Bacteria and Archaea

Introduction

Bacteria and Archaea are two major domains of prokaryotic life. These organisms are among the most abundant and diverse forms of life on Earth, capable of surviving in a wide range of environments, including extreme conditions. Understanding their structure, diversity, and evolutionary adaptations is fundamental to general biology.

Prokaryotes

Definition and Abundance

• Prokaryotes are unicellular organisms that lack a membrane-bound nucleus and organelles.

• The two domains of prokaryotes are Bacteria and Archaea.

• Prokaryotes can live in diverse environments: water, land, with or without oxygen, and in extreme conditions (e.g., high salinity, temperature, or acidity).

• They are the most abundant organisms on Earth. For example, a handful of soil contains more bacteria than the total number of humans who have ever lived.

• Human bodies contain more bacterial cells than human cells, with bacteria outnumbering human cells by approximately 90%.

Structure of a Prokaryotic Cell

Key Features

• Prokaryotes are single-celled organisms.

• They lack membrane-bound organelles (such as mitochondria or endoplasmic reticulum).

• Their genetic material is located in a region called the nucleoid, not enclosed by a membrane.

• They possess a cell wall to maintain structure and protect against environmental stress.

• Some have pili, which are structures that help cells attach to surfaces or each other and can facilitate DNA transfer.

• Flagella are present in many prokaryotes, enabling movement (motility).

Special Adaptations of Prokaryotes

Adaptations for Survival and Evolution

• Small Size: Prokaryotes are much smaller than eukaryotic cells (typically 0.5–5 μm in diameter), allowing for rapid nutrient uptake and growth.

• Rapid Reproduction: They reproduce quickly by binary fission, sometimes doubling in as little as 20 minutes, leading to large populations in short periods.

• Genetic Diversity: High mutation rates and genetic recombination contribute to their evolutionary adaptability.

• Diverse Adaptations: Prokaryotes can survive in extreme environments (e.g., high temperature, salinity, acidity), especially Archaea.

Evolution and Shapes of Bacteria

Origins and Morphology

• Prokaryotes were the first organisms to inhabit Earth, appearing over 3.5 billion years ago.

• They exhibit a variety of shapes, including:

  • Spherical (cocci): e.g., Streptococcus pyogenes (causes strep throat)

  • Rod-shaped (bacilli): e.g., Escherichia coli (E. coli), Lactobacillus acidophilus (a probiotic bacterium)

  • Spiral-shaped (spirilla/spirochetes): e.g., Borrelia burgdorferi (causes Lyme disease)

• Some bacteria are beneficial (probiotics), while others are pathogenic.

Cell Surface Structures

Cell Wall and Related Features

• Cell Wall: Provides structural support and protection. In bacteria, the cell wall contains peptidoglycan, a polymer of sugars and amino acids.

• Gram Staining: A method to classify bacteria based on cell wall composition:

  • Gram-positive: Thick peptidoglycan layer; stains purple.

  • Gram-negative: Thin peptidoglycan layer and an outer membrane; stains pink/red. More resistant to antibiotics due to the outer membrane.

• Capsule: A sticky, protective layer outside the cell wall that helps bacteria adhere to surfaces and evade the immune system.

• Endospores: Dormant, tough structures formed by some bacteria to survive harsh conditions. Can remain viable for centuries and are highly resistant to heat and chemicals.

• Pili (Fimbriae): Hair-like structures that help bacteria attach to surfaces or each other. Specialized pili (sex pili) are involved in DNA transfer during conjugation.

• Flagella: Tail-like structures used for movement (motility). Bacterial and archaeal flagella differ in structure and mechanism.

Internal Organization and DNA

Genetic Material and Ribosomes

• Prokaryotes lack a true nucleus; their DNA is found in the nucleoid region.

• Most have a single, circular chromosome; some have additional small DNA molecules called plasmids.

• Prokaryotic ribosomes are 70S (smaller than eukaryotic 80S ribosomes).

Reproduction and Genetic Diversity

Binary Fission and Genetic Variation

• Binary Fission: Asexual reproduction where one cell divides into two identical cells.

• Rapid reproduction leads to large populations and increased opportunities for mutation.

• Genetic Diversity: Achieved through mutation and genetic recombination.

Mechanisms of Genetic Recombination

• Transformation: Uptake of foreign DNA from the environment by a bacterial cell, resulting in genetic changes.

• Transduction: Transfer of bacterial genes by bacteriophages (viruses that infect bacteria).

• Conjugation: Direct transfer of DNA between two bacterial cells joined by a pilus. Often involves plasmids (e.g., F factor, R plasmids).

Conjugation Details

• F factor (fertility factor): A plasmid that enables a bacterium to form a pilus and transfer DNA to another cell.

• R plasmids: Plasmids carrying antibiotic resistance genes, which can spread rapidly among bacteria.

Archaea: Unique Adaptations

Extremophiles and Methanogens

• Many archaea are extremophiles, thriving in extreme environments such as high salinity (halophiles), high temperature (thermophiles), or acidic conditions.

• Methanogens: Archaea that produce methane as a metabolic byproduct. They are strict anaerobes and are found in environments like the guts of cows and termites.

Comparing Bacteria and Archaea

Key Differences

Probiotics and Human Health

Role of Beneficial Bacteria

• Probiotics: Live bacteria that are beneficial to human health, especially in the digestive system (e.g., Lactobacillus acidophilus).

• Probiotics help maintain gut health, prevent "leaky gut," and support the immune system.

• Dietary fiber (cellulose) supports the growth of beneficial bacteria in the gut.

Summary Table: Mechanisms of Genetic Recombination in Prokaryotes

Key Equations and Concepts

• Binary Fission Doubling Time: If a bacterial population doubles every t minutes, the number of cells after n doublings is:

• Where is the final number of cells, is the initial number, and is the number of generations.

Additional info: Some details, such as the structure of the cell wall, mechanisms of antibiotic resistance, and the role of probiotics, have been expanded for academic completeness and clarity.