Introduction to Microbiology: Classification and Diversity of Microorganisms

Chapter 1: The Microbial World and You

Introduction

This chapter introduces the field of microbiology, focusing on the classification and diversity of microorganisms. It covers the historical development of taxonomy, the three-domain system, and the major groups of microbes.

Classifying Living Organisms

Historical Development of Taxonomy

Taxonomy is the science of classifying living organisms. Over time, the classification systems have evolved as scientific understanding and technology have advanced.

• 1758: Carolus Linnaeus developed a two-kingdom system (plants and animals) and introduced binomial nomenclature.

• 1866: Ernst Haeckel proposed a three-kingdom system by adding Protista.

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

Key Point: The classification of living organisms is continually refined as new information becomes available.

The Three Domain System

Woese and Fox's Phylogenetic Tree

The three-domain system is a modern classification scheme based on genetic relationships, particularly ribosomal RNA sequences. It divides all life into three domains:

• Bacteria: Includes all typical prokaryotic bacteria.

• Archaea: Includes prokaryotes that often live in extreme environments and have unique genetic and biochemical characteristics.

• Eukarya: Includes all eukaryotic organisms (animals, plants, fungi, and protists).

Key Point: Domains Archaea and Bacteria contain all prokaryotic organisms, while Eukarya contains all eukaryotic organisms.

Comparison of the Three Domains

Domain	Cell Type	Cell Wall Composition	Examples
Bacteria	Prokaryotic	Peptidoglycan	Escherichia coli, Staphylococcus aureus
Archaea	Prokaryotic	No peptidoglycan	Methanogens, Halophiles, Thermophiles
Eukarya	Eukaryotic	Varies (cellulose in plants, chitin in fungi, none in animals)	Animals, Plants, Fungi, Protists

Scientific Nomenclature

Each organism is given a two-part scientific name (binomial nomenclature):

• Genus: Capitalized and italicized (or underlined).

• Specific epithet: Lowercase and italicized (or underlined).

• Example: Escherichia coli (abbreviated as E. coli after first use).

Key Point: Scientific names are used worldwide and provide a standardized way to refer to organisms.

Types of Microorganisms

Major Groups of Microbes

• Bacteria: Prokaryotic, peptidoglycan cell walls, reproduce by binary fission, diverse metabolism (organic/inorganic chemicals, photosynthesis).

• Archaea: Prokaryotic, lack peptidoglycan, often live in extreme environments (e.g., methanogens, halophiles, thermophiles).

• Fungi: Eukaryotic, chitin cell walls, absorb organic chemicals, include yeasts (unicellular) and molds/mushrooms (multicellular).

• Protozoa: Eukaryotic, absorb or ingest organic chemicals, may move by pseudopods, cilia, or flagella (e.g., Giardia lamblia).

• Algae: Eukaryotic, cellulose cell walls, photosynthetic, produce oxygen and organic compounds.

• Viruses: Acellular, consist of DNA or RNA core surrounded by a protein coat (sometimes with a lipid envelope), replicate only inside living host cells.

• Multicellular Animal Parasites: Eukaryotic, multicellular animals (helminths: flatworms and roundworms), some stages are microscopic.

Size Range of Microbes

• Viruses: ~100 nm (nanometers)

• Bacteria: ~1 μm (micrometer)

• Plant/Animal Cells: ~10–100 μm

• Objects must be ~100 μm to be visible without a microscope.

Examples and Applications

• Escherichia coli (E. coli): Found in the large intestine of humans.

• Staphylococcus aureus (S. aureus): Found on human skin.

• Members of the Coronavirus family: Cause respiratory infections such as the common cold, SARS, and MERS.

Additional info: The classification and study of microorganisms are foundational to understanding their roles in health, disease, and the environment.