Microbial Cell Structure and Function

The Cell Envelope

The cell envelope is a series of layered structures that surround the cytoplasm of microbial cells, governing interactions with the environment. It is essential for maintaining cell integrity, mediating transport, and providing protection.

• Cytoplasmic membrane: The innermost boundary, crucial for selective permeability.

• Cell wall: Provides rigidity and shape, protecting against osmotic pressure.

• Outer membrane: Present in gram-negative bacteria, adds an extra layer of protection.

• S-layers: Protein or glycoprotein layers found in some Bacteria and Archaea.

2.1 The Cytoplasmic Membrane

The cytoplasmic membrane surrounds the cytoplasm, separating it from the environment. Its main function is selective permeability, allowing the cell to control the movement of substances in and out.

• Structure: Phospholipid bilayer (8–10 nm wide) with embedded proteins.

• Membrane proteins:

  • Integral membrane proteins: Embedded within the membrane.

  • Transmembrane proteins: Span the entire membrane.

  • Peripheral membrane proteins: Loosely attached to the membrane surface.

• Archaeal membranes: Contain isoprenoid chains instead of fatty acids and ether linkages, which can form lipid monolayers for increased stability.

• Functions:

  • Permeability barrier: Only allows specific molecules to pass.

  • Protein anchor: Holds transport and other proteins in place.

  • Energy conservation: Site of proton motive force generation.

2.2 Transporting Nutrients into the Cell

Microbial cells use specialized transport systems to accumulate nutrients against concentration gradients. These systems are energy-dependent and highly specific.

• Simple transport: Driven by the proton motive force; includes symport (same direction) and antiport (opposite direction) mechanisms.

• Group translocation: The transported substance is chemically modified during transport (e.g., phosphotransferase system in E. coli).

• ABC transporter systems: Consist of a substrate-binding protein, a transmembrane transporter, and an ATP-hydrolyzing protein; use ATP for transport.

2.3 The Cell Wall

The cell wall provides structural support, maintains cell shape, and prevents lysis due to osmotic pressure. Bacteria are classified as gram-positive or gram-negative based on cell wall structure and Gram stain reaction.

• Gram-positive: Thick peptidoglycan layer, often with teichoic acids; susceptible to lysozyme and penicillin.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), and periplasmic space.

• Peptidoglycan: Rigid polysaccharide unique to Bacteria, providing strength and shape.

2.4 LPS: The Outer Membrane

The outer membrane of gram-negative bacteria contains lipopolysaccharide (LPS), which is important for surface recognition, virulence, and structural integrity.

• LPS structure: Composed of lipid A (endotoxin), core polysaccharide, and O-polysaccharide.

• Porins: Protein channels that allow the passage of small molecules.

• Periplasm: Space between the cytoplasmic and outer membranes, containing various proteins.

2.5 Diversity of Cell Envelope Structure

Microbial cell envelopes show significant diversity, especially among Archaea and some specialized Bacteria.

• S-layers: Paracrystalline protein or glycoprotein layers providing strength and protection.

• Alternative configurations: Some Bacteria and Archaea lack cell walls but have tough membranes (e.g., Mycoplasmas, Thermoplasma).

• Archaeal cell walls: Lack peptidoglycan; may have S-layers or pseudomurein.

Cell Surface Structures and Inclusions

2.6 Cell Surface Structures

Microbial cells possess various surface structures that aid in attachment, protection, and genetic exchange.

• Capsules and slime layers: Sticky polysaccharide coats that assist in attachment, biofilm formation, and protection from desiccation.

• Fimbriae and pili: Protein appendages for attachment, biofilm formation, and genetic exchange (conjugation).

• Hami: Archaeal surface structures resembling grappling hooks, aiding in attachment and biofilm formation.

2.7 Cell Inclusions

Cell inclusions serve as storage sites for energy reserves and other important compounds, reducing osmotic stress.

• Carbon storage polymers: Polyhydroxybutyric acid (PHB), polyhydroxyalkanoate (PHA), and glycogen.

• Polyphosphate, sulfur, and carbonate minerals: Serve as reservoirs for phosphorus, sulfur, and biomineralization products.

• Gas vesicles: Protein-bound structures that confer buoyancy.

• Magnetosomes: Allow orientation within magnetic fields (magnetotaxis).

2.8 Endospores

Endospores are highly differentiated, dormant cells formed by some gram-positive bacteria to survive extreme conditions. They are resistant to heat, radiation, chemicals, and desiccation.

• Formation: Triggered by nutrient limitation; involves asymmetric division and multiple protective layers.

• Structure: Core, inner membrane, cortex, outer membrane, coat, and exosporium; contains dipicolinic acid and small acid-soluble spore proteins (SASPs).

• Germination: Occurs in three steps—activation, germination, and outgrowth—when conditions become favorable.

Cell Locomotion

2.9 Flagella, Archaella, and Swimming Motility

Flagella and archaella are appendages that enable swimming motility in Bacteria and Archaea, respectively. They are powered by rotary motors and allow cells to move toward or away from stimuli.

• Flagella structure: Composed of a filament (flagellin), hook, and basal body (motor).

• Arrangements: Polar (single or tuft at one end), lophotrichous (tuft at one or both ends), peritrichous (distributed over the cell).

• Archaella: Structurally distinct from bacterial flagella, powered by ATP hydrolysis, generally smaller and slower.

2.10 Surface Motility

Some bacteria move across surfaces using mechanisms other than flagella, such as twitching and gliding motility.

• Twitching motility: Extension and retraction of pili to pull the cell forward.

• Gliding motility: Smooth movement along surfaces, often involving protein tracks and motors.

2.11 Chemotaxis and Other Forms of Taxis

Taxis refers to directed movement in response to environmental stimuli, allowing microbes to optimize their position for survival and growth.

• Chemotaxis: Movement in response to chemicals; involves 'run and tumble' behavior in peritrichously flagellated bacteria like E. coli.

• Phototaxis: Movement in response to light, important for phototrophic organisms.

• Aerotaxis: Movement in response to oxygen concentration.

• Other forms: Osmotaxis (ionic strength), hydrotaxis (water), magnetotaxis (magnetic fields).