Introduction to Prokaryotic Cells

Overview

Prokaryotic cells are fundamental to the study of microbiology, representing the simplest and most ancient forms of life. This section explores their classification, structure, diversity, and clinical significance.

Bacteria and Archaea: Domains of Prokaryotic Life

Classification and Evolution

• Prokaryotic cells originated approximately 3.8 billion years ago.

• They are classified into two domains: Bacteria and Archaea.

• Eukaryotic cells make up the domain Eukarya.

Example: The evolutionary tree separates Bacteria, Archaea, and Eukarya from a shared ancestor.

Prokaryotic Cell Structure

General Features

• Prokaryotes are unicellular.

• Lack a membrane-bound nucleus and membrane-bound organelles.

Sizes, Shapes, and Arrangements

Importance in Identification

• Understanding prokaryotic morphology aids in pathogen identification.

• Monomorphic bacteria have one shape; Pleomorphic bacteria can take on different forms, enhancing survival and transmission.

Sizes

• Range: 0.2–750 μm in diameter (most are 0.5–2.0 μm).

• Mycoplasma species: among the smallest (>1 μm).

• Thiomargarita magnifica: among the largest (<1 cm).

• Size is limited by surface area-to-volume ratio; larger ratios favor efficient diffusion of nutrients.

Shapes

• Bacilli: rod-shaped

• Cocci: spherical

• Vibrio: comma-shaped

• Stella: star-shaped

• Coccobacilli: ovoid

• Spirochetes: spiral-shaped, corkscrew motion

Arrangements

• Diplococci: paired cocci

• Streptococci: chains of cocci

• Staphylococci: grape-like clusters

• Diplobacilli: paired bacilli

• Streptobacilli: chains of bacilli

• Palisade: clusters of bacilli

Prokaryotic Cell Division

Binary Fission

• Prokaryotes reproduce by binary fission, a form of asexual reproduction producing two identical daughter cells.

• Steps of binary fission:

  1. DNA is copied

  2. Cell grows

  3. Chromosomes move to opposite ends

  4. Septum forms at the midpoint

  5. Septum partitions, resulting in two cells

Cell Boundaries: Plasma Membrane and Cell Wall

Plasma Membrane

• Thin, flexible phospholipid bilayer acting as a selective barrier.

• Proteins (transporters, anchors, receptors, enzymes) can make up half the membrane mass.

• Site for metabolic reactions (e.g., electron transport chain for ATP synthesis).

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

Membrane Fluidity

• Described as a fluid-mosaic model; lipids and proteins move within the layer.

• Fluidity is essential for function and is affected by:

  • Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

  • Fatty acid content: Unsaturated fatty acids increase fluidity; saturated fatty acids decrease it.

Bacterial vs. Archaeal Membranes

• Bacteria: linear fatty acids in bilayers.

• Archaea: long-branched fatty acids; some form monolayers for extreme environments.

Cell Wall Structure

• Provides rigidity and protection.

• Bacteria: peptidoglycan is the main component.

• Archaea: pseudopeptidoglycan is the main component.

Gram Staining and Clinical Relevance

Gram Stain Classification

• Distinguishes bacteria based on cell wall structure:

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

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

Clinical Implications

• Gram-negative bacteria are harder to kill due to their outer membrane, which acts as a selective barrier (protects against lysozyme, drugs, detergents).

• Outer membrane contains porins (protein channels) that allow passage of certain molecules but exclude harmful substances.

• Gram-positive cell walls are more sensitive to agents targeting peptidoglycan but retain moisture and resist mechanical stress due to teichoic acids.

Antibiotic Targets and Resistance

Main Targets

• Antibiotics target cell wall synthesis, protein synthesis, DNA/RNA synthesis, and metabolic pathways.

Resistance Mechanisms

• Bacteria can develop resistance through modifying targets, inactivating drugs, or preventing drug entry.

Acid-Fast Staining

Principle and Clinical Importance

• Detects mycolic acid (waxy lipid) in cell walls of certain bacteria (e.g., Nocardia, Mycobacterium).

• Acid-fast cells stain red/pink; grow slowly due to limited nutrient/gas diffusion.

• Require long, multidrug therapies as drugs do not easily penetrate the waxy wall.

Mycoplasma and L-Forms

Unique Cell Wall Features

• Mycoplasma species lack a cell wall, have sterol-rich membranes, and are pleomorphic.

• L-forms are bacteria that lost their cell wall, contributing to persistent infections and resistance to some stresses.

Transport Across Prokaryotic Membranes

Passive Transport

• Diffusion: Movement from high to low concentration.

• Simple diffusion: Small, noncharged molecules, gases, and lipid-soluble substances.

• Facilitated diffusion: Uses channel or carrier proteins for larger/polar molecules.

Osmosis

• Water moves from low to high solute concentration.

• Isotonic: No net water movement.

• Hypertonic: Cell loses water (plasmolysis).

• Hypotonic: Cell gains water (osmotic lysis if wall is damaged).

Active Transport

• Requires energy (ATP or ion gradients).

• Can move substances against concentration gradients.

• Three main types:

  • Primary active transport: Direct use of ATP.

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

  • Phosphotransferase systems: Group translocation, transferring phosphate to the transported substance (e.g., glucose uptake).