Chapter 1: The Microbial World and You – Foundations of Microbiology

Introduction to Microbiology

The Microbial World and Its Importance

Microbiology is the study of organisms too small to be seen with the unaided eye, including bacteria, archaea, fungi, protozoa, algae, viruses, and prions. Microbes play essential roles in ecosystems, human health, industry, and disease.

• Pathogenicity: Some microbes cause diseases in humans, animals, and plants.

• Beneficial Roles: Many microbes decompose organic waste, recycle vital elements, produce oxygen, and are used in food production and biotechnology.

• Industrial Applications: Microbes are used to produce chemicals (e.g., ethanol, acetone), fermented foods (e.g., cheese, yogurt), and pharmaceuticals (e.g., insulin).

The Microbiome and Human Health

Normal Microbiota and Microbiome

The microbiome refers to the community of microbes living stably on and in the human body. These microbes help maintain health, prevent pathogen colonization, and train the immune system. Normal microbiota are acquired before birth and throughout life, while transient microbiota are present temporarily.

• Human Microbiome Project (2007–2016): Mapped typical microbiota and their roles in health and disease.

• National Microbiome Initiative (2016–): Studies microbial roles in diverse ecosystems.

Naming and Classifying Microorganisms

Scientific Nomenclature

Microorganisms are named using a binomial system established by Carolus Linnaeus in 1735. Each organism has a genus (capitalized) and a specific epithet (lowercase), both italicized or underlined (e.g., Escherichia coli).

• Genus: The first part of the name, always capitalized.

• Specific epithet: The second part, always lowercase.

• Names may honor scientists or describe characteristics/habitats.

• After first use, names may be abbreviated (e.g., E. coli).

Types of Microorganisms

Major Groups of Microbes

Microorganisms are classified into several groups based on cellular structure, metabolism, and genetics.

• Bacteria: Prokaryotic, unicellular, peptidoglycan cell walls, reproduce by binary fission, diverse metabolism, may have flagella.

• Archaea: Prokaryotic, lack peptidoglycan, often extremophiles (e.g., methanogens, halophiles, thermophiles), not known to cause disease.

• Fungi: Eukaryotic, chitin cell walls, absorb nutrients, include unicellular yeasts and multicellular molds/mushrooms.

• Protozoa: Eukaryotic, absorb/ingest nutrients, motile via pseudopods, cilia, or flagella, free-living or parasitic, some photosynthetic.

• Algae: Eukaryotic, cellulose cell walls, photosynthetic, found in aquatic/soil environments, produce oxygen and carbohydrates.

• Viruses: Acellular, DNA or RNA core, protein coat (sometimes lipid envelope), replicate only in living hosts.

• Multicellular Animal Parasites: Eukaryotic, include helminths (flatworms, roundworms), microscopic stages in life cycle.

Bacteria

• Prokaryotic, unicellular, peptidoglycan cell walls.

• Reproduce by binary fission.

• Obtain energy from organic/inorganic chemicals or photosynthesis.

• May be motile via flagella.

Fungi

• Eukaryotic, chitin cell walls.

• Absorb organic chemicals for energy.

• Yeasts are unicellular; molds and mushrooms are multicellular.

• Molds consist of mycelia (masses of hyphae).

Protozoa

• Eukaryotic, absorb or ingest organic chemicals.

• Motile via pseudopods, cilia, or flagella.

• Free-living or parasitic; some are photosynthetic.

• Reproduce sexually or asexually.

Algae

• Eukaryotic, cellulose cell walls.

• Photosynthetic, produce oxygen and carbohydrates.

• Found in freshwater, saltwater, and soil.

• Reproduce sexually and asexually.

Viruses

• Acellular, consist of DNA or RNA core surrounded by protein coat (sometimes lipid envelope).

• Replicate only inside living host cells; inert outside hosts.

Classification of Microorganisms

The Three Domains

Carl Woese (1978) classified all life into three domains based on cellular organization and genetics:

• Bacteria

• Archaea

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

History of Microbiology

Early Observations and Cell Theory

Robert Hooke (1665) observed "cells" in cork, marking the beginning of cell theory: all living things are composed of cells. Anton van Leeuwenhoek (1673–1723) was the first to observe and document microbes, which he called "animalcules."

Spontaneous Generation vs. Biogenesis

Debate existed over whether life could arise spontaneously from nonliving matter (spontaneous generation) or only from preexisting life (biogenesis).

• Redi, Needham, and Spallanzani performed experiments to test these hypotheses.

• Louis Pasteur (1861) disproved spontaneous generation with his S-shaped flask experiment, showing that microbes originate from the air, not mystical forces.

The First Golden Age of Microbiology (1857–1914)

• Fermentation: Pasteur showed microbes convert sugars to alcohol and cause spoilage.

• Pasteurization: Heating kills spoilage microbes without evaporating alcohol.

• Germ Theory of Disease: Microbes cause disease (Bassi, Pasteur, Lister, Koch).

• Koch's Postulates: Experimental steps to link a specific microbe to a specific disease.

Discovery of Antibiotics

• Paul Ehrlich (1910): Developed salvarsan, a synthetic drug for syphilis.

• Alexander Fleming (1928): Discovered penicillin, the first antibiotic.

Branches of Microbiology

Specialized Fields

• Bacteriology: Study of bacteria.

• Mycology: Study of fungi.

• Parasitology: Study of protozoa and parasitic worms.

• Immunology: Study of immunity.

• Virology: Study of viruses.

Genetics and Molecular Biology

• Microbial genetics: How microbes inherit traits.

• Molecular biology: How genetic information is carried in DNA.

• Genomics: Study of an organism's genes; enables classification and understanding of microbiomes.

• Recombinant DNA: DNA from different sources combined to produce proteins or modify organisms.

Microbes and Human Welfare

Beneficial Activities of Microorganisms

• Recycling Elements: Bacteria convert carbon, nitrogen, sulfur, and phosphorus for use by plants and animals.

• Sewage Treatment: Microbes break down organic matter in sewage, recycling water.

• Bioremediation: Microbes degrade pollutants (e.g., oil, mercury).

• Insect Pest Control: Microbes like Bacillus thuringiensis are used as biological pesticides.

• Biotechnology: Use of microbes for practical applications (e.g., producing foods, chemicals, and pharmaceuticals).

• Recombinant DNA Technology: Enables production of proteins, vaccines, and genetically modified organisms.

Microbes and Human Disease

Normal Microbiota, Resistance, and Biofilms

• Normal microbiota: Microbes normally present in and on the human body, preventing pathogen growth and producing vitamins.

• Resistance: The body's ability to ward off disease, involving skin, stomach acid, and immune chemicals.

• Biofilms: Microbes attached to surfaces, forming complex communities. Can be beneficial (protect mucous membranes) or harmful (cause infections, resist antibiotics).

Emerging Infectious Diseases (EIDs)

• Definition: New or increasing diseases caused by pathogens overcoming host resistance.

• Contributing Factors: Evolution (e.g., antibiotic resistance), global travel, environmental changes, and increased human exposure.

• Examples: COVID-19, Monkeypox, Zika virus, H1N1 influenza, Avian influenza, MRSA, Ebola, Marburg virus.