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Summary Basic molecular bacteriology
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Molecular Basis of Bacterial Infections Evelien FloorBasic molecular bacteriology
The cell wall
The bacterial cell wall consists of all the structures outside the plasma membrane. The cell wall is the
most essential structure of the bacterium. It provides structural integrity; it prevents bacterial lysis
from turgor pressure. The cell wall is also the main determinant of bacterial cell shape. It allows
diffusion of gasses and small molecules in and out of the cell. It will also keep out toxics and prevent
water loss.
The cell wall allows interaction and communication with the environment including the host. The cell
wall acts as a scaffold for other cell wall structures:
• Lipopolysaccharide
• Flagella
• Teichoic acids
• Proteins
The bacterial cell wall is a proven target for vaccines and antibiotics. Almost half of all antibiotics used
are targets of the cell wall: -lactams, glycopeptides, polymyxins and teixobactins. Besides that, it is a
target of vaccine-induced antibodies.
Infectious diseases are a significant cause of global death. Because of antibiotic resistance the amount
of deaths caused by infectious diseases will increase. Therefore, understanding of molecular
composition and biosynthesis processes of the cell wall opens up opportunities for new therapies.
Composition of the cell wall
Peptidoglycan
The main structural component of both Gram-negative and Gram-positive bacteria is peptidoglycan.
The peptidoglycan layer of Gram-positive bacteria is thicker than in Gram-negative bacteria. This is the
reason why Gram-negative bacteria stain pink with Gram-staining and Gram-positive bacteria stain
purple.
Peptidoglycan consists of very long polysaccharide sugar chains
that are crosslinked by peptide sidechains. Peptidoglycan is a
helical structure; this makes it elastic and porous. The sugars in
the polysaccharide backbone are N-acetylglucosamine (NAG)
and N-acetylmuramic acid (NAM). Attached to those building
blocks are peptide sidechains which contain D-amino acids.
There are two main types of crosslinking of peptide chains:
direct and indirect via a peptide interbridge. It depends on the
species which type of crosslinking is used. A direct cross-link is
characterized by connecting the carboxyl group of an amino
acid in one stem peptide to the
amino group of an amino acid in
another stem peptide. Bacteria that
have indirect linkage use a peptide
interbridge, a short chain of amino
acids that links the stem peptide of
one peptidoglycan strand to that of
another. Cross-linking results in one
dense, interconnected network of
peptidoglycan strands.
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, Molecular Basis of Bacterial Infections Evelien Floor
There is structure variation of peptidoglycan in:
• Length of sugar strands
• Amount of crosslinking
• Diversity in peptide composition
This variation affects permeability for molecules including nutrients, but also toxins or antibiotics.
Biosynthesis of peptidoglycan
Biosynthesis of peptidoglycan starts with synthesis of sugar building blocks:
1. NAM and NAG are synthesized while attached to uridine diphosphate (UDP)
2. Amino acids are sequentially added to UDP-NAM to form pentapeptide
3. The start of the actual biosynthesis occurs at the membrane: the UDP-NAM-pentapeptide is
attached to lipid carrier Bactoprenol: Lipid I
4. NAG is transferred to Lipid I to form Lipid II
5. Lipid II is flipped (transported) across the membrane
6. The NAG-NAM-pentapeptide is attached to the growing end of the peptidoglycan chain
7. Bactoprenol donor is recycled for another round of synthesis
8. Peptide crosslinks between peptidoglycan chains: transpeptidation via Penicillin-binding
proteins (PBPs) → direct crosslinking is inhibited by penicillin’s and -lactams.
Most glycan structures are synthesized at the inside of the membrane but in this case, it happens at
the plasma membrane. Lipid II is the real building block of peptidoglycan and after synthesis it is flipped
(transported) across the membrane. The molecule doing this is still unknown but would be a good
target for antibiotics.
Other cell wall structures
Gram-negative bacteria consist of lipopolysaccharides (LPS) and Gram-positive bacteria consist of
teichoic acids. Other cell structures that both bacteria consist of are: flagella, a capsule, glycoproteins
and pili.
Lipopolysaccharide
LPS is also called endotoxin and is a major component
of the outer membrane of Gram-negative bacteria
(75% of the surface). It is a glycolipid composed of
three parts:
• Lipid A: embedded in membrane
• Core oligosaccharide
• O-antigen: glycan polymer
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