Structure and Features of Prokaryotes: Bacteria and Archaea

Domain Bacteria

Overview of Bacteria

Bacteria are one of the two main types of prokaryotic organisms. They are single-celled microorganisms lacking a nucleus and other membrane-bound organelles. Bacteria are found in diverse environments and play essential roles in ecosystems, industry, and human health.

• Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.

• Bacteria: One of the two domains of prokaryotes (the other is Archaea).

Bacterial Cell Wall

Structure and Composition

The bacterial cell wall is a rigid structure that provides shape and protection to the cell. It is primarily composed of peptidoglycan, a complex polymer of sugars and amino acids. The structure and thickness of the cell wall are key features used in bacterial classification.

• Peptidoglycan: A mesh-like polymer consisting of long chains of alternating sugars (N-acetylglucosamine, NAG, and N-acetylmuramic acid, NAM) cross-linked by short peptide chains.

• Function: Maintains cell shape, prevents osmotic lysis, and is involved in cell division.

• Classification: The structural form of the cell wall is a major criterion for classifying bacteria (e.g., Gram-positive vs. Gram-negative).

Example: The Gram stain technique differentiates bacteria based on cell wall structure.

Gram-Positive and Gram-Negative Bacteria

Gram-Positive Bacteria

Gram-positive bacteria have a thick cell wall composed of multiple layers of peptidoglycan. This structure retains the crystal violet stain used in the Gram staining procedure, resulting in a purple appearance under the microscope.

• Thick peptidoglycan layer: Provides rigidity and protection.

• Teichoic acids: Polymers embedded in the cell wall, contributing to cell wall maintenance and ion transport.

• No outer membrane: Unlike Gram-negative bacteria, Gram-positive bacteria lack an external lipid membrane.

Gram-Negative Bacteria

Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane containing lipopolysaccharide (LPS). During Gram staining, these bacteria do not retain the crystal violet stain and appear pink after counterstaining.

• Thin peptidoglycan layer: Only a few layers thick, located in the periplasmic space.

• Outer membrane: Contains lipopolysaccharide (LPS), which contributes to the structural integrity and protects against certain antibiotics.

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

Comparison Table: Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick (many layers)	Thin (few layers)
Outer Membrane	Absent	Present (contains LPS)
Teichoic Acids	Present	Absent
Gram Stain Result	Purple	Pink/Red

Bacterial Cell Shapes

Common Morphologies

Bacteria exhibit a variety of cell shapes, which are important for identification and classification.

• Coccus (plural: cocci): Spherical-shaped bacteria (e.g., Staphylococcus).

• Bacillus (plural: bacilli): Rod-shaped bacteria (e.g., Bacillus subtilis).

• Spirilla (singular: spirillum): Spiral or helical-shaped bacteria (e.g., Spirillum volutans).

Example: Streptococcus (chains of cocci), Escherichia coli (bacillus), and Spirillum (spirilla).

Bacterial Appendages

Types and Functions

Bacteria possess various surface structures called appendages that aid in movement, attachment, and genetic exchange.

• Flagellum (plural: flagella): Long, whip-like structures anchored in the cell wall and membrane, responsible for motility.

• Fimbria (plural: fimbriae): Short, bristle-like structures used for attachment to surfaces and other cells; may also be involved in limited movement (retractable).

• Pilus (plural: pili): Longer, protein tubes used for the transfer of DNA between bacterial cells (conjugation).

Example: Escherichia coli uses fimbriae to adhere to host tissues and flagella for swimming.

Other Features of Some Bacteria

Glycocalyx (Capsule)

The glycocalyx is an external layer of polysaccharide that surrounds some bacterial cells, providing protection and aiding in adherence to surfaces.

• Capsule: A well-organized, dense glycocalyx that protects against phagocytosis and desiccation.

• Slime layer: A loosely organized glycocalyx that aids in adherence and biofilm formation.

Endospore Formation

Some bacteria can form endospores, which are dormant, highly resistant structures that allow survival in harsh conditions. Endospores are not reproductive structures but serve as a survival mechanism.

• Endospore: A tough, non-reproductive structure formed by genera such as Bacillus and Clostridium.

• Function: Resistance to heat, desiccation, chemicals, and radiation.

Domain Archaea

Overview and Unique Features

Archaea are a distinct domain of prokaryotes, differing from bacteria in several fundamental ways. They are often found in extreme environments and have unique molecular and structural characteristics.

• Ribosomal RNA and proteins: Distinct from those of bacteria.

• Cytoplasmic membrane lipids: Composed of unique ether-linked lipids.

• Cell wall: Lacks peptidoglycan; may contain pseudopeptidoglycan or other polymers.

• Unusual shapes: Some archaea have unique cell morphologies not seen in bacteria.

• Unique appendages: Structures such as the "hamus" are found only in archaea.

• Ecology: Many archaea are extremophiles (e.g., thermophiles, halophiles); none are known to be pathogenic.

Biofilms

Formation and Significance

Biofilms are complex communities of microorganisms that adhere to surfaces and are embedded in a self-produced extracellular matrix. Biofilms can include bacteria, archaea, and other microorganisms.

• Matrix: Composed of polysaccharides, proteins, and DNA; provides structural support and protection.

• Adherence: Biofilms adhere firmly to surfaces and are resistant to removal.

• Protection: Biofilms protect resident cells from detergents, antibiotics, antibodies, and phagocytes.

• Metabolic diversity: Cells within a biofilm may exhibit different metabolic states.

Example: Dental plaque is a biofilm on teeth; biofilms also form on medical devices such as catheters and pacemakers, contributing to persistent infections.

Table: Examples and Significance of Biofilms

Location	Microorganisms Involved	Significance
Dental plaque	Streptococcus, Staphylococcus, Escherichia	Tooth decay, gum disease
Medical devices (e.g., catheters)	Various bacteria	Source of persistent infections

Additional info: Pelagic (free-living) cells can leave the biofilm to colonize new surfaces, contributing to the spread of infection.