Introduction to Microbiology

Course Overview

This course provides a comprehensive survey of the biology of microorganisms, with an emphasis on bacteria. Microorganisms are essential for human life, and only a minority cause diseases. The course covers the structure, function, genetics, evolution, diversity, ecology, and applications of microbes.

• Microorganisms are life forms too small to be seen by the human eye.

• They are the oldest form of life and inhabit every environment that supports life.

• Microbes differ from plants and animals (macroorganisms) in their cellular structure and function.

• The cell is the fundamental unit of life; viruses are not cells.

The Microbial World

Microbial Communities and Diversity

Microorganisms often live in communities and display remarkable diversity in form and function. They are found in every environment, including extreme habitats unsuitable for most other organisms.

• Microbial biomass far exceeds human biomass.

• Microbes play critical roles in biogeochemical cycles (carbon, nitrogen, phosphorus).

• They produce oxygen, clean up waste, and are essential for life on Earth.

Winogradsky Column: Microbial Ecology in Action

The Winogradsky column is a classic tool for studying microbial communities and their metabolic diversity. It demonstrates how different microbes occupy distinct ecological niches based on their metabolic requirements.

• Aerobic zone: Cyanobacteria and algae dominate.

• Microaerophilic zone: Nonsulfur photosynthetic bacteria (e.g., Rhodomicrobium).

• Anaerobic zone: Purple and green photosynthetic bacteria (e.g., Chromatium, Chlorobium).

• Microbial stratification is driven by gradients of oxygen and hydrogen sulfide.

Structure and Activities of Microbial Cells

Prokaryotic vs. Eukaryotic Cells

Microbial cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Bacteria and Archaea; lack membrane-bound organelles and nucleus; usually have a single, circular chromosome; reproduce asexually.

• Eukaryotes: Plants, animals, algae, protozoa, fungi; possess membrane-bound organelles and nucleus; chromosomes are linear and enclosed; often reproduce sexually.

• Prokaryotic cells are generally smaller and simpler than eukaryotic cells.

Properties of Microbial Cells

All cells share certain fundamental properties, while some possess specialized functions.

• Metabolism: Cells take up nutrients, transform them, and expel wastes.

• Genetic functions: Replication, transcription, translation.

• Catalytic functions: Energy generation, biosynthesis.

• Growth: Conversion of nutrients into new cell material.

• Differentiation: Formation of new cell structures (e.g., spores).

• Communication: Interaction via chemical messengers.

• Genetic exchange: Exchange of genes by various mechanisms.

• Motility: Self-propulsion (e.g., flagella).

• Evolution: Cells evolve to display new properties; phylogenetic trees capture evolutionary relationships.

Evolutionary Relationships and the Tree of Life

Three Domains of Life

Microbial cells are found in all three domains of life: Bacteria, Archaea, and Eukarya. The phylogenetic tree of life, based on ribosomal RNA gene sequences, reveals evolutionary relationships among organisms.

• Bacteria: Includes most prokaryotes.

• Archaea: Prokaryotes distinct from bacteria, often found in extreme environments.

• Eukarya: Includes plants, animals, fungi, algae, and protozoa.

Shared Features of All Life

Universal Traits

All living organisms share fundamental traits, reflecting their common evolutionary origin.

• DNA: Double-stranded, composed of nucleotides, uses 3-base codons, replicates by conserved mechanisms.

• RNA: Includes tRNA, rRNA, mRNA; transcription mechanisms are conserved.

• Translation: Ribosomes, amino acids, anticodons.

• Cell membranes: Bi-layer lipid structure.

• Metabolic pathways: Catabolic and anabolic mechanisms, fermentation, energy storage (ATP), electron carriers (NADH), oxidative phosphorylation, substrate-level phosphorylation.

• Signal transduction: Proton gradients drive ATP synthesis.

Why is so much shared? Many functions were conserved through evolutionary time as more complex organisms evolved from simpler ones.

Microbial Evolution and the History of Life

Timeline of Life on Earth

Earth is approximately 4.6 billion years old. The first cells appeared between 3.8 and 4.3 billion years ago, and life was exclusively microbial until about 1 billion years ago. The atmosphere was anoxic until ~2.6 billion years ago, and plants and animals appeared around 0.5 billion years ago.

• Evolution: The process of genetic change over time resulting in new varieties and species.

The Impact of Microorganisms on Human Society

Microorganisms as Disease Agents

While most microorganisms are beneficial, some cause infectious diseases. The incidence of infectious diseases has declined in the US, but outbreaks still occur globally.

• Examples: Ebola outbreaks in Africa, bubonic plague cases in the US.

• Microbes can cause symptoms such as internal bleeding (Ebola) or severe tissue damage (plague).

Microorganisms and Water Quality

Microbes play a role in water quality, both positively and negatively. Toxic blooms of algae and cyanobacteria are linked to fertilizer overuse and can impact drinking water safety.

• Cyanobacteria: Oxygenated the planet but can cause trouble with drinking water.

• Algal blooms: Occur in lakes and reservoirs, often due to nutrient pollution.

Microorganisms in Agriculture, Food, Energy, and Environment

Microbial Roles in Agriculture

Microorganisms cycle nutrients in agriculture, contributing to plant growth and animal nutrition. They can have both positive and negative impacts.

• Positive impacts: Nitrogen-fixing bacteria, nutrient regeneration, waste removal, cellulose-degrading microbes in the rumen, gut microbiome for digestion.

• Negative impacts: Diseases in plants and animals.

Microorganisms and Food

Microbes are essential in the production of fermented foods and can also cause food spoilage.

• Fermentation: Microbial processes produce propionic acid, acetic acid, lactic acid, ethanol, and CO2.

• Food spoilage: Caused by microbial growth on food products.

Microorganisms, Energy, and the Environment

Microbes are involved in biofuel production, soil bioremediation, and wastewater treatment. They are critical to natural remediation processes, including the breakdown of organic pollutants and microplastics.

• Biofuels: Methane, ethanol, hydrogen produced by microbial metabolism.

• Bioremediation: Microbial processes clean up oil spills and other pollutants.

• Wastewater treatment: Microbes degrade organic matter in treatment plants.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Key Equations in Microbial Metabolism

Microbial metabolism involves energy transformations and biosynthetic reactions. Two key equations:

• ATP synthesis via oxidative phosphorylation:

• Fermentation (example: glucose to lactic acid):

Conclusion

Microbiology is a foundational discipline that explores the diversity, structure, function, and impact of microorganisms. Understanding microbes is essential for advances in health, agriculture, biotechnology, and environmental science.