alternating 1–4 linked
b-N-acetylmuramic acid (NAM) and b-N-acetylglucosamine (NAG) units
(Figure A2.2). Tetrapeptide chains are attached through the lactic acid residues
of the NAM units of these polysaccharide chains.
In Gram-positive bacteria, the tetrapeptide chains of one polysaccharide
chain are cross linked by pentaglycine peptide bridges from the g-amino
group of lysine to the terminal alanine of the tetrapeptide chains of a second
polysaccharide chain to form a net-like polymer (Figure A2.3(a)). The penta-
glycinepeptide bridges occasionally contain other residues. In Gram-negative
bacteria the tetrapeptide chains are directly linked by amide group (peptide
bond) bridges (Figure A2.3(b)). The structure is denser than that found in the
Gram-positive cell wall because the peptidoglycan chains are closer together.
The exterior surface of Gram-positive bacteria is covered byteichoic acids.
These are ribitol–phosphate or glycerol–phosphate polymer chains that are
frequently substituted by alanine and glycosidically linked monosaccharides
(Figure A2.4). They are attached to the peptidoglycan by a phosphate diester
link. Teichoic acids can act as receptors to bacteriophages and some appear to
have antigenic properties.
APPENDIX 2 BACTERIA 241