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Chapter 27: Bacteria and Archaea

General Overview

Instructor: Dr. Adam Hrincevich

Learning Objectives

• Describe the structure of the tree of all life and its significance in understanding evolutionary relationships among organisms.

• Explain structural and functional adaptations of prokaryotes that allow them to thrive in diverse environments.

• Identify sources of genetic diversity in prokaryotes, focusing on mechanisms of genetic exchange.

• Provide examples of nutritional and metabolic adaptations that enable prokaryotes to exploit various ecological niches.

• Identify major phylogenetic groups of prokaryotes and outline their characteristics.

• Describe the ecological roles that prokaryotes play in different environments, including their interactions within ecosystems.

• Give examples of both beneficial and harmful effects of prokaryotes on humans, emphasizing their dual nature.

Suggestion:

To enhance understanding, complete the crossword puzzle related to key terminology in this chapter to reinforce essential concepts.

Concept 27.1: Structural and Functional Adaptations of Prokaryotes

• Prokaryotes were the first organisms on Earth, emerging approximately 3.5 billion years ago, and remain the most abundant form of life, colonizing a vast range of habitats.

• They belong to two distinct domains: Bacteria and Archaea, which differ in their genetic, biochemical, and physiological characteristics.

• Prokaryotes often excel in extreme environments (such as acidic hot springs, deep-sea vents, and highly saline lakes), leading to a greater understanding of life’s adaptability.

• Most prokaryotes are unicellular, though some can form complex multicellular associations, exhibiting various forms of colony structures.

• The average size of prokaryotic cells is between 0.5–5 µm, indicating they are significantly smaller than eukaryotic cells, which typically range from 10-100 µm.

Common Shapes of Prokaryotic Cells

• Cocci (spherical): ~1 µm in diameter; can exist as single cells or in arrangements like chains (streptococci) or clusters (staphylococci).

• Bacilli (rod-shaped): ~3 µm in length; may also form chains (e.g., Bacillus cereus).

• Spirilli (spiral-shaped): such as Treponema, which causes syphilis; can be rigid or flexible.

Cell Surface Structures

• The cell wall is crucial for maintaining cell shape, providing structural support, and preventing lysis under hypotonic conditions.

Bacterial Cell Walls:

• Composed mainly of peptidoglycan, which consists of sugar polymers linked by peptides, providing rigidity and protecting against environmental stressors.

Archaea:

• Their cell walls are composed of different polysaccharides and proteins, which lack peptidoglycan, showcasing the structural divergence from bacteria.

Gram Staining Classifications

1. Gram-positive:

   • Characterized by a simple cell wall with a large amount of peptidoglycan, which retains the crystal violet stain during Gram staining, thus appearing purple under a microscope.

   • Antibiotics effective against Gram-positive bacteria include Erythromycin and Penicillin, which target the peptidoglycan synthesis.

2. Gram-negative:

   • Have a more complex cell wall, with a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides (LPS), which causes them to stain pink in Gram staining.

   • Antibiotics effective against Gram-negative bacteria include Ciprofloxacin and Gentamicin, often necessitating different approaches compared to Gram-positive counterparts.

Antibiotics and Bacterial Susceptibility

• Antibiotics primarily target peptidoglycan, compromising bacterial cell walls, particularly effective against Gram-positive bacteria due to their thicker walls.

• Certain prokaryotes possess capsules that enhance adherence to surfaces and provide a protective barrier against immune system attacks, contributing to pathogenicity in some strains.

Concept 27.2: Genetic Diversity in Prokaryotes

Rapid Reproduction and Mutation

• Prokaryotes reproduce through binary fission, allowing for exponential growth under optimal conditions (as quickly as every 1-3 hours).

• While mutation rates are low, they can be accelerated by environmental factors or by large population sizes fostering rapid adaptation.

Genetic Recombination

• Horizontal Gene Transfer is a key process in prokaryotic genetic diversity that includes the following methods:

  • Transformation: Uptake and incorporation of foreign DNA from the environment into a prokaryotic genome.

  • Transduction: Transfer of genetic material between prokaryotes via bacteriophages (viruses that infect bacteria).

  • Conjugation: Direct transfer of DNA between prokaryotic cells via a specialized structure called a pilus, facilitating the spread of beneficial traits such as antibiotic resistance.

R Plasmids and Antibiotic Resistance

• R Plasmids are circular DNA molecules that confer antibiotic resistance, allowing for survival against antibiotic treatments.

• Natural selection plays a significant role in increasing the prevalence of resistant strains in populations exposed to antibiotics, leading to the concerning global issue of antibiotic resistance.

Concept 27.3: Nutritional and Metabolic Adaptations

Categories of Prokaryotes by Energy and Carbon Sources

• Photoautotrophs: Utilize light energy for photosynthesis, with CO2 serving as a carbon source (e.g., cyanobacteria are crucial in oxygen production).

• Chemoautotrophs: Depend on inorganic substances as energy sources, exemplified by organisms such as Sulfolobus in extreme environments.

• Photoheterotrophs: Use light energy to metabolize organic compounds for carbon (e.g., Rhodobacter).

• Chemoheterotrophs: Acquire both energy and carbon from organic compounds (e.g., Clostridium species, some of which can produce toxins).

Metabolism of Oxygen and Nitrogen

• Obligate aerobes: Require oxygen for survival, using it for aerobic respiration.

• Obligate anaerobes: Poisoned by oxygen, utilizing fermentation or anaerobic respiration.

• Facultative anaerobes: Can switch between aerobic and anaerobic processes depending on available oxygen.

• Certain prokaryotes possess nitrogen fixation capabilities, converting atmospheric nitrogen (N2) into ammonia (NH3), a crucial process for nutrient cycling in ecosystems.

Metabolic Cooperation

• Cooperation among prokaryotic species enables the utilization of resources inaccessible to single cells; for example, the cyanobacterium Anabaena consists of photosynthetic cells and specialized nitrogen-fixing cells that exchange metabolic byproducts, exemplifying metabolic mutualism.

Concept 27.4: Diversity of Prokaryotes

• Prokaryotes have continuously evolved over billions of years, leading to their adaptation in diverse habitats, including extreme conditions.

• Proteobacteria: A major group displaying diverse metabolic capabilities that includes subgroups such as:

  • Alpha Proteobacteria: (e.g., Rhizobium), important in nitrogen fixation.

  • Beta Proteobacteria: (e.g., Nitrosomonas), significant in nitrogen cycling.

  • Gamma Proteobacteria: Includes Escherichia coli, a model organism in microbiological research.

  • Delta Proteobacteria: Comprising myxobacteria, known for their complex social behavior.

  • Epsilon Proteobacteria: Includes pathogens like Campylobacter, which is associated with gastrointestinal disease.

Unique Groups of Prokaryotes

• Chlamydias: Obligate parasites, such as Chlamydia trachomatis, causing sexually transmitted infections.

• Spirochetes: Morphologically unique bacteria (e.g., Treponema pallidum, responsible for syphilis).

• Cyanobacteria: Essential for aquatic ecosystems and thought to be ancestors of the chloroplasts in plants due to their photosynthetic abilities.

• Gram-positive Bacteria: Includes important pathogens such as Bacillus anthracis (causes anthrax) and Clostridium botulinum (produces a potent neurotoxin).

Archaea: Unique Features

• Archaeal organisms share characteristics with both Bacteria and Eukarya, reflecting their ancient evolutionary lineage. They often inhabit extreme environments, earning the title of extremophiles.

• Methanogens are a subgroup of Archaea that produce methane as a byproduct of their metabolic process, typically found in anaerobic environments like marshes and the guts of ruminants.

Concept 27.5: Ecological Roles

• Prokaryotes are indispensable for biosphere health; their absence would severely disrupt ecosystems, leading to the collapse of many food webs.

• They play critical roles in nutrient cycling, breaking down organic matter, and recycling elements such as carbon, nitrogen, sulfur, and phosphorus, essential for life.

Symbiotic Relationships

• Mutualism: Both organisms benefit from the interaction (e.g., gut microbiota aiding human digestion).

• Commensalism: One organism benefits while the other remains unaffected (e.g., skin bacteria).

• Parasitism: One organism benefits at the expense of the other (e.g., pathogenic bacteria causing disease).

Concept 27.6: Prokaryotes and Human Interaction

• While some prokaryotes are pathogenic, causing diseases such as tuberculosis and cholera, many play vital roles in human health and are used in biotechnology.

• Pathogens are responsible for a wide range of human diseases; transmission can occur through various means such as direct contact, contaminated food and water, and insect vectors (e.g., Lyme disease).

Mechanisms of Pathogenicity

• Pathogenic prokaryotes can produce exotoxins (secreted proteins that can damage host tissues) and endotoxins (component of the Gram-negative bacterial cell wall that can trigger severe immune responses) to establish infection and cause disease.

Applications of Prokaryotes

• The importance of prokaryotes extends to research and technology, including applications in biotechnology and bioremediation.

• E. coli is a widely utilized model organism in genetic research, gene cloning, CRISPR technology, and the production of biofuels and biopolymers, highlighting their utility in scientific advancements and industrial applications.