Introduction to Microbiology: The Microbial World and Classification

Chapter 1: The Microbial World and You

Classifying Living Organisms

Classification of living organisms is a foundational concept in microbiology, providing a framework for understanding the diversity and relationships among all forms of life. Over time, the classification systems have evolved as scientific knowledge and technology have advanced.

• Carolus Linnaeus (1758): Developed a two-kingdom system (plants and animals) and introduced binomial nomenclature for naming organisms.

• Ernst Haeckel (1866): Proposed a three-kingdom system by adding Protista to include microorganisms.

• Robert Whittaker (1969): Suggested a five-kingdom system, adding Fungi as a separate kingdom.

Key Terms:

• Taxonomy: The science of classifying organisms.

• Binomial Nomenclature: The system of naming organisms with two names: genus and species (specific epithet).

Example: Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) are named using binomial nomenclature.

The Three Domain System

The three domain system is the most widely accepted classification scheme for all life forms, based on differences in ribosomal RNA (rRNA) sequences. This system was proposed by Carl Woese and George Fox in the late 20th century.

• Bacteria: Prokaryotic organisms with peptidoglycan cell walls.

• Archaea: Prokaryotic organisms lacking peptidoglycan, often found in extreme environments.

• Eukarya: Eukaryotic organisms, including plants, animals, fungi, and protists.

Phylogenetic Tree: Shows evolutionary relationships among organisms based on genetic similarities and differences.

Domain	Cell Type	Cell Wall Composition	Examples
Bacteria	Prokaryotic	Peptidoglycan	Escherichia coli, Bacillus subtilis
Archaea	Prokaryotic	No peptidoglycan	Methanogens, Halophiles, Thermophiles
Eukarya	Eukaryotic	Varied (cellulose, chitin, none)	Plants, Animals, Fungi, Protists

Key Points:

• Prokaryotes: Organisms without a nucleus (Bacteria and Archaea).

• Eukaryotes: Organisms with a nucleus (Eukarya).

• Domains Archaea and Bacteria: Contain all prokaryotic organisms.

• Domain Eukarya: Contains all eukaryotic organisms.

Example: The star on the phylogenetic tree indicates the position of humans (Eukarya).

Additional info: The three domain system is based on molecular evidence, primarily rRNA gene sequences, which provide a more accurate reflection of evolutionary relationships than previous classification systems.