Chapter 3 Notes: External Structures for Motility, Pili, Nanotubes, Glycocalyx, and Biofilms

Flagellum: Structure and Function

• The flagellum is the locomotory structure in bacteria that enables movement in response to environmental cues.
• Three distinct parts:
  • Filament: long, whip-like propeller; hollow core; composed of flagellin subunits that wind helically; grows from the tip onward.
  • Hook: short curved segment that connects the filament to the basal body.
  • Basal body: motor complex embedded in the cell envelope; in Gram-negative bacteria, it associates with multiple layers of the cell wall (outer membrane, peptidoglycan) and the cell membrane; contains the motor machinery.
• Growth and assembly:
  • Filament synthesized in the cytoplasm, moves through the hollow core, and is added at the tip as the flagellum grows to full length.
  • Basal body serves as the motor and anchor; rings/motors are composed of proteins.
• Structural context:
  • In Gram-negative cells, the basal body spans the outer membrane, peptidoglycan layer, and inner cell membrane; the illustration shows the filament extending outward from the cell through the rod-like basal body motor.
  • Inner cellular location: flagellin proteins are synthesized inside the cell and exported through the hollow core of the growing filament.
• Variability among bacteria:
  • Some species have a single flagellum; others have many flagella.
  • Flagella can be arranged in different patterns on the cell surface (see below).
• Visualization and staining:
  • Bacterial flagella are too small to visualize with light microscopy directly.
  • Flagella staining thickens the filament so it can be seen under a light microscope (not performed in the lab in this course).
  • Electron microscopy can reveal basal bodies and the detailed structure.

Flagellar Arrangements: How flagella attach to the cell

• Polar attachment refers to flagella attached at or near one end (pole) of the cell.
• Monotrichous: a single flagellum (polar flagellation).
• Lophotrichous: small tufts or bunches of flagella at one pole (tufts).
• Amphitrichous: single flagella at both ends (one flagellum at each end or a tuft at each end).
• Peritrichous: flagella dispersed around the entire cell surface (random distribution).
• The number and location of flagella are useful for bacterial identification.
• Visualization notes:
  • Monotrichous or polar flagellation can be seen in electron micrographs (and some light micrographs after staining).
  • Peritrichous flagellation can be seen with a flagella stain on light microscopy.

Chemotaxis and motility in response to chemicals

• Chemotaxis: movement of bacteria in response to chemical signals (chemo- implies chemical cues; taxis = movement).
• The process relies on membrane-bound receptors that detect attractants or repellents.
• Attractant binding leads to movement toward the chemical (positive chemotaxis); repellents drive movement away (negative chemotaxis).
• Movement patterns:
  • Run: a straight, smooth movement in one direction.
  • Tumble: a reorientation event that changes direction.
• Flagellar rotation governs movement:
  • Counterclockwise (CCW) rotation produces a run (straight propulsion).
  • Clockwise (CW) rotation causes a tumble (reorientation).
• The energy source for flagellar rotation:
  • Powered by a proton concentration gradient across the membrane (proton motive force), not directly by ATP.
  • Chapter 7 (electron transport and ATP synthesis) will cover proton gradients in more detail.
• Relevance:
  • Helps explain how bacteria navigate toward nutrients and away from harmful substances in their environment.
  • Provides context for comparing to other motility forms (e.g., twitching, gliding).

Periplasmic flagella and spirochetes

• Some bacteria use an internal flagellar apparatus located in the periplasmic space, termed axial filaments.
• Organisms with periplasmic flagella are called spirochetes (e.g., Treponema pallidum and Borrelia burgdorferi).
• Structure:
  • Basal body, hook, and filament are present but wrapped around inside the cell (periplasmic space).
  • Axial fibrils (axial filaments) run within periplasmic space and enable the corkscrew-like movement.
• Movement:
  • On liquid media, spirochetes rotate in a corkscrew manner.
  • On moist surfaces, movement can resemble inchworm-like locomotion.
• Notable pathogenic examples:
  • Treponema pallidum (syphilis).
  • Borrelia burgdorferi (Lyme disease).
• Functional significance:
  • Internal flagella provide motility without exterior flagella, contributing to distinct invasive strategies and tissue penetration.

Other motility mechanisms: twitching and gliding

• Twitching motility:
  • Mediated by Type IV pili (long, thin, hollow filaments composed of pilin).
  • Movement is jerky and occurs as pili extend, attach to a surface, and retract, pulling the cell forward.
  • Requires cell-to-cell contact and surface contact; described as social motility because coordination often involves neighboring cells.
  • Powered by ATP.
  • Type IV pili also contribute to DNA uptake (transformation) and attachment to surfaces, aiding in biofilm formation.
• Gliding motility:
  • Smooth movement across surfaces, often without direct contact with other cells.
  • May involve secretion of slime to facilitate forward movement (slime layer) and/or internal motor proteins moving along cytoskeletal tracks.
  • Mechanisms can include adhesion proteins and cytoskeletal interactions enabling forward motion.
  • Also independent of external flagella, contrasting with twitching.
• Key contrasts:
  • Twitching requires surface contact and is jerky; gliding is smooth and can occur without direct cell contact.
  • Energy sources include ATP and cytoskeletal motor activities, rather than membrane-proton gradients driving flagellar rotation.

Fimbriae and pili: attachment, DNA transfer, and social surfaces

• Fimbriae (singular fimbrial structure):
  • Small, bristle-like fibers that sprout from the bacterial surface.
  • Facilitate attachment to surfaces, enabling colonization and potential infection.
  • Typically numerous and contribute to adhesion during biofilm formation.
• Pili (pilus, singular):
  • Longer than fimbriae; a subset exists with distinct functions.
  • F pilus (sex pilus): a specialized pilus used in conjugation (horizontal gene transfer) between Gram-negative bacteria.
  • Conjugation: one cell transfers DNA to another via a pilus bridge; can disseminate antibiotic resistance genes and other traits.
  • Transformation: Type IV pili can also take up extracellular DNA from the environment (natural transformation)
  • Role in biofilms: pili aid in adhesion and microcolony formation, contributing to biofilm development.
• Summary of functional overlap:
  • Both fimbriae and pili promote surface attachment and contribute to biofilm formation.
  • Type IV pili are multifunctional: motility (twitching), DNA uptake (transformation), and adhesion.

Nanowires and nanotubes: intercellular and environmental electron transfer

• Nanotubes and nanowires are extremely thin, long extensions of the cytoplasmic membrane (and sometimes periplasmic structures).
• Functions described:
  • Act as channels to transfer amino acids and other metabolites between neighboring cells.
  • Some bacteria use these structures to harvest energy by shuttling electrons along the nanotubes.
  • Electrons can be transferred to external acceptors such as rocks or minerals (e.g., iron-containing substrates), enabling alternative respiratory strategies.
• Conceptual takeaway:
  • These structures expand the range of environmental interactions and energy acquisition strategies beyond conventional oxygen-based respiration.

The glycocalyx: extracellular coating outside the cell wall

• Definition:
  • A coating secreted outside the cell wall, composed of repeating polysaccharide units or glycoproteins (polysaccharide or glycoprotein components).
  • Made up of sugar polymers (e.g., polysaccharides) and/or sugar-linked proteins.
• Two main forms:
  • Slime layer: loosely attached, diffuse, unorganized; easily rinsed off.
  • Capsule: dense, well-organized, thick, mucoid layer; not easily washed away; often shiny and sticky in colonies.
• Capsule stain (visualization):
  • Capsule appears as a clear halo around stained cells because capsule material does not take up the stain; the background is stained to reveal the capsule as a non-stained area.
  • Capsule staining is described in lectures and short PowerPoints; not performed in this lab.
• Functions and consequences:
  • Adhesion: capsules and slime facilitate attachment to tissues and surfaces, aiding colonization and infection.
  • Pathogenicity: encapsulation enhances virulence by promoting adherence and establishing infection sites.
  • Evasion of phagocytosis: capsules impede opsonization (antibody and complement tagging) and phagocytosis, increasing survival in hosts.
  • Protection from chemicals and desiccation: glycocalyx retains water and can shield against antibiotics and some viruses.
  • Interference with bacteriophages: capsules can hinder phage attachment if receptors are shielded.
  • Biofilm formation: glycocalyx is a major component of the extracellular matrix in biofilms, promoting surface attachment and community structure.
• Capsule and slime in disease context:
  • Many pathogenic bacteria have capsules that enhance colonization and persistence in the host.
  • Biofilm formation on medical devices (catheters, implants, IUDs) is often mediated by the glycocalyx.

S-layer: surface layer proteins

• S-layer definition:
  • A crystalline array composed of thousands of copies of a single protein, forming a protective surface layer.
  • Often described as chain-mail-like due to the highly regular protein network.
• Environmental response:
  • S-layers are produced when bacteria encounter hostile or stressful environments.
• Localization and function:
  • In Archaea, the S-layer can function as part of the cell wall or as a protective glycocalyx-like layer.
  • In some bacteria, S-layer lies between the glycocalyx and the cell wall, contributing to protection and structural integrity.
• Relationship to other structures:
  • The S-layer can provide similar protective or structural roles to the glycocalyx and cell wall components under certain conditions.

Biofilms: communities of microorganisms on surfaces

• Definition and significance:
  • Biofilms are communities of microorganisms that adhere to surfaces and produce extracellular matrix within which they live.
• Common examples:
  • Dental plaque on teeth.
  • Medical devices: catheters, pacemakers, implants, IUDs, etc.
• Structure and protection:
  • Organisms in a biofilm are embedded in a glycocalyx-like extracellular matrix that traps nutrients and protects inner cells.
  • Nutrients diffuse through the matrix; inner cells are sheltered from some antibiotics and immune effectors.
• Sloughing and recurrence:
  • Outer cells can slough off (planktonic cells), potentially causing recurrent infections when antibiotics kill surface-adhered cells but not all embedded cells.
• Clinical relevance:
  • Biofilms are linked to chronic infections due to their resistance to antibiotics and immune clearance.
• Formation sequence (simplified):
  • Surface conditioning with organic matter → initial attachment (via pili/fimbriae) → production of extracellular polymeric substances → maturation into a multi-layered community → dispersion of cells to seed new sites.

Capsule staining and clinical relevance

• Capsule staining highlights the capsule as a clear halo around cells, illustrating the capsule’s presence and rough organization.
• Capsule features discussed: stickiness, protection from immune responses, and contribution to biofilm formation and persistence in infections.

Summary of key concepts and connections to broader biology

• Flagella and motility: structural components (filament, hook, basal body) and patterns of flagellar arrangement; energy source (proton gradient) and chemotaxis mechanisms for navigating chemical landscapes.
• Alternative motility: twitching and gliding reveal that bacteria can move without external flagella; Type IV pili are central to both twitching motility and genetic exchange (transformation) and biofilm development.
• Periplasmic flagella in spirochetes provide a distinct motility strategy that enables corkscrew-like motion and tissue penetration, with medically important examples like syphilis and Lyme disease.
• Fimbriae and pili: diversity of surface appendages for adhesion, DNA transfer (conjugation, F-pili), and genetic diversification (horizontal gene transfer); significance for antibiotic resistance spread and biofilm formation.
• Nanowires and nanotubes: intercellular channels and energy-harvesting structures that expand microbial metabolic capabilities, including electron transfer to minerals (breathing rocks) and potential cross-species electron exchange.
• Glycocalyx: protective and adhesive extracellular layer with slime and capsule forms; critical roles in adherence, immune evasion, hydration, and biofilm robustness; linked to pathogenicity and resistance to environmental stresses.
• S-layer: an additional protective proteinaceous layer in certain bacteria and archaea, with functional parallels to the glycocalyx and cell wall in hostile environments.
• Biofilms: a major mode of microbial life on natural and clinical surfaces; their resilience explains chronic infections and device-associated complications; strategies to disrupt biofilms are of clinical importance.

Upcoming topics preview

• In the next recording, we will discuss the structure of Gram-positive and Gram-negative cell walls to contrast how these external structures interact with the cell envelope in different organisms.