Introduction to Microbiology: The Microbial World and You

Course Overview

The Microbial World and You

What is a Microbe?

Microbes are organisms that are typically too small to be seen with the unaided eye. They include bacteria, fungi, protozoa, microscopic algae, viruses, and some multicellular animal parasites.

• Not all microbes are "living" in the traditional sense (e.g., viruses are acellular).

• Microbes play essential roles in ecosystems and human health.

Microbes in Our Lives

Microorganisms have diverse roles and significant impacts on the environment and human society.

• Pathogenicity: Only a few microbes cause disease.

• Decomposition: Decompose organic waste, recycling nutrients.

• Photosynthesis: Generates oxygen and carbohydrates.

• Industrial Applications: Produce chemicals (ethanol, acetone, vitamins), fermented foods (vinegar, cheese, bread), and products for manufacturing and medicine (cellulose, insulin).

• Knowledge Benefits: Prevent food spoilage, prevent disease, and understand disease transmission to prevent epidemics.

The Microbiome

The microbiome refers to the collection of microbes normally present in and on the human body, such as normal intestinal bacteria. These microbes protect the host and produce essential nutrients.

Naming and Classifying Microorganisms

Scientific Nomenclature

Carolus Linnaeus established the system of scientific nomenclature in 1735. Each organism has two names: the genus and the specific epithet.

• Names are italicized or underlined.

• The genus is capitalized; the specific epithet is lowercase.

• Example: Escherichia coli

Classification of Microorganisms

Microorganisms are classified into three domains based on cellular organization (developed by Carl Woese in 1978):

Domain	Groups
Bacteria	Prokaryotes
Archaea	Prokaryotes (extremophiles)
Eukarya	Protists, Fungi, Plants, Animals

Major Groups of Microorganisms

Bacteria

Bacteria are single-celled prokaryotes with peptidoglycan cell walls. They divide by binary fission and may derive nutrition from organic/inorganic chemicals or photosynthesis. Some bacteria are motile via flagella.

Archaea

Archaea are prokaryotes that lack peptidoglycan cell walls and may live in extreme environments. Types include methanogens, extreme halophiles, and extreme thermophiles. They are generally not known to cause disease in humans.

Fungi

Fungi are eukaryotes with chitin cell walls. They absorb organic chemicals for energy. Yeasts are unicellular, while molds and mushrooms are multicellular. Molds consist of masses of mycelia made of hyphae.

Protozoa

Protozoa are eukaryotes that absorb or ingest organic chemicals. They may be motile via pseudopods, cilia, or flagella. Protozoa can be free-living or parasitic and reproduce sexually or asexually.

Algae

Algae are eukaryotes with cellulose cell walls, found in freshwater, saltwater, and soil. They use photosynthesis for energy, producing oxygen and carbohydrates. Both sexual and asexual reproduction are possible.

Viruses

Viruses are acellular entities consisting of a DNA or RNA core surrounded by a protein coat, sometimes enclosed in a lipid envelope. They replicate only within living host cells and are inert outside hosts.

Multicellular Animal Parasites

These are eukaryotic multicellular animals, not strictly microorganisms. Parasitic flatworms and roundworms (helminths) have some microscopic stages in their life cycles.

Historical Milestones in Microbiology

The First Observations

• 1665: Robert Hooke reported that living things are composed of cells, marking the beginning of cell theory.

• 1623-1673: Anton van Leeuwenhoek observed microbes ("animalcules") through magnifying lenses.

The Debate over Spontaneous Generation

• Spontaneous generation: Life arises from nonliving matter; a "vital force" is necessary for life.

• Biogenesis: Living cells arise only from preexisting living cells (Rudolf Virchow, 1858).

Golden Age of Microbiology

• Pasteur: Fermentation, disproved spontaneous generation, pasteurization.

• Lister: Aseptic surgery.

• Koch: Germ theory of disease, Koch's postulates.

• Other milestones: Gram staining, Petri dish, discovery of pathogens and immunity.

The Germ Theory of Disease

• 1835: Bassi - fungus caused silkworm disease.

• 1865: Pasteur - protozoan caused another silkworm disease.

• 1840s: Semmelweis - handwashing to prevent puerperal fever.

• 1860s: Lister - chemical antiseptic for surgical wound infections.

• 1876: Koch - anthrax caused by bacterium; developed Koch's postulates.

Prevention and Treatment of Diseases

Vaccination

• 1796: Edward Jenner inoculated a person with cowpox virus, conferring immunity to smallpox.

Chemotherapy

• Treatment of disease with chemicals (synthetic drugs or antibiotics).

• Antibiotics are produced by bacteria and fungi to inhibit or kill other microbes.

• Quinine from tree bark used to treat malaria.

• 1910: Ehrlich developed synthetic arsenic drug (salvarsan) for syphilis.

Antibiotics

• 1928: Alexander Fleming discovered penicillin from Penicillium fungus.

• 1940s: Penicillin was clinically tested and mass-produced.

Fields of Microbiology

Field	Focus
Bacteriology	Study of bacteria
Mycology	Study of fungi
Parasitology	Study of protozoa and parasitic worms
Immunology	Study of immunity
Virology	Study of viruses
Microbial genetics	Study of how microbes inherit traits
Molecular biology	Study of how DNA directs protein synthesis

Modern Microbiology: Ecology and Biotechnology

Recycling Vital Elements

• Microbial ecology: Study of relationships between microorganisms and their environment.

• Bacteria convert carbon, oxygen, nitrogen, sulfur, and phosphorus into forms usable by plants and animals.

Bioremediation

• Bacteria degrade organic matter in sewage.

• Bacteria detoxify pollutants such as oil and mercury.

Insect Pest Control

• Bacillus thuringiensis infections are fatal to many insects but harmless to animals and plants.

Biotechnology and Recombinant DNA Technology

• Use of microbes for practical applications (food, chemicals).

• Recombinant DNA technology enables production of proteins, vaccines, and enzymes by bacteria and fungi.

Normal Microbiota and Biofilms

Normal Microbiota

• Microbes normally present in and on the human body; protect and produce nutrients.

• Resistance: Ability of the body to ward off disease.

Biofilms

• Microbes attach to solid surfaces and grow into masses (biofilms).

• Biofilms grow on rocks, pipes, teeth, and medical implants.

• Biofilms can cause infections and are often resistant to antibiotics.

Emerging Infectious Diseases (EIDs)

Definition and Examples

Emerging infectious diseases are new diseases or diseases increasing in incidence, often due to pathogens overcoming host resistance.

• Viral: Zika virus, MERS, SARS, SARS-CoV-2/COVID-19, H1N1/H5N1 influenza, Ebola, Marburg, HIV

• Bacterial: Methicillin-resistant Staphylococcus aureus (MRSA), Clostridium difficile

Summary Table: Microbial Groups and Key Features

Group	Cell Type	Cell Wall	Reproduction	Nutrition
Bacteria	Prokaryote	Peptidoglycan	Binary fission	Organic/inorganic/photosynthesis
Archaea	Prokaryote	No peptidoglycan	Binary fission	Varied/extreme environments
Fungi	Eukaryote	Chitin	Sexual/asexual	Absorb organic chemicals
Protozoa	Eukaryote	None	Sexual/asexual	Absorb/ingest organic chemicals
Algae	Eukaryote	Cellulose	Sexual/asexual	Photosynthesis
Viruses	Acellular	None	Host-dependent	Host-dependent
Helminths	Eukaryote	None	Sexual/asexual	Parasitic

Additional info: These notes expand on the introductory chapter of a college-level microbiology course, providing context, definitions, and examples for foundational concepts in the field.