Chapter 1: The Microbial World and You – Study Notes

The Microbial World and You

Introduction to Microbiology

Microbiology is the study of microorganisms, which are organisms too small to be seen with the unaided eye. This field encompasses a wide variety of life forms, including bacteria, archaea, fungi, protozoa, algae, viruses, and prions. Microorganisms play essential roles in ecosystems, human health, and industry.

Microbes in Our Lives

• Pathogenicity: Only a small fraction of microbes are pathogenic (disease-causing).

• Food Spoilage: Some microbes cause food spoilage.

• Ecological Roles: Microbes form the basis of food chains in aquatic environments, decompose organic waste, and recycle vital elements such as nitrogen and carbon.

• Photosynthesis: Certain microbes generate oxygen through photosynthesis.

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

Definitions

• Microbiome: The collection of all microorganisms living in association with the human body.

• Normal Microbiota: Microbes that colonize the body and contribute to health.

• Transient Microbiota: Microbes that temporarily colonize the body.

The Human Microbiome

The human body contains approximately 30 trillion human cells and 40 trillion bacterial cells. The microbiome helps maintain health, prevents the growth of pathogens, and trains the immune system. Colonization by microbes depends on the availability of nutrients and suitable environmental conditions.

Naming and Classifying Microorganisms

• Scientific Nomenclature: Established by Carolus Linnaeus in 1735, each organism is given a two-part name: the genus (capitalized) and the specific epithet (lowercase), both italicized or underlined (e.g., Escherichia coli).

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

Types of Microorganisms

Microorganisms are classified into several groups based on cellular structure and function:

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

• Archaea: Prokaryotic, lack peptidoglycan, often inhabit extreme environments, not known to cause disease.

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

• Protozoa: Eukaryotic, absorb/ingest organic chemicals, may be motile, free-living or parasitic.

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

• Viruses: Acellular, DNA or RNA core, protein coat, replicate only in living hosts.

• Multicellular Animal Parasites: Eukaryotic, include helminths (parasitic worms), some stages are microscopic.

Bacteria

• Prokaryotic, unicellular, peptidoglycan cell walls.

• Reproduce by binary fission.

• Obtain nutrition 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 made 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 a protein coat (sometimes with a lipid envelope).

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

Multicellular Animal Parasites

• Eukaryotic, multicellular animals.

• Include helminths (parasitic flatworms and roundworms).

• Some life stages are microscopic.

Classification of Microorganisms

• Developed by Carl Woese in 1978.

• Three domains based on cellular organization:

  • Bacteria

  • Archaea

  • Eukarya (includes protists, fungi, plants, animals)

A Brief History of Microbiology

Early Observations and Cell Theory

• 1665: Robert Hooke observed "cells" in cork, marking the beginning of cell theory (all living things are composed of cells).

• 1673–1723: Anton van Leeuwenhoek observed and described microorganisms ("animalcules") using simple microscopes.

Spontaneous Generation vs. Biogenesis

• Spontaneous Generation: Hypothesis that life arises from nonliving matter.

• Biogenesis: Hypothesis that living cells arise only from preexisting living cells.

• Key experiments:

  • 1668: Francesco Redi – maggots do not arise from decaying meat without flies.

  • 1745: John Needham – claimed spontaneous generation after boiling broth.

  • 1765: Lazzaro Spallanzani – no growth in sealed, boiled flasks.

  • 1858: Rudolf Virchow – proposed biogenesis.

  • 1861: Louis Pasteur – used S-shaped flasks to show that microbes come from the air, not spontaneous generation.

The First Golden Age of Microbiology (1857–1914)

• Relationship between microbes and disease established.

• Development of vaccines, aseptic techniques, and chemotherapeutic drugs.

• Fermentation: Microbial conversion of sugar to alcohol in the absence of air.

• Pasteurization: Application of heat to kill harmful microbes in beverages.

The Germ Theory of Disease

• Microorganisms can cause disease in animals and humans.

• Joseph Lister used phenol for antiseptic surgery.

• Robert Koch developed Koch's postulates to link specific microbes to specific diseases (e.g., anthrax).

Vaccination and Chemotherapy

• 1796: Edward Jenner developed the first vaccine (cowpox for smallpox).

• Paul Ehrlich developed the first synthetic drug (salvarsan) for syphilis.

• 1928: Alexander Fleming discovered penicillin, the first antibiotic.

Modern Fields in Microbiology

• Bacteriology: Study of bacteria.

• Mycology: Study of fungi.

• Parasitology: Study of protozoa and parasitic worms.

• Immunology: Study of immunity.

• Virology: Study of viruses.

Genetics, Molecular Biology, and Genomics

• Microbial Genetics: Study of inheritance in microbes.

• Molecular Biology: Study of how genetic information is carried in DNA.

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

• Recombinant DNA: DNA made from two different sources, enabling genetic engineering and biotechnology.

Microbes and Human Welfare

Recycling Vital Elements

• Microbial Ecology: Study of the relationship between microbes and their environment.

• Bacteria recycle elements such as carbon, nitrogen, sulfur, and phosphorus for use by plants and animals.

Sewage Treatment and Bioremediation

• Microbes are used to treat sewage and recycle water by converting organic materials into harmless by-products.

• Bacteria degrade pollutants such as oil and mercury (bioremediation).

Insect Pest Control

• Microbes such as Bacillus thuringiensis are used as biological insecticides, producing toxins harmful to insects but safe for animals and plants.

Biotechnology and Recombinant DNA Technology

• Biotechnology: Use of microbes for practical applications (e.g., food production, chemical synthesis).

• Recombinant DNA Technology: Genetic engineering to produce proteins, vaccines, and enzymes; gene therapy; genetically modified crops.

Microbes and Human Disease

Normal Microbiota and Resistance

• Normal Microbiota: Microbes that inhabit the human body and prevent pathogen growth.

• Produce essential growth factors (e.g., vitamins B and K).

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

Biofilms

• Microbes attach to surfaces and form complex communities called biofilms.

• Biofilms can be beneficial (protect mucous membranes, provide food in aquatic systems) or harmful (clog pipes, cause infections, resist antibiotics).

Emerging Infectious Diseases (EIDs)

• Emerging Infectious Diseases: New or increasing diseases caused by pathogens overcoming host resistance.

• Contributing factors: antibiotic resistance, global travel, environmental changes, increased human exposure.

• Examples: COVID-19, Mpox, Zika virus, H1N1 influenza, avian influenza, antibiotic-resistant infections (MRSA, Clostridium difficile, MDR-TB), Ebola, Marburg virus.