Microbiology: The Evolution of Microorganisms and Microbiology

Chapter 1: The Evolution of Microorganisms and Microbiology

Learning Objectives

• Define microbiology.

• Explain Carl Woese's contributions to the three-domain system for classifying cellular life.

• Determine the type of microbe (bacterium, fungus, etc.) from a description.

• Provide examples of the importance of major microbe types to humans.

• Explain the RNA world hypothesis and supporting evidence.

• Design experiments to place a newly discovered cellular microbe on a phylogenetic tree using small subunit (SSU) rRNA sequences.

• Compare and contrast the evolution of mitochondria and chloroplasts.

• Evaluate contributions by Hooke, Leeuwenhoek, Pasteur, Lister, Koch, Beijerinck, von Behring, Kitasato, Metchnikoff, and Winogradsky.

• Outline experiments to determine if a particular microbe is the causative agent of a disease.

• Predict difficulties in using Koch's postulates to determine if a microbe causes a disease unique to humans.

Divisions and Types of Microbes

• Cellular:

  • Bacteria: Example: Escherichia coli

  • Archaea: Example: Methanogens

  • Fungi:

    • Yeasts

    • Molds

  • Protists:

    • Algae

    • Protozoa

    • Slime molds

• Acellular:

  • Viruses: Composed of protein and nucleic acid

  • Viroids: Composed of RNA

  • Satellites: Nucleic acid enclosed in a protein shell

  • Prions

Diversity of Microorganisms

• There are more microbes on Earth than stars in the known universe (10^{30} microbial cells).

• Microbes are found in diverse habitats:

  • Hydrothermal vents

  • Human body

  • Radioactive waste

  • Hot springs

The Challenge of Microbial Taxonomy

• Cells have two possible floor plans:

  • Prokaryotic cells: Open floor plan.

  • Eukaryotic cells: Have a membrane-enclosed nucleus, larger and more complex.

• All microorganisms (except viruses and other acellular infectious agents) were initially placed in three kingdoms.

• Organisms with prokaryotic cells were placed in the kingdom Monera.

We Live in a Microbial World

• Carl Woese (1928 – 2012) compared ribosomal RNA (rRNA) nucleic acid sequences starting in the 1970s.

• All cellular life shares a shared evolutionary history, and some genes are shared by all organisms.

• The sequence of those genes can be used as a genetic fingerprint for different organisms.

• There are two very different groups of organisms with prokaryotic cells: Bacteria and Archaea.

Domain Bacteria

• Usually single-celled.

• Most have a cell wall with peptidoglycan.

• Most lack a membrane-bound nucleus.

• Live in diverse environments, including extreme ones and the human body.

• Include both disease-causing and non-disease-causing organisms.

Domain Archaea

• Distinguished from Bacteria by unique rRNA sequences.

• Have unique membrane lipids.

• Some have unusual metabolic characteristics.

• Many live in extreme environments.

• Do not directly cause disease in humans.

Domain Eukarya

• Protists:

  • Unicellular, but generally larger than Bacteria and Archaea.

  • Protozoa: Animal-like metabolism.

  • Algae: Photosynthetic.

• Fungi:

  • Unicellular or multicellular.

  • Yeast: Unicellular.

  • Molds and mushrooms: Multicellular.

Acellular Agents

• Viruses:

  • Extremely small (smallest is 10,000 times smaller than a typical bacterium).

  • Cause many animal and plant diseases (COVID-19, rabies, influenza, AIDS, common cold, etc.).

• Viroids: Cause numerous plant diseases.

• Satellites:

  • Require coinfection with a helper virus to complete their life cycle.

  • Cause both animal and plant diseases (e.g., hepatitis D).

• Prions: Responsible for causing neurological diseases such as scrapie and