Micro: Lecture 1: 1/12

Introduction to Microbiology

Microbes and Their Essential Role on Earth

Microbes are fundamental to life on Earth, playing critical roles in biogeochemical cycles and the development of life. Their ubiquity and diversity have shaped the planet for billions of years.

• Biogeochemical Cycles: Microbes cycle carbon, nitrogen, sulfur, hydrogen, and oxygen, maintaining ecosystem balance.

• Dominance: Microbes have dominated Earth for over 3 billion years, with an estimated $5 \times 10^{31}$ microbial cells present.

• Human Development: Microbes have provided insights into fundamental biological processes, tools for molecular biology, antibiotics, and have influenced health and disease through the microbiome.

• Climate Control: Microbial activity contributes to climate regulation.

Microbiology as an Interdisciplinary Field

Microbiology integrates multiple scientific disciplines to address questions about microbial function and interactions.

• Disciplines Involved: Chemistry, biochemistry, genetics, molecular biology, biophysics, cell biology, bioinformatics, immunology.

• Evolution of the Field: Incorporation of molecular tools has expanded the scope and depth of microbiological research.

Historical Foundations

The Golden Age of Microbiology (1854–1914)

This era marked the acceptance of microbes as living organisms and the discovery of many disease-causing bacteria.

• Microscopy: The invention of the microscope (Leeuwenhoek, 17th century) enabled the study of microorganisms.

• Disease Prevention: Research led to the identification of pathogens and the development of treatments.

• Viruses: Early work on viruses began during this period.

Koch's Postulates and Germ Theory

Robert Koch validated the germ theory of disease and established criteria for identifying causative agents of disease.

1. The microbe is found in all cases of the disease but absent from healthy individuals.

2. The microbe is isolated from the diseased host and grown in pure culture.

3. When the microbe is introduced into a healthy, susceptible host, the same disease occurs.

4. The same strain of microbe is obtained from the newly diseased host.

Discovery of DNA as Genetic Material

Griffith (1928) and Avery (1944) demonstrated that DNA is the transforming material responsible for heredity.

• Transformation Experiments: Showed that genetic traits could be transferred between bacteria via DNA.

The Era of Antibiotics

Alexander Fleming discovered penicillin in 1929, revolutionizing the treatment of bacterial infections.

• Penicillin: Effective against Gram-positive bacteria.

• Nobel Prize: Fleming shared the Nobel Prize for medicine in 1945.

Viruses as Filterable Infective Particles

Viruses were identified as infectious agents that could pass through filters, distinguishing them from bacteria.

• Tobacco Mosaic Virus: Early studies revealed the structure and infectivity of viruses.

• Components: Viruses consist of RNA or DNA surrounded by a protein capsid.

Microbial Ecology and Evolution

Microbial Ecology

Microbes support natural ecosystems and drive Earth's biogeochemical cycles.

• Relatedness and Evolution: Microbes are central to understanding evolutionary processes and the discovery of life's three domains.

Microbiomes in Health and Disease

Microbiomes, the communities of microbes living in and on organisms, are critical for health and disease.

• Human Microbiome: Influences digestion, immunity, and disease susceptibility.

DNA Revolution in Microbiology

Bacterial research has driven advances in molecular biology, including DNA structure, sequencing, and amplification.

• Key Milestones: Structure (1953), sequencing (1977), PCR amplification (1988).

• Applications: Bacterial enzymes and gene regulation models have been extended to animals and plants.

Microbes as Model Organisms

Microbes are ideal model organisms due to their conserved building blocks, genetic properties, and ease of manipulation.

• Metabolism: Conserved across higher forms of life.

• Growth: Rapid growth and high numbers facilitate experimentation.

Microbes' Ongoing Impact

• Medical Progress: Bacterial research accelerates medical advancements.

• Biotechnology: Microbes are used as reagents, in biofuel production, and for soil remediation.

Origins and Classification of Microbes

Early Life and Geological Evidence

Microbes were the earliest life forms, with evidence dating back 3.8 billion years.

• Origin of Life: Confirmed by genetics, geology, and biochemistry.

• Microbial Metabolisms: Shaped Earth's crust and atmosphere.

Geological Evidence Table

Type of Evidence	Advantages	Limitations
Stromatolites	Fossil evidence, layered formations	Some formations may be abiotic
Microfossils	Visible and measurable microstructures	Microscopic rock formations may be abiotic
Isotope Ratios	Highly reproducible, indicate biological activity	Cannot prove absolute biotic origin
Certain organic molecules	Biomarkers suggest specific life forms	Abiogenic synthesis possible
Oxidation states	Indicate metabolic activity	Must be linked to biological processes

Microbial Classification

Microbes are classified based on morphology, metabolism, and genetic relatedness.

• Morphology: Physical differences observed under microscopy.

• Metabolism: Commercial kits and tests assess metabolic capabilities.

• Clinical Significance: Ease of classification is important for medical applications.

Metabolic Classification

Microbes are categorized by their energy and carbon sources.

• Autotrophs: Obtain carbon from CO2

  • Chemoautotrophs (Lithotrophs)

  • Photoautotrophs

• Organotrophs (Heterotrophs): Obtain carbon from organic molecules

  • Chemoheterotrophs

  • Photoheterotrophs

• Culturability: Not all microbes can be cultured in the laboratory.

Molecular Phylogeny and Evolutionary Relationships

Molecular Phylogeny

Modern biology classifies organisms based on evolutionary relationships determined by genetic relatedness.

• Three Domains of Life: Bacteria, Archaea, and Eukarya (Carl Woese, 1977).

Molecular Clocks

Molecular clocks use mutation rates to estimate evolutionary divergence and generate phylogenetic trees.

• Assumptions:

  • Gene function is conserved across organisms.

  • Generation time is consistent.

  • Mutation rate is constant across generations.

Properties of Phylogenetic Markers

• Present in all organisms of interest

• Not transferred between species

• Appropriate sequence conservation

• Large enough to contain historical information

16S Ribosomal RNA as a Marker

16S rRNA is a key phylogenetic marker used in microbial classification.

• Part of the ribosome

• Required for translation

• Present in all living cells

Principles of Phylogenetic Trees

Phylogenetic trees are constructed by aligning homologous regions, minimizing mismatches, and modeling evolutionary divergence.

• Phylogenetic Tree: A model representing evolutionary relationships, not absolute proof.

Current Tree of Life

The root of the tree of life remains undefined due to complexities in evolutionary history.

Critical Thinking in Phylogeny

• Realities such as horizontal gene transfer and variable mutation rates can distort molecular clock validity.

• Phylogenetic trees are models supported by genetic, biochemical, and geological evidence.

Review Questions

Ribosomal RNAs as Phylogenetic Markers

• Found in all living things

• Evolve slowly but not too slowly

• Can be sequenced relatively easily

Models for Origin of First Cells

• Address environment, energy generation, and hereditary material

Molecular Clock Tool

• Equates with time when assumptions are met

• Factors: Ribosomal RNA sequence, horizontal gene transfer, sequence conservation, mutations

Additional info: These notes expand on the provided slides and images, integrating foundational concepts from introductory microbiology, including microbial diversity, classification, and evolutionary biology.