The Prokaryotes: Eubacteria and Archaea – Structure, Classification, and Importance

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The Six Kingdoms of Life

Overview of Biological Classification

The six-kingdom system is a widely accepted method for classifying all living organisms based on cellular organization and modes of nutrition. The kingdoms include Plantae, Animalia, Fungi, Protista, Eubacteria, and Archaebacteria. Eubacteria and Archaebacteria are prokaryotic, while the other four kingdoms are eukaryotic.

Diagram of the six kingdoms of life

Importance of Prokaryotes

Harmful Effects

Pathogenic bacteria cause diseases in humans, animals, and plants. Examples include cholera, leprosy, typhoid fever, strep throat, salmonella poisoning, and tuberculosis.
Bacterial infections can impact agriculture by infecting livestock and crops.
Diseases that harm one species may benefit another by altering ecological balances (e.g., weakening predators can benefit prey populations).

Beneficial Effects

Decomposers: Break down dead organic matter, recycling nutrients in ecosystems.
Nitrogen fixation: Convert atmospheric nitrogen into forms usable by plants, essential for soil fertility.
Photosynthetic bacteria: Major producers of atmospheric oxygen, especially in marine environments (e.g., cyanobacteria).
Reside in animal intestines, producing vitamins such as K and B12 for humans.

Photosynthetic bacteria and cyanobacteria cell structure

Commercial and Medical Uses

Essential in food production: vinegar, butter, cheese, yogurt, and sourdough bread.
Production of antibiotics and medically valuable compounds (e.g., insulin, human growth hormone).
Used in sewage treatment and odor control by digesting organic waste.

Examples of foods produced by bacteria (cheese, yogurt, etc.) Medical use of bacteria for insulin production

Classification of Bacteria

Two Prokaryotic Kingdoms

Eubacteria: True bacteria, found in most environments.
Archaebacteria: Ancient bacteria, often found in extreme environments (e.g., hot springs, salt lakes).

Illustration of the six kingdoms, highlighting Eubacteria and Archaebacteria

Key Features of Eubacteria

General Characteristics

Prokaryotic: Lack a nucleus and membrane-bound organelles.
Unicellular with a rigid cell wall (some form colonies).
Genetic material is a single circular DNA loop (chromosome) in the nucleoid region.
Reproduce asexually by binary fission; some exchange genetic material via conjugation.
Metabolism: Some are autotrophic (make their own food), others are heterotrophic (consume organic material).

Structure of a Typical Bacterial Cell

Cell wall made of peptidoglycan (unique to bacteria).
No membrane-bound organelles.
Some have a capsule for protection against water loss and high temperatures.
May contain plasmids—small loops of DNA carrying extra genes.
Ribosomes scattered in the cytoplasm for protein synthesis.
Movement via flagella; attachment via pili (singular: pilus).

Structure of a typical bacterial cell

Bacterial Shapes and Arrangements

Common Shapes

Cocci (singular: coccus): Spherical, resist drying.
Bacilli (singular: bacillus): Rod-shaped, absorb nutrients efficiently.
Spirilli (singular: spirillum): Spiral-shaped, move easily through fluids.

Bacterial shapes: cocci, bacilli, spirilli

Groupings and Naming Conventions

Diplo-: Arranged in pairs (e.g., diplococcus).
Staphylo-: Arranged in clusters (e.g., staphylococcus).
Strepto-: Arranged in chains (e.g., streptococcus).

Bacterial arrangements: diplo, staphylo, strepto

Gram Staining and Cell Wall Structure

Gram Stain Technique

The Gram stain, developed by Hans Gram in 1884, is a differential staining technique that distinguishes bacteria based on cell wall composition:

Gram-positive bacteria: Thick peptidoglycan layer, stain purple.
Gram-negative bacteria: Thin peptidoglycan layer and an outer lipid membrane, stain pink.

Gram-negative vs. Gram-positive bacteria under microscope Gram-negative bacteria under microscope

Structural Differences

Gram-positive: Thick peptidoglycan, no outer lipid membrane.
Gram-negative: Thin peptidoglycan, outer lipid membrane present.

Diagram of Gram-positive and Gram-negative cell walls

Other Classification Methods: Bacterial Respiration

Oxygen Requirements

Aerobic bacteria: Require oxygen for survival and growth.
Obligate aerobes: Cannot survive without oxygen (e.g., most animals and plants).
Anaerobic bacteria: Do not use oxygen for respiration.
Obligate anaerobes: Cannot survive in the presence of oxygen.
Facultative anaerobes: Can survive with or without oxygen, switching metabolic pathways as needed.

Summary Table: Bacterial Classification and Features

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Outer Lipid Membrane	Absent	Present
Stain Color	Purple	Pink

Key Terms

Prokaryote: An organism lacking a nucleus and membrane-bound organelles.
Peptidoglycan: A polymer forming the cell wall of most bacteria.
Plasmid: Small, circular DNA molecule in bacteria, often carrying beneficial genes.
Binary fission: Asexual reproduction method in prokaryotes.
Conjugation: Transfer of genetic material between bacterial cells.
Pathogen: Disease-causing organism.