Prokaryotic Cell Structure: The Cell Wall

II. THE PROKARYOTIC CELL: BACTERIA

B. PROKARYOTIC CELL STRUCTURE

2. The Peptidoglycan Cell Wall

The overall purpose of this Learning Object is:
1) to learn the chemical makeup and function of peptidoglycan in the cell wall of organisms in the domain Bacteria;
2) understand how bacteria synthesize new peptidoglycan as a part of normal replication; and
3) introduce how some antibiotics function by interfering with bacterial cell wall synthesis.

LEARNING OBJECTIVES FOR THIS SECTION

In this section on Prokaryotic Cell Structure we are looking at the various organelles or structures that make up a bacterium. As mentioned in the introduction to this section, a typical bacterium usually consists of:

a cytoplasmic membrane surrounded by a peptidoglycan cell wall and maybe an outer membrane;
a fluid cytoplasm containing a nuclear region (nucleoid) and numerous ribosomes; and
often various external structures such as a glycocalyx, flagella, and pili.

We will now look at the peptidoglycan cell wall found in members of the domain Bacteria.

The Peptidoglycan Cell Wall (def)

The mycoplasmas, discussed later in this unit, are the only bacteria that naturally lack a cell wall. Mycoplasmas maintain a nearly even pressure between the outside environment and the cytoplasm by actively pumping out sodium ions. Their cytoplasmic membranes also contain sterols that most likely provide added strength. All other bacteria have a cell wall.

The Bacteria (eubacteria), with the exception of the Chlamydias discussed later in this unit, have a semirigid cell wall containing peptidoglycan. The Archaea (archaebacteria), that are often found growing in extreme environments, also have a semirigid cell wall but it is composed of chemicals distinct from peptidoglycan such as protein or pseudomurein. The eubacterial cell wall will be discussed here.

A. Structure and Composition of Peptidoglycan

With the exceptions above, members of the domain Bacteria have a cell wall containing a semirigid, tightknit molecular complex called peptidoglycan (def).

Peptidoglycan, also called murein, is a vast polymer consisting of interlocking chains of identical peptidoglycan monomers (see Fig. 1A and Fig. 1B). A peptidoglycan monomer (def) consists of two joined amino sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), with a pentapeptide (def) coming off of the NAM (see Fig. 2A and Fig. 2B). The types and the order of amino acids in the pentapeptide, while almost identical in gram-positive and gram-negative bacteria, show some slight variation among the domain Bacteria.

The peptidoglycan monomers (def) are synthesized in the cytosol of the bacterium where they attach to a membrane carrier molecule called bactoprenol (see Fig. 3, step-1, Fig. 3, step-2, and Fig. 3, step-3). The bactoprenols transport the peptidoglycan monomers across the cytoplasmic membrane (see Fig. 3, step-4) and work with the enzymes discussed below to insert the monomers into existing peptidoglycan enabling bacterial growth following binary fission.

Once the new peptidoglycan monomers are inserted, glycosidic bonds then link these monomers into the growing chains of peptidoglycan. These long sugar chains are then joined to one another by means of peptide cross-links between the peptides coming off of the NAMs. By linking the rows and layers of sugars together in this manner, the peptide cross-links provide tremendous strength to the cell wall, enabling it to function similar to a molecular chain link fence around the bacterium (see Fig. 1A and Fig. 1B).

B. Function of Peptidoglycan

Peptidoglycan prevents osmotic lysis (def). As seen earlier under the cytoplasmic membrane, bacteria concentrate dissolved nutrients (solute) through active transport. As a result, the bacterium's cytoplasm is usually hypertonic to its surrounding environment and the net flow of free water is into the bacterium. Without a strong cell wall, the bacterium would burst from the osmotic pressure of the water flowing into the cell.

For More Information: Cytoplasmic Membrane from Unit 1.

C. Synthesis of Peptidoglycan

In order for bacteria to increase their size following binary fission, links in the peptidoglycan must be broken, new peptidoglycan monomers must be inserted, and the peptide cross links must be resealed.

The following sequence of events occur:

1. Bacterial enzymes called autolysins:

a) Break the glycosidic bonds between the peptidoglycan monomers at the point of growth along the existing peptidoglycan (see Fig. 5, steps 1-3); and

b) Break the peptide cross-bridges that link the rows of sugars together (see Fig. 5, steps 1-3).

2. Transglycosidase enzymes then insert and link new peptidoglycan monomers into the breaks in the peptidoglycan (see Fig. 4, step 1, Fig. 4, step 2, and Fig. 4, step 3).

3. Finally, transpeptidase enzymes reform the peptide cross-links between the rows and layers of peptidoglycan to make the wall strong (see Fig. 4, step 4).

In Escherichia coli, the terminal D-alanine is cleaved from the pentapeptides to form a tetrapeptides. This provides the energy to bond the D-alanine of one tetrapeptide to the diaminopimelic acid of another tetrapeptide (see Fig. 1B). In the case of Staphylococcus aureus, the terminal D-alanine is cleaved from the pentapeptides to form a tetrapeptides. This provides the energy to bond a pentaglycine bridge (5 molecules of the amino acid glycine) from the D-alanine of one tetrapeptide to the L-lysine of another (see Fig. 1A).

GIF animation showing the synthesis of peptidoglycan.

Flash animation showing the synthesis of peptidoglycan.
© Juliet V. Spencer, Stephanie K.M. Wong, authors, Licensed for use, ASM MicrobeLibrary.

D. Using Antimicrobial Agents that Inhibit Peptidoglycan Synthesis to Control Bacteria

Many antibiotics work by inhibiting normal synthesis of peptidoglycan in bacteria causing them to burst as a result of osmotic lysis (def).

As just mentioned, in order for bacteria to increase their size following binary fission, enzymes called autolysins break the peptide cross links in the peptidoglycan, transglycosidase enzymes then insert and link new peptidoglycan monomers into the breaks in the peptidoglycan, and transpeptidase enzymes reform the peptide cross-links between the rows and layers of peptidoglycan to make the wall strong.

Interference with this process results in a weak cell wall and lysis of the bacterium from osmotic pressure. Examples include the penicillins (penicillin G, methicillin, oxacillin, ampicillin, amoxicillin, ticarcillin, etc.), the cephalosporins (cephalothin, cefazolin, cefoxitin, cefotaxime, cefaclor, cefoperazone, cefixime, ceftriaxone, cefuroxime, etc.), the carbapenems (imipenem, metropenem), the monobactems (aztreonem), the carbacephems (loracarbef), and the glycopeptides (vancomycin, teichoplanin).

For example, penicillins and cephalosporins bind to the transpeptidase enzymes (also called penicillin-binding proteins) responsible for resealing the cell wall as new peptidoglycan monomers are added during bacterial cell growth. This blocks the transpeptidase enzymes from cross-linking the sugar chains and results in a weak cell wall and subsequent osmotic lysis of the bacterium (see Fig. 6).

GIF animation illustrating how penicillins inhibit peptidoglycan synthesis.

Flash animation showing how penicillins inhibit peptidoglycan synthesis.
© Juliet V. Spencer, Stephanie K.M. Wong, authors, Licensed for use, ASM MicrobeLibrary.

To view a Quick Time video of penicillin killing a bacterium, see the CELL'S ALIVE web page.

Antimicrobial chemotherapy will be discussed in greater detail later in Unit 2 under Control of Bacteria by Using Antibiotics and Disinfectants.

Preview of "Chemotherapeutic Control of Bacteria" from Unit 2.

E. Gram-Positive, Gram-Negative, and Acid-Fast Bacteria

Most bacteria can be placed into one of three groups based on their color after specific staining procedures are performed: gram-positive, gram-negative, or acid-fast.

gram-positive: retain the initial dye crystal violet during the Gram stain procedure and appear purple when observed through the microscope. Common gram-positive bacteria of medical importance include Streptococcus pyogenes, Streptococcus pneumoniae, Staphylococcus aureus, Enterococcus faecalis, and Clostridium species.

gram-negative: decolorize during the Gram stain procedure, pick up the counterstain safranin, and appear pink when observed through the microscope. Common Gram-negative bacteria of medical importance include Salmonella species, Shigella species, Neisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae, Escherichia coli, Klebsiella pneumoniae, Proteus species, and Pseudomonas aeruginosa. Also see gram stain of a mixture of gram-positive and gram-negative bacteria.

The Gram Stain from Lab 6.

acid-fast: resist decolorization with an acid-alcohol mixture during the acid-fast stain procedure, retain the initial dye carbolfuchsin and appear red when observed through the microscope. Common acid-fast bacteria of medical importance include Mycobacterium tuberculosis, Mycobacterium leprae, and Mycobacterium avium-intracellulare complex. For additional acid-fast stains of pathogenic Mycobacteria, scroll down to Mycobacteria under the AIDS pathology section of WebPath's web page.

The Acid-Fast Stain from Lab 17.

These staining reactions are due to fundamental differences in their cell wall as will be discussed in Lab 6 and Lab 16. We will now look at each of these three bacterial cell wall types.

For further information on the bacterial cell wall and its functions see the online Microbiology Web Textbook at the University of Wisconsin-Madison.