Midterm

Introduction to Microbiology

Definition and Scope

Microbiology is the study of microorganisms, which are organisms too small to be seen with the naked eye. These include bacteria, archaea, fungi, protozoa, viruses, and some multicellular parasites. Microbes are ubiquitous and play essential roles in ecosystems, human health, and disease.

• Microorganism (Microbe): Any microscopic organism, including bacteria, archaea, fungi, protozoa, and viruses.

• Germ: A term often used for microbes that can cause disease.

• Viruses: Acellular entities often included in the study of microbiology due to their impact on living organisms.

This image illustrates the relative sizes of various microbes (bacteria, archaea, fungi, protozoa, viruses) compared to a human hair, highlighting the microscopic scale of these organisms.

Classification of Microorganisms

Prokaryotes vs. Eukaryotes

Microorganisms are classified based on cellular organization into prokaryotes and eukaryotes.

• Prokaryotic Cells: Lack a membrane-bound nucleus and organelles. Includes Bacteria and Archaea.

• Eukaryotic Cells: Possess a membrane-bound nucleus and organelles. Includes animals, plants, fungi, and protists.

This diagram compares the internal structures of prokaryotic and eukaryotic cells, emphasizing the presence of organelles and a nucleus in eukaryotes.

Prokaryotic Microorganisms

Bacteria

Bacteria are unicellular prokaryotes with diverse metabolic capabilities and structural features.

• Cell wall typically contains peptidoglycan.

• Reproduce by binary fission (asexual reproduction).

• Genetic material is usually a single, circular DNA molecule.

• Can be autotrophic (photosynthetic) or heterotrophic.

SEM image showing rod-shaped bacteria, a common bacterial morphology.

Archaea

Archaea are prokaryotes distinct from bacteria, often found in extreme environments.

• Cell wall lacks peptidoglycan; contains pseudomurein.

• Many are extremophiles (e.g., thermophiles, halophiles, methanogens).

• Reproduce by binary fission and possess circular DNA.

Eukaryotic Microorganisms

Fungi

Fungi are eukaryotic organisms that can be unicellular (yeasts) or multicellular (molds, mushrooms).

• Cell wall composed of chitin.

• Heterotrophic; obtain nutrients by absorption.

• Reproduce sexually or asexually via spores.

SEM image of yeast cells, a unicellular form of fungi.

Protozoa

Protozoa are unicellular eukaryotes, often motile and heterotrophic.

• Usually lack cell walls.

• Move using pseudopods, flagella, or cilia.

• Can reproduce sexually or asexually.

• Some are pathogenic, e.g., Trypanosoma causes African sleeping sickness.

Microscopic images showing the diversity of protozoan morphology.

Viruses

General Characteristics

Viruses are acellular infectious agents that require a host cell for replication.

• Composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid).

• Some have an additional lipid envelope derived from the host cell membrane.

• Obligate intracellular parasites; lack independent metabolism.

Illustration of various virus morphologies, including spherical, rod-like, and complex (bacteriophage) forms.

Multicellular Animal Parasites

Helminths

Helminths are multicellular eukaryotic parasites, including flatworms and roundworms, with complex life cycles involving microscopic stages.

• No cell wall; heterotrophic.

• Reproduce sexually or asexually.

• Some stages are microscopic and relevant to microbiology.

Microbial Diversity and Ubiquity

Microbes in the Environment

Microbes are found in virtually every environment on Earth, from deep in the Earth's crust to polar ice caps, oceans, and within living organisms. They are essential for nutrient cycling, decomposition, and maintaining ecosystem balance.

Illustration showing the role of microbes in global biogeochemical cycles and their ubiquity in diverse environments.

Taxonomy and Nomenclature

Classification and Naming

Microorganisms are classified using a hierarchical system: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Scientific names are assigned using binomial nomenclature (Genus species), which is italicized or underlined.

• Taxonomy: Science of classifying organisms.

• Nomenclature: Assignment of scientific names.

• Classification: Arrangement into hierarchical groups.

• Identification: Determining the identity of an organism.

Example of binomial nomenclature, showing the distinction between common and scientific names.

Cell Structure and Function

Components of All Cells

All cells share certain fundamental structures:

• Plasma (Cell) Membrane: Phospholipid bilayer separating the cell from its environment.

• Chromosomes: DNA molecules carrying genetic information.

• Ribosomes: Sites of protein synthesis.

• Cytosol: Semi-fluid substance inside the cell membrane.

Prokaryotic vs. Eukaryotic Cell Structure

Key differences between prokaryotic and eukaryotic cells include the presence of a nucleus, organelles, and the structure of genetic material.

• Prokaryotes: Circular DNA, no nucleus, no membrane-bound organelles.

• Eukaryotes: Linear chromosomes, nucleus, membrane-bound organelles.

Summary Table: Major Groups of Microorganisms

Group	Cell Type	Cell Wall	Genetic Material	Reproduction	Examples
Bacteria	Prokaryotic	Peptidoglycan	Circular DNA	Binary fission	Escherichia coli
Archaea	Prokaryotic	Pseudomurein	Circular DNA	Binary fission	Halophiles, Methanogens
Fungi	Eukaryotic	Chitin	Linear DNA	Sexual/Asexual	Yeasts, Molds
Protozoa	Eukaryotic	Usually none	Linear DNA	Sexual/Asexual	Trypanosoma
Viruses	Acellular	None	DNA or RNA	Host-dependent	Influenza virus, HIV
Helminths	Eukaryotic	None	Linear DNA	Sexual/Asexual	Flatworms, Roundworms

Additional info: This summary table provides a quick reference for distinguishing the major groups of microorganisms by their cellular structure, genetic material, and reproductive strategies.