Ch 5: Bacterial cell wall

Peptidoglycan Components (specific to bacteria)

1) Glycan Chains

2) Short Peptides

Glycan chains (Peptidoglycan component)

Parallel polymers of disaccharides

Short Peptides (Peptidoglycan component)

Cross-link the glycan chains

What color does Gram Positive stain?

Purple

Gram positive components:

1) Thick cell wall

• many peptidoglycan layers

2) Teichoic acids

3) Lipoteichoic acids

• Also attach to cell membrane

Teichoic acids

• Carbohydrate chains in the peptidoglycan layers

• Negative charge

• Roles in cell division, adhesion, resistance

Effects of diverse Teichoic acids (gram positive)

Provide means for Identification and be recognized by our Immune system

Lancefield Grouping

Serological (antibody-based) testing of Streptococci

What color does Gram negative stain:

Stains reddish-pink

Gram negative components

1) Thin cell wall

2) Periplasm

3) Outer membrane

4) S-layer: extra protein layer in some species

Thin cell wall (Gram negative)

• Few Peptidoglycan layers

• Murein Lipoproteins attach OM and peptidoglycan

Periplasm (gram negative)

• Between the membranes

• Extracellular enzymes- busy place!

Outer membrane (OM)

Porins: transporters

Lipopolysaccharides (LPS)

Lipopolysaccharides (3 parts)

1) O polysaccharide

2) Core polysaccharide

3) Lipid A

O Polysaccharide (aka O antigen)

Is variable between strains

• Used for identification

• Might help a pathogen interact with host or evade immune system

Core Polysaccharide

More conserved/similar between strains

Lipid A

Embedded in the outer membrane

• Endotoxin

• Cations

Endotoxin (lipid A in Gram negative)

Free Lipid A binds receptors on immune cells and easily triggers inflammnation, possibly overstimulating our immune system → Sepsis

Cations (gram negative: lipid A)

Interact with lipid A, helps stabilize the membrane

β-Lactam antibiotics (Penicillin)

Inhibit formation of the peptide chains (Breaks down cell wall)

• β-Lactams are enzyme inhibitors

Lysozyme

Human enzyme

• Breaks down the glycan chain as a general defense

• Found in sweat, tears, and saliva

Differential staining

Distinguishes between 2 groups of bacteria

Step 4: Decolorization is key

Alcohol dehydrates the thick peptidoglycan layer (gram +) into a dense mesh like barrier

• Crystal violet is retained

What happens to the color of gram positive and negative after decolorization

Gram positive: Remain Purple

Gram negative: CV stain is removed, and then counterstain is pink.

Gram-variable

Bacterial species that cannot be reliably identified with gram staining

What does gram staining work best on?

Freshly cultured bacteria

• “old” cells: some species will stain variable after they begin to run out of nutrients

PG structure in gram staining?

Some Lactobacillus species are not distinguishable

• PG layer is thinner than most Gram positive → poor crystal violet retention

Mycobacteria (Gram stain exception)

• Complex cell wall with peptidoglycan and waxy Mycolic acids

  • Resistant to gram staining

  • Help resist digestion by phagocytes (WBC)

• Phylogentically related to gram-positives

What is used to identify Mycobacteria

Acid-fast staining

What pathogens do mycobacteria include?

1) Mycobacterium Tuberculosis (TB)

2) Mycobacterium leprae (leprosy)

Mycoplasma structure? (5 things)

1) No cell wall

• Cell membrane only, no peptidoglycan or outer membrane

2) Small and very in cell shape (pleomorphic)

• Can be a issue in filtration

3) Stain negative but are related to gram positives

4) Stronger cell membrane that contains Sterols

5) Many are pathogens

• Mycoplasma pneumoniae

Archaea walls/gram stain?

1) Pseudomurein (NOT peptidoglycan, but similar)

2) S-layers (surface layers)

• Sheets of protein or glycoproteins; some bacteria also have S-layers

3) Gram stain is not reliable or informative

4) Some Archaea lack cell walls

Algae

• Polysaccharides (often cellulose or Pectins)

• Diatoms are unique - use Silica and Calcium

Fungi

Polysaccharides (often Chitin) and glycoproteins