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Introduction to Microbiology: Microorganisms, Classification, and Domains of Life

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Microbiology: The Study of Microbes

Definition and Scope

Microbiology is the scientific study of microbes, which includes both organisms and infectious agents too small to be seen by the naked eye. Microbes encompass living organisms (microorganisms) and non-living infectious agents.

  • Microorganism: Any living organism too small to see without a microscope. Can be unicellular or multicellular.

  • Microbe: Includes both microorganisms and infectious agents (such as viruses) that are microscopic.

  • Cell: The smallest, most basic/fundamental unit of life.

Example: SARS-CoV-2 (Covid-19 virus), Escherichia coli (bacterium), Nector americanus (hookworm), Dinobryon divergens (Golden Algae).

Additional info: Microorganisms can be bacteria, archaea, fungi, protozoa, algae, and viruses.

Discovering Microorganisms

Historical Milestones

The existence of microorganisms was discovered between 1665 and 1674.

  • Robert Hooke (1665): First to visualize and depict a microorganism. Described a common bread mold (Mucor) as a "microscopical mushroom."

  • Antonie van Leeuwenhoek (1674): Dutch merchant who used a simple microscope to observe and describe bacteria and protozoa, calling them "animalcules."

Example: Leeuwenhoek observed algae, bacteria, protozoa, and yeast cells.

Additional info: Early microscopes were not powerful enough to observe viruses.

Classification of Life: Taxonomy

Taxonomic Hierarchy

Taxonomy is the branch of science that classifies, identifies, and names organisms. Organisms are classified into hierarchical categories:

  • Domain

  • Kingdom

  • Phylum

  • Class

  • Order

  • Family

  • Genus

  • Species

Example: Taxonomy is concerned with naming and classifying organisms.

The Three Domains of Life

Overview

The broadest and most inclusive categories of life are the three domains:

  • Bacteria: Prokaryotic cells (do not contain a nucleus).

  • Archaea: Prokaryotic cells (do not contain a nucleus), often found in extreme environments.

  • Eukarya: Eukaryotic cells (contain a nucleus).

Example: The most recent species to evolve on Earth is an ancestor of group "A" (Eukarya).

Kingdoms of the Eukarya Domain

Organisms in each domain are further subdivided into kingdoms. The Domain Eukarya includes:

  • Kingdom Animalia: Multicellular, heterotrophic organisms.

  • Kingdom Plantae: Multicellular, autotrophic organisms.

  • Kingdom Fungi: Mostly multicellular, heterotrophic organisms.

  • Kingdom Protista: Mostly unicellular, some multicellular.

Example: Kingdom Eubacteria is NOT part of the Eukaryotic Domain.

Categorizing Life Based on Energy Acquisition

Autotrophs vs. Heterotrophs

Living organisms are categorized based on how they acquire energy:

  • Autotrophs: Acquire energy by making their own food (e.g., photosynthesis).

  • Heterotrophs: Acquire energy by eating other living organisms.

  • Decomposers: Acquire energy from wastes and dead organisms.

Most energy utilized by life originates from the sun, but with every transfer, some energy is lost as heat.

Example: Autotrophs are also called producers or synthesizers.

Summary Table: Domains and Kingdoms

Domain

Cell Type

Kingdoms

Examples

Bacteria

Prokaryotic

Eubacteria

Escherichia coli

Archaea

Prokaryotic

Archaebacteria

Halophiles, Thermophiles

Eukarya

Eukaryotic

Animalia, Plantae, Fungi, Protista

Humans, Plants, Mushrooms, Amoeba

Key Terms and Concepts

  • Microbe: Any microscopic organism or infectious agent.

  • Microorganism: Living microbe (bacteria, fungi, protozoa, algae).

  • Virus: Non-living infectious agent, requires host cell to replicate.

  • Taxonomy: Science of classification, identification, and naming of organisms.

  • Autotroph: Organism that produces its own food from inorganic substances.

  • Heterotroph: Organism that obtains energy by consuming other organisms.

Important Equations

  • Photosynthesis (autotroph energy acquisition):

Practice Questions (for Review)

  • Which of these is considered a microbe but NOT a microorganism?

  • Which are characteristics of microorganisms?

  • Which branch of biology is concerned with naming and classifying organisms?

  • Which of the following kingdoms is NOT part of the Eukaryotic Domain?

  • Biologists can divide living organisms into two groups: autotrophs and heterotrophs, which differ in their method of obtaining energy.

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