Prokaryotic Structure & Function: Adhesion, Motility, and Secretion

Prokaryotic Structure & Function: Adhesion, Motility & Secretion

Attachment - Fimbriae

Fimbriae are short, fine, hair-like appendages found on the surface of many prokaryotic cells, primarily bacteria. They play a crucial role in the ability of bacteria to adhere to surfaces and host tissues.

• Structure: Composed of protein; up to 1000 fimbriae per cell.

• Functions:

  • Attachment to solid surfaces (e.g., rocks in streams, host tissues).

  • Type IV fimbriae are responsible for twitching motility on moist surfaces.

• Example: Neisseria meningitidis uses Type IV pili for attachment, contributing to diseases such as meningococcal meningitis and pneumonia.

Motility - Flagella

Flagella are thread-like locomotor appendages that enable bacterial movement. The arrangement and number of flagella vary among species and influence motility patterns.

• Distribution Types:

  • Monotrichous: Single flagellum at one pole.

  • Amphitrichous: Single flagellum at both poles.

  • Lophotrichous: Cluster of flagella at one or both poles.

  • Peritrichous: Flagella distributed over the entire cell surface.

• Example: Pseudomonas species exhibit monotrichous flagellation, while Escherichia coli are peritrichous.

Flagellar Movement & Energy Source

Bacterial flagella rotate to propel the cell, functioning as a rotary engine powered by a proton gradient.

• Mechanism:

  • Flagellar rotation is driven by proton motive force (PMF), generated by the flow of protons (hydrogen ions) across the bacterial cell membrane.

  • Energy for movement comes from the concentration gradient set up by the cell's metabolism.

• Equation: $\text{Proton Motive Force (PMF)} = \Delta \psi - (2.303 \frac{RT}{F} \Delta pH)$ where $\Delta \psi$ is the membrane potential, $R$ is the gas constant, $T$ is temperature, $F$ is Faraday's constant, and $\Delta pH$ is the pH gradient.

Chemotaxis

Chemotaxis is the directed movement of bacteria in response to chemical gradients, allowing them to move toward attractants and away from repellents.

• Detection: Attractants and repellents are detected by chemoreceptors on the cell surface.

• Other Environmental Cues:

  • Temperature (thermotaxis)

  • Light (phototaxis)

  • Oxygen (aerotaxis)

• Example: Escherichia coli uses chemotaxis to locate nutrients and avoid harmful substances.

Protein Trafficking in Bacteria

Protein trafficking refers to the synthesis and localization of proteins within bacterial cells, a process essential for cellular function and adaptation.

• Sites of Synthesis: Proteins are synthesized by ribosomes in the cytoplasm.

• Destinations: Proteins may function in the cytoplasm, be integrated into membranes, or be secreted outside the cell.

• Key Structures:

  • Lipopolysaccharide (outer membrane)

  • Cell wall

  • Periplasm

  • Inner membrane (cell membrane)

  • Ribosome

Overview of Bacterial Protein Secretion

Bacterial protein secretion is a complex process, with different challenges faced by Gram-positive and Gram-negative bacteria. All secretion pathways require energy, either from ATP hydrolysis or proton motive force.

• Gram-positive vs. Gram-negative: Gram-negative bacteria must transport proteins across both inner and outer membranes, while Gram-positive bacteria only have a single membrane.

• Energy Requirement:

  • ATP hydrolysis: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

  • Proton motive force

Sec-dependent Pathway

The Sec-dependent pathway is ubiquitous in bacteria and is responsible for translocating proteins across the plasma membrane or integrating them into the membrane.

• Mechanism:

  • Proteins are synthesized with a signal peptide.

  • SecB chaperone binds the preprotein.

  • SecA pushes the protein through the SecYEG translocator, hydrolyzing ATP for energy.

• Equation: $\text{SecA} + \text{ATP} \rightarrow \text{SecA} + \text{ADP} + \text{P}_i$

Protein Secretion Systems of Gram-negative Bacteria

Gram-negative bacteria possess specialized secretion systems to transport proteins across the outer membrane, many of which are involved in virulence.

• Types:

  • Type I: ABC transporter

  • Type III: Injectisome

  • Type IV: Conjugation system

• Function: Many virulence proteins are secreted using these systems.

Type I Secretion System: Hly ABC Transporter

The Type I secretion system utilizes the ABC transporter family to export a variety of substances, including toxins, proteases, lipases, and antimicrobial drugs.

• Mechanism: Direct transport from cytoplasm to extracellular space, bypassing the periplasm.

• Example: Hemolysin secretion by Staphylococcus aureus causes hemolysis of red blood cells.

Type III Secretion System: Yersinia pestis

The Type III secretion system acts as a molecular syringe, injecting virulence factors directly into host cells.

• Mechanism: Multi-protein complex spans both bacterial membranes and the host cell membrane.

• Example: Yersinia pestis uses this system to deliver Yops effectors, disrupting host cell processes.

Type IV Secretion System: Agrobacterium tumefaciens

The Type IV secretion system can transfer both proteins and DNA, playing a role in horizontal gene transfer and pathogenesis.

• Mechanism: Transfers T-DNA and proteins into plant cells.

• Example: Agrobacterium tumefaciens causes crown gall disease in plants by transferring T-DNA into the host genome.

Additional info: These notes expand on the original slides by providing definitions, mechanisms, and examples for each secretion system and motility structure, ensuring a comprehensive understanding suitable for exam preparation.