Microbial Cell Structure and Function

The Cell Envelope

The cell envelope is a complex, multi-layered structure that protects microbial cells and mediates interactions with their environment. It consists of the cytoplasmic membrane, cell wall, and, in some cases, an outer membrane. The composition and function of these components differ between Bacteria and Archaea.

• Cytoplasmic Membrane: The cytoplasmic membrane is a selectively permeable barrier composed primarily of phospholipids and proteins. In Bacteria, it contains ester-linked fatty acids, while in Archaea, it features ether-linked lipids and can form monolayers for increased stability.

• Differences Between Bacterial and Archaeal Membranes:

  • Bacterial membranes: Contain phospholipids with fatty acids linked by ester bonds to glycerol.

  • Archaeal membranes: Contain isoprenoid chains linked by ether bonds to glycerol; some Archaea have monolayer membranes for extreme environments.

• Cell Wall: The cell wall provides structural support and shape. In Bacteria, it is mainly composed of peptidoglycan, a polymer of sugars and amino acids. In Archaea, cell walls lack peptidoglycan and may contain pseudopeptidoglycan or other polymers.

• Outer Membrane (LPS): In Gram-negative Bacteria, the outer membrane contains lipopolysaccharide (LPS), which acts as a barrier to toxins and antibiotics. LPS consists of lipid A, a core polysaccharide, and an O-specific polysaccharide.

Example: Escherichia coli is a Gram-negative bacterium with a cell envelope consisting of a cytoplasmic membrane, a thin peptidoglycan cell wall, and an outer membrane containing LPS.

Cell Surface Structures and Inclusions

Microbial cells possess various surface structures and internal inclusions that aid in survival, attachment, and storage. These features are often adaptations to environmental challenges.

• Cell Surface Structures:

  • Polysaccharide Layers: Capsules and slime layers protect against desiccation and phagocytosis, and aid in attachment.

  • Fimbriae: Short, hair-like appendages that facilitate attachment to surfaces.

  • Pili: Longer structures involved in motility and genetic exchange (conjugation).

• Inclusions: Intracellular storage bodies for nutrients and energy reserves.

  • Polyhydroxyalkanoates (PHA): Carbon storage.

  • Glycogen: Energy storage.

  • Volutin (polyphosphate granules): Phosphate storage.

  • Sulfur granules: Sulfur storage.

Example: Many aquatic bacteria possess gas vesicles, which are protein-bound inclusions that provide buoyancy.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria in response to adverse conditions. They ensure survival until favorable conditions return.

• Role of Endospore Formation: Endospores allow bacteria to withstand extreme heat, desiccation, radiation, and chemicals.

• Endospore Structures:

  • Core: Contains DNA, ribosomes, and dipicolinic acid for stability.

  • Cortex: Thick layer of peptidoglycan.

  • Spore Coat: Protein layers providing chemical and enzymatic resistance.

  • Exosporium: Outermost layer, sometimes present.

• Sporulation Cycle: Endospore formation involves a complex developmental process, including DNA replication, asymmetric cell division, engulfment, and maturation.

• Comparison to Vegetative Cells: Vegetative cells are metabolically active and sensitive to environmental stress, while endospores are dormant and highly resistant.

Example: Bacillus subtilis forms endospores when nutrients are depleted.

Table: Comparison of Bacterial and Archaeal Cell Envelope Components

Additional info: Archaeal cell walls are highly variable and may include S-layers (proteinaceous surface layers) instead of peptidoglycan.