Introduction to Microbiology: The Microbial World and You

Chapter Overview: The Microbial World and You

Introduction

This chapter introduces the foundational concepts of microbiology, focusing on the classification of living organisms, the development of taxonomy, and the major domains of life. Understanding these concepts is essential for studying the diversity, structure, and function of microorganisms.

Classifying Living Organisms

Historical Development of Taxonomy

Taxonomy is the science of classifying living organisms. Over centuries, 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.

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

• Robert Whittaker (1969): Proposed 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

Woese and Fox's Phylogenetic Tree

Modern classification recognizes three domains of life based on differences in ribosomal RNA sequences and cell structure. This system reflects evolutionary relationships among organisms.

• Bacteria: Prokaryotic organisms with peptidoglycan cell walls. Includes groups such as Proteobacteria, Cyanobacteria, and Thermotogae.

• Archaea: Prokaryotic organisms lacking peptidoglycan. Often found in extreme environments. Includes methanogens, halophiles, and thermophiles.

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

Key Points:

• Domains Bacteria and Archaea contain all prokaryotic organisms.

• Domain Eukarya contains all eukaryotic organisms.

Example: Humans belong to the domain Eukarya, while Escherichia coli belongs to Bacteria, and Halobacterium belongs to Archaea.

Scientific Nomenclature

Rules and Usage

Scientific names are standardized to avoid confusion and are used worldwide. Each organism is given a two-part name:

• Genus: Capitalized and italicized or underlined.

• Specific epithet: Lowercase and italicized or underlined.

After the first use, the genus name may be abbreviated to its initial (e.g., E. coli).

Example: Staphylococcus aureus is found on the skin; Escherichia coli is found in the large intestine.

Types of Microorganisms

Major Groups

Microorganisms are classified into several major groups based on their cellular structure, metabolism, and ecological roles.

• Bacteria: Prokaryotes with peptidoglycan cell walls. Reproduce by binary fission. Obtain energy from organic/inorganic chemicals or photosynthesis.

• Archaea: Prokaryotes lacking peptidoglycan. Live in extreme environments (e.g., methanogens, halophiles, thermophiles).

• Fungi: Eukaryotes with chitin cell walls. Obtain energy by absorbing organic chemicals. Includes molds (multicellular, with mycelia and hyphae) and yeasts (unicellular).

• Protozoa: Eukaryotes that absorb or ingest organic chemicals. May move via pseudopods, cilia, or flagella. Example: Giardia lamblia causes severe diarrhea.

• Algae: Eukaryotes with cellulose cell walls. Use photosynthesis for energy. Produce oxygen and organic compounds.

• Viruses: Acellular entities consisting 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 such as helminths (flatworms and roundworms) with complex life cycles.

Size Range of Microbes

Relative Sizes

Microorganisms vary greatly in size:

• Viruses: ~100 nm (nanometers), much smaller than bacteria.

• Bacteria: ~1 μm (micrometer), about 10 times larger than viruses.

• Plant/Animal Cells: ~10-100 μm, about 10 times larger than bacteria.

• Visibility: Objects must be about 100 μm to be visible without a microscope.

Historical Observations and Experiments

Discovery of Cells and Microorganisms

• Robert Hooke (1665): Reported that living things are composed of cells ("little boxes"). Published Micrographia.

• Anton van Leeuwenhoek (1673-1723): First to observe live microorganisms using a microscope.

Spontaneous Generation vs. Biogenesis

Debate over the origin of life:

• Spontaneous Generation: Hypothesis that living organisms arise from nonliving matter.

• Biogenesis: Hypothesis that living organisms arise from preexisting life.

Key Experiments:

• Francesco Redi (1668): Showed that maggots only appeared in meat exposed to flies, supporting biogenesis.

• Lazzaro Spallanzani (1765): Boiled nutrient solutions in sealed flasks; no microbial growth occurred, supporting biogenesis.

• Louis Pasteur (1857-1914): Used swan-neck flasks to show that microorganisms do not arise spontaneously but come from other microbes in the air.

Fermentation and Disease

Fermentation

Pasteur demonstrated that fermentation is caused by microorganisms, not by air. This led to advances in food preservation and industrial microbiology.

Germ Theory of Disease

The idea that microorganisms cause disease was established through several key discoveries:

• Agostino Bassi (1835): Showed that a fungus causes silkworm disease.

• Louis Pasteur (1855): Linked protozoans to silkworm disease.

• Ignaz Semmelweis (1840s): Advocated handwashing to prevent puerperal fever.

• Joseph Lister (1860s): Used chemical disinfectants to prevent surgical wound infections.

• Robert Koch: Proved that specific bacteria cause specific diseases and developed Koch's Postulates to establish causation.

Koch's Postulates

• The microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

• The microorganism must be isolated from a diseased organism and grown in pure culture.

• The cultured microorganism should cause disease when introduced into a healthy organism.

• The microorganism must be re-isolated from the experimentally infected host and identified as being identical to the original specific causative agent.

Vaccination and Chemotherapy

Vaccination

• Edward Jenner (1796): Inoculated a person with cowpox virus, providing protection from smallpox. The term "vaccination" is derived from vacca (cow).

• Immunity: The protection conferred by vaccination.

Chemotherapy and Antibiotics

• Chemotherapy: Treatment of disease with chemicals.

• Antibiotics: Chemicals produced by bacteria and fungi that inhibit or kill other microbes.

• Alexander Fleming (1928): Discovered penicillin, the first antibiotic, produced by the fungus Penicillium.

Modern Developments in Microbiology

Subfields of Microbiology

• Bacteriology: Study of bacteria.

• Mycology: Study of fungi.

• Virology: Study of viruses.

• Parasitology: Study of protozoa and parasitic worms.

• Immunology: Study of immunity. Includes research on vaccines and interferons.

Microbial Ecology

Bacteria play essential roles in recycling carbon, nutrients, sulfur, and phosphorus, making these elements available to plants and animals.

Antibiotic Resistance

MRSA and Related Strains

• MRSA: Methicillin-resistant Staphylococcus aureus. Resistance to penicillin developed in the 1950s, methicillin in the 1980s, and vancomycin in the 1990s.

• VISA: Vancomycin-intermediate-resistant S. aureus.

• VRSA: Vancomycin-resistant S. aureus.

Summary Table: Domains of Life

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

Key Equations and Concepts

• Binary Fission (Bacterial Growth): $N = N_0 imes 2^n$ Where $N$ is the final number of cells, $N_0$ is the initial number, and $n$ is the number of generations.

Additional info: The above notes expand on the provided materials with academic context, definitions, and examples to ensure completeness and clarity for college-level microbiology students.