The Microbial World and You

Importance of Microbiomes

Microbiomes are communities of microorganisms that inhabit various environments, including the human body. They play essential roles in maintaining health, supporting bodily functions, and influencing disease states.

• Microorganisms (or microbes): Organisms too small to be seen with the naked eye, including bacteria, fungi, protozoa, algae, and viruses.

• Microbes are crucial for processes such as nitrogen fixation, decomposition, oxygen generation, and food production.

• Some microbes are pathogenic (disease-causing), but most are beneficial or harmless.

• The microbiome refers to the collective genomes of the microbes in a particular environment, such as the human body.

• Normal microbiota: Microorganisms that colonize the body without causing disease; they help prevent the growth of pathogens and may train the immune system.

• Transient microbiota: Microbes present in the body for a short period.

Examples of Microbial Benefits:

• Decomposition of organic waste

• Production of fermented foods (e.g., cheese, yogurt, bread)

• Manufacture of products like insulin and cellulose

• Basis of aquatic food chains

Human Microbiome Project (2007–2016): Aimed to characterize the human microbiota and understand its role in health and disease.

National Microbiome Initiative (2016–): Explores the role of microbes in various ecosystems.

Functional Anatomy and Classification of Microorganisms

Prokaryotes vs. Eukaryotes

Microorganisms are classified based on cellular structure:

• Prokaryotes: Lack a true nucleus; DNA is usually a single circular chromosome. Examples: Bacteria, Archaea.

• Eukaryotes: Have a true nucleus and membrane-bound organelles. Examples: Fungi, Protozoa, Algae.

Key Differences:

• Prokaryotes: Simple cell wall, no nucleus, divide by binary fission.

• Eukaryotes: Complex cell wall (if present), nucleus, divide by mitosis or meiosis.

Types of Microorganisms

• Bacteria: Prokaryotic, single-celled, peptidoglycan cell walls, reproduce by binary fission, may be motile via flagella.

• Archaea: Prokaryotic, often live in extreme environments, cell walls lack peptidoglycan, include methanogens and thermophiles, not known to cause disease in humans.

• Fungi: Eukaryotic, can be unicellular (yeasts) or multicellular (molds, mushrooms), cell walls contain chitin, absorb organic material for energy.

• Protozoa: Eukaryotic, unicellular, move by pseudopods, cilia, or flagella, may be free-living or parasitic.

• Algae: Eukaryotic, cellulose cell walls, photosynthetic, found in aquatic environments.

• Viruses: Acellular, consist of DNA or RNA core surrounded by a protein coat, require a host cell to replicate.

• Helminths: Multicellular parasitic worms, some stages are microscopic.

Naming and Classification

Scientific nomenclature assigns each organism a two-part name (binomial nomenclature):

• Genus (capitalized) and species (not capitalized), both italicized or underlined (e.g., Staphylococcus aureus).

• Established by Carolus Linnaeus in 1735.

Three Domains (Woese, 1978):

• Bacteria

• Archaea

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

Observing Microorganisms and the Cell Theory

Historical Discoveries

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

• 1673–1723: Anton van Leeuwenhoek observed "animalcules" (bacteria, protozoa) with a microscope.

Spontaneous Generation vs. Biogenesis

Debate over the origin of life:

• Spontaneous generation: Life arises from non-living matter.

• Biogenesis: Living cells arise only from pre-existing living cells.

Key experiments:

• 1668: Francesco Redi showed that maggots do not arise from decaying meat unless flies lay eggs.

• 1745: John Needham observed microbial growth in heated, unsealed broth.

• 1765: Lazzaro Spallanzani found no microbial growth in broth heated and sealed immediately.

• 1858: Rudolf Virchow proposed biogenesis.

• 1861: Louis Pasteur disproved spontaneous generation using S-shaped flasks.

Golden Ages of Microbiology

First Golden Age (1857–1914)

• Pasteur demonstrated fermentation and pasteurization.

• Joseph Lister introduced antiseptic surgery.

• Robert Koch established Koch's postulates for linking microbes to disease.

• Development of vaccines and chemotherapeutic drugs.

Second Golden Age: Chemotherapy and Antibiotics

• Paul Ehrlich developed Salvarsan for syphilis (1910).

• 1930s: Sulfonamides synthesized.

• 1928: Alexander Fleming discovered penicillin.

• 1940s: Mass production of penicillin.

• Problems: Toxicity and resistance to antibiotics.

Third Golden Age: Genomics and Molecular Biology

• Genomics: Study of organismal genes, enabling classification and understanding of microbiomes.

Chemical Principles and Microbial Metabolism

Fermentation and Pasteurization

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

• Pasteurization: Application of high heat for a short time to kill harmful microbes in beverages without evaporating alcohol.

Microbial Genetics

Key Discoveries in Molecular Genetics

• 1941: Beadle and Tatum showed genes encode enzymes.

• 1944: Avery, MacLeod, and McCarty demonstrated DNA is hereditary material.

• 1953: Watson and Crick proposed the DNA double helix model.

• 1961: Jacob and Monod discovered mRNA's role in protein synthesis.

Applied and Environmental Microbiology

Sewage Treatment: Using Microbes to Recycle Water

Microbes play a vital role in treating sewage by removing contaminants and recycling water.

• Sewage is mostly water with a small percentage of suspended solids.

• Treatment removes solids physically and uses microbes to convert organic materials into by-products such as carbon dioxide, nitrates, phosphates, sulfates, ammonia, hydrogen sulfide, and methane.

Insect Pest Control by Microorganisms

Microbes can be used as biological alternatives to chemical pesticides, reducing environmental impact and preventing crop damage.

• Bacillus thuringiensis produces protein crystals toxic to insects but harmless to animals and plants.

• The toxin gene has been inserted into plants for insect resistance.

Biotechnology and Recombinant DNA Technology

Biotechnology uses microbes for practical applications, such as producing foods, chemicals, and medicines. Recombinant DNA technology enables the genetic modification of organisms to produce proteins, vaccines, and enzymes.

• Gene therapy: Replacing defective genes in human cells.

• Agricultural applications: Genetically modified bacteria protect crops from insects and freezing.

Emerging Infectious Diseases

Antibiotic-Resistant Infections

• Methicillin-resistant Staphylococcus aureus (MRSA): Developed resistance to penicillin (1950s), methicillin (1980s), and vancomycin (1990s).

• VISA: Vancomycin-intermediate S. aureus

• VRSA: Vancomycin-resistant S. aureus

• Clostridium difficile: New antibiotic-resistant strain emerged in 2004.

• Mycobacterium tuberculosis: Multidrug-resistant strains (MDR-TB).

Other Emerging Diseases

• COVID-19 (SARS-CoV-2): Declared a pandemic in 2020.

• Monkeypox (mpox): Orthopoxvirus, outbreaks in non-endemic countries.

• Zika virus: Spread by mosquitoes, can cause birth defects.

• H1N1 influenza (Swine flu): Pandemic in 2009.

• Avian flu (H5N7): Primarily in birds, limited human transmission.

• Ebola and Marburg viruses: Cause hemorrhagic fevers, outbreaks in Africa.

Microbial Ecology and Environmental Roles

Recycling Vital Elements

• Microbial ecology studies the relationship between microbes and their environment.

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

• Microbes are used in sewage treatment, bioremediation, and pest control.

Biofilms

• Biofilms are complex microbial communities attached to surfaces (e.g., rocks, teeth, medical implants).

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

Normal Microbiota and Resistance

• Normal microbiota prevent pathogen growth and 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.