Bacterial Structures: Flagella, Pili, and Capsules

• Bacteria use diverse cell surface proteins and structures for communication, sensory input, and survival, compensating for the lack of complex features found in larger organisms.
• Extracellular proteins and structures facilitate bacterial movement, environmental sensing, and immune system evasion.

Flagella

• Flagella enable bacteria to move independently of the environment.
• Flagella are made of flagellin proteins.
• Not all bacteria have flagella.
• Types of flagellated bacteria:
  • Monotrichous: single flagellum at one end
  • Amphitrichous: single flagellum at both ends
  • Peritrichous: flagella around the entire cell
  • Lophotrichous: multiple flagella

Fimbriae and Pili

• Fimbriae are thin microfibrils (1 to 1.5 nm thick) on the bacterial surface that help anchor the bacteria to surfaces
• Pili serve a similar purpose

Conjugation Pili (Sex Pili)

• Conjugation pili facilitate the transfer of DNA, plasmids, or proteins between bacteria.

Capsule

• Capsules are polymer layers around the cell that protect against environmental conditions and phagocytic attacks by immune cells.
• Microscopy can help visualize capsules for bacterial identification
• Klebsiella pneumoniae is a capsule-positive microbe

Importance of Bacterial Cell Surface Proteins

• Identifying bacterial strains for appropriate treatment plans.
• Enhancing virulence and pathogenicity by helping bacteria evade immune attacks.
• Enabling bacteria to adhere to and invade host cells.
• Staph. aureus utilizes surface protein structures to increase nasal carriage.
• Biofilm formation in Staph. aureus contributes to antibiotic resistance and immune evasion.
• Bacterial flagella enhance mobility, immune cell evasion, and entry into host cells.

Capsule/Slime Layer Functions

• Not specified in text

Biofilm

• Biofilm formation has been linked to nosocomial and chronic infections.
• Biofilms increase antibiotic resistance and reduce immune penetration.
• Biofilm formation in the lungs and vessels can lead to chronic inflammatory responses.
• Biofilm formation increases specific surface proteins that increases antibiotic resistance

Additional Surface Structures

• Pili: hair-like structures for attaching to other cells.
• Fimbriae: shorter structures that help bacteria adhere to surfaces.

Recap of bacterial cell structures

• Cell envelope: collective layers surrounding bacterial cells.
• Cell wall: protects the bacteria and maintains their shape and is present in both gram-positive and gram-negative bacteria.
• Capsule: an extra layer on top of the cell wall, a major virulence factor in some bacteria.
• Plasma membrane: encloses the cytoplasm and defines the cell, with respiration, photosynthesis, and synthesis of lipids and cell wall constituents.
• Cytoplasm: contains:
  • Ribosomes: the site of protein synthesis.
  • Nucleoid: contains the genetic material.
  • Plasmids: contain genes that confer a selective advantage to the bacterium.