Lesson 11 - Bacterial Cell Envelope cntd. & Specialized Structures

Review: What are the major features of Gram-negative bacterial cell envelopes, and how is LPS involved in serotyping?

• Gram-negative bacteria are diderm, with inner and outer membranes separated by a periplasm containing thin peptidoglycan.

• The outer membrane contains LPS, porins, and lipoproteins.

• LPS consists of O-antigen (variable), core polysaccharide (conserved), and Lipid A (membrane anchor).

• LPS acts as an endotoxin when released from lysed bacterial cells.

• Differences in LPS O-antigens help distinguish serovars of the same species through serotyping.

What makes the mycobacterial cell envelope unique?

• Extremely complex envelope with characteristics of Gram-positive and Gram-negative bacteria alongside its own unique features.

• Extreme resistance to environmental impacts

  • Ex. Mycobacterium tuberculosis, Mycobacterium leprae

• ~1,000X less permeable than E. coli; Prevent the accurate use of Gram stain

What are the distinguishing characteristics of mycobacterial cell wall?

• Thicker than many other prokaryotes

• Consists of hydrophobic, waxy “mycomembrane"

  • Rich in mycolic acids* (contains 2 HC chains of different lengths) interleaved with sugar mycolates

  • A final outer layer of hydrophobic phenolic glycolipids

• Mycolic acid layer and a peptidoglycan layer held together by the polysaccharide, arabinogalactan* (arabinose and galactose)

• Characterized and stained using the Ziehl-Neelsen acid-fast stain that targets mycolic acids; important diagnostic test

• Bold = Unique to mycobacteria

What are the advantages and disadvantages of the mycobacterial cell envelope?

• Advantages:

  • Thick, waxy mycomembrane resists dryness, osmotic stress, detergents, antiseptics, and many antibiotics.

  • Helps resist phagocytosis by host cell and host immune defences.

• Disadvantages:

  • Slow growth.

  • Cell envelope is energetically expensive to synthesize and maintain.

How do archaeal cell walls differ from bacterial cell walls?

• Semi-rigid structure for protection (like Bacteria)

• Archaea lack bacterial peptidoglycan.

• Archaea have variety of cell wall types; adapted for specific environment; some archaea lack cell walls entirely

• Most have a proteinaceous S-layer, sometimes their only wall component.

• A few species contain pseudomurein, which uses NAT instead of NAM and forms stronger peptide interbridges.

• Methanochondroitin is a cell wall polymer in some archaea

  • Similar to connective tissue component (chondroitin) in vertebrae

Describe the Proteinaceous S-layer

• Often anchored to cell membrane

• In some species S-layer is the only cell wall component

• Some species contain a protein sheath in addition to the S-layer

What is a bacterial capsule, and what are its main functions?

• An additional coating exterior to the cell wall in Gram-positive or Gram-negative bacteria.

• Composed of polysaccharides and glycoproteins that bind water and form a hydration layer.

• Can coexist with an S-layer; if so the capsule is external to the S-layer.

• Difficult to stain, so it may appear as a clear halo around the cell.

• Functions:

  • Protects against phagocytosis by the immune system.

  • Assists with adherence to surfaces.

  • Protects against dehydration.

What factors contribute to determining bacterial cell shape?

• Cell wall: Provides structural support.

• Turgor pressure: Internal pressure that contributes to maintaining cell shape.

• Bacterial cytoskeletal proteins: Contribute to cell shape and organization.

• Bacterial shape is a complex phenomenon involving multiple factors.

What is FtsZ, and how does it form the Z-ring?

• FtsZ is a bacterial cytoskeletal protein.

• FtsZ proteins polymerize to form a Z-ring.

• The lecture associates the Z-ring with maintenance of cell diameter.

• Additional context: The Z-ring plays a central role in bacterial cell division by organizing constriction at the division site.

What is the role of MreB in bacterial cells?

• MreB is a bacterial cytoskeletal protein.

• Required for elongation of rod-shaped bacterial cells.

• Helps support the maintenance of rod-shaped cell morphology.

• Comparison: FtsZ forms the Z-ring, whereas MreB is required for rod-shaped cell elongation.

What are bacterial flagella, and how are they powered?

• Flagella are specialized structures that allow many bacteria to swim.

• They provide motility in changing environments.

• Bacterial flagella rotate to propel the cell.

• Rotation is powered by the transmembrane proton motive force (electrochemical H+ gradient).

• Example: Flagellated Salmonella.

What are pili/fimbriae, and why are they important?

• Short, hair-like protein appendages on bacterial cells.

• Mediate attachment (adherence) to surfaces and host tissues.

• Adherence is an important first step in colonization and infection.

• Adhesins are molecules that facilitate attachment.

• Example: E. coli pili/fimbriae are important in urinary tract infections (UTIs).

Compare the cell envelopes of Gram-positive, Gram-negative, and mycobacterial bacteria.

1. Gram-positive

• One cell membrane (monoderm).

• Thick, multilayered peptidoglycan (20–80 nm).

• Contains teichoic and lipoteichoic acids.

• No outer membrane; peptidoglycan is exposed.

• Stains purple; more accessible to lysozyme and some antibiotics.

2. Gram-negative

• Two membranes (diderm) separated by a periplasm.

• Thin peptidoglycan (1–2 sheets) in the periplasm.

• Outer membrane contains LPS, porins, and lipoproteins.

• Outer membrane protects peptidoglycan and blocks many toxic substances.

• Stains pink/red.

3. Mycobacteria

• Complex envelope with a thick, waxy mycomembrane rich in mycolic acids.

• Arabinogalactan connects the mycolic acid layer to peptidoglycan.

• Extremely low permeability (~1,000× less than E. coli).

• Resistant to drying, detergents, many antibiotics, and host defences.

• Standard Gram stain is unreliable; use acid-fast staining.