Introduction to Prokaryotic Cells: Structure, Function, and Clinical Relevance

Prokaryotic Cell Basics

Overview of Prokaryotic Domains

Prokaryotic cells are fundamental to the study of microbiology and are classified into two main domains: Bacteria and Archaea. These domains share certain characteristics but also exhibit key differences.

• Bacteria and Archaea are both unicellular organisms lacking a membrane-bound nucleus and membrane-bound organelles.

• Difference: Bacteria have cell walls containing peptidoglycan, while Archaea have cell walls composed of pseudopeptidoglycan or other unique polymers.

• Similarity: Both domains reproduce asexually, primarily through binary fission.

• Prokaryotic cells are generally small, typically ranging from 0.2–2.0 μm in diameter, which allows efficient nutrient uptake due to a high surface area-to-volume ratio.

Shapes and Arrangements of Prokaryotes

Prokaryotic cells display a variety of shapes and arrangements, which are important for identification and classification.

• Bacilli (rod-shaped)

• Cocci (spherical)

• Vibrio (comma-shaped)

• Stella (star-shaped)

• Coccobacilli (ovoid)

• Spirochetes (spiral-shaped, corkscrew motion)

Common arrangements include:

• Diplococci: pairs of cocci

• Streptococci: chains of cocci

• Staphylococci: grape-like clusters

• Diplobacilli: pairs of bacilli

• Streptobacilli: chains of bacilli

• Palisade: clusters of bacilli

Pleomorphism

Pleomorphic bacteria can alter their shape or size in response to environmental conditions. This property can enhance their ability to evade the immune system and adapt to various environments, potentially increasing their pathogenicity.

Binary Fission

Prokaryotic cells reproduce by binary fission, a simple form of asexual reproduction.

1. DNA is replicated.

2. The cell grows and elongates.

3. Chromosomes are segregated to opposite ends.

4. A septum forms at the midpoint.

5. The cell divides into two genetically identical daughter cells.

Prokaryotic Cell Structure

Plasma Membrane

The plasma membrane is a thin, flexible phospholipid bilayer that acts as a selective barrier, controlling the movement of substances in and out of the cell.

• Contains proteins that function as transporters, anchors, receptors, and enzymes.

• Exhibits selective permeability: small, noncharged molecules and gases diffuse freely, while ions and large polar molecules require transport proteins.

• Membrane fluidity is influenced by temperature and fatty acid composition (unsaturated fatty acids increase fluidity; saturated fatty acids decrease it).

Bacterial membranes contain linear fatty acids, while archaeal membranes have long-branched fatty acids and may form lipid monolayers for stability in extreme environments.

Cell Wall

The cell wall provides structural support and protection.

• Bacteria: Cell walls contain peptidoglycan.

• Archaea: Cell walls contain pseudopeptidoglycan or other unique polymers.

Gram Staining and Cell Wall Types

Gram staining differentiates bacteria based on cell wall structure:

• Gram-positive: Thick peptidoglycan layer, no outer membrane, stains purple.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide, stains red/pink.

Gram-negative bacteria are generally more resistant to chemicals and antibiotics due to their outer membrane, which contains porins that exclude harmful substances.

Gram-positive cell walls retain moisture, provide mechanical protection, and contain teichoic acids for stability and ion transport.

Acid-Fast Bacteria

Acid-fast staining detects bacteria with waxy cell walls containing mycolic acid (e.g., Mycobacterium, Nocardia). Acid-fast cells appear red/pink after staining and are resistant to many drugs, requiring multidrug therapies.

Mycoplasma and L-Forms

• Mycoplasma: Lack a cell wall, have sterol-enriched membranes, and are pleomorphic.

• L-forms: Bacteria that have lost their cell wall, often resistant to antibiotics targeting cell wall synthesis.

Transport Mechanisms in Prokaryotes

Passive Transport

• Simple diffusion: Movement of small, noncharged molecules from high to low concentration.

• Facilitated diffusion: Movement along the concentration gradient via membrane proteins (channels or carriers).

• Osmosis: Diffusion of water across a selectively permeable membrane.

Effects of different solutions:

• Hypertonic: Cell loses water, plasmolysis may occur.

• Hypotonic: Cell gains water, risk of osmotic lysis if the wall is damaged.

• Isotonic: No net water movement.

Active Transport

• Primary active transport: Uses ATP to move substances against their concentration gradient.

• Secondary active transport: Uses ion gradients (symport and antiport mechanisms).

• Phosphotransferase system: Group translocation where a phosphate group is transferred to the transported substance.

Extracellular Structures

Flagella

Flagella are filamentous structures used for motility, composed of the protein flagellin. They function like rotary propellers and are anchored differently in Gram-positive (two rings) and Gram-negative (four rings) bacteria.

• Monotrichous: Single flagellum

• Lophotrichous: Tuft at one pole

• Amphitrichous: Flagella at both poles

• Peritrichous: Flagella all over the surface

Movement types include chemotaxis (chemical stimuli), phototaxis (light), and aerotaxis (oxygen).

Periplasmic flagella (axial filaments) are located between the plasma membrane and cell wall, enabling spirochetes to move in a corkscrew motion.

Fimbriae and Pili

• Fimbriae: Short, bristle-like structures for adhesion and biofilm formation, common in Gram-negative bacteria.

• Pili: Longer, less numerous structures involved in adhesion, movement, and gene transfer (conjugation).

Glycocalyx

The glycocalyx is a sticky, carbohydrate-rich layer that aids in adhesion, protection from desiccation, and resistance to antibiotics and disinfectants.

• Slime layer: Loosely organized and unstructured.

• Capsule: Well-organized and tightly associated with the cell.

Intracellular Structures

Nucleoid

The nucleoid is the region where the prokaryotic chromosome (usually a single, circular DNA molecule) is located.

Ribosomes

Prokaryotic ribosomes (70S) are composed of a 50S large subunit and a 30S small subunit. They are responsible for protein synthesis by linking amino acids.

Cytoskeleton

The prokaryotic cytoskeleton consists of protein filaments that provide structural support and shape to the cell.

Inclusion Bodies

Inclusion bodies are storage sites for nutrients and other substances. Examples include:

• Carboxysomes: Contain carbon-fixing enzymes.

• Magnetosomes: Contain magnetic iron for orientation in magnetic fields.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (notably Bacillus and Clostridium species) to survive harsh conditions.

• Resistant to heat, desiccation, radiation, and chemicals.

• Can persist for long periods and germinate when conditions improve.

Sporulation is the process of endospore formation, involving DNA replication, packaging, and the development of protective layers before release.

Clinical Relevance: Case Study Example

A clinical case involving Streptococcus pyogenes (Gram-positive, nonmotile, encapsulated) demonstrates the importance of understanding prokaryotic cell structure for diagnosis and treatment. Key diagnostic features include cell shape (cocci), arrangement (chains), Gram status, and the absence of endospores or acid-fast properties.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (20–80 nm)	Thin (2–8 nm)
Outer Membrane	Absent	Present (with LPS)
Teichoic Acids	Present	Absent
Stain Color	Purple	Red/Pink
Antibiotic Sensitivity	More sensitive to cell wall-targeting drugs	More resistant due to outer membrane

Additional info: Understanding these structural differences is crucial for selecting appropriate antibiotics and interpreting laboratory diagnostic tests.