Introduction to Microbiology: Microbial Life, Structure, and Methods

Microorganisms and Microbial Life

Overview of Microorganisms

Microorganisms, or microbes, are organisms too small to be seen with the naked eye. They are highly diverse in form and function and inhabit environments that support life.

• Diversity: Includes many single-celled organisms, some with complex structures, and some multicellular forms.

• Communities: Microbes often live in communities and are among the oldest forms of life.

• Biomass: Major fraction of Earth's biomass, surrounding plants and animals.

• Impact: Affect human life significantly (disease, soil, water, food, animal health).

Tools for Studying Microbes

• Microscopy: Observing cells in nutrient medium.

• Culture: Growing cells in nutrient media.

• Medium: Liquid/solid mixture of nutrients for culturing microbes.

• Growth: Increase in cell numbers from cell division.

Structure and Activities of Microbial Cells

Basic Cell Structure

The cell is a living compartment that interacts with the environment and other cells.

• Cytoplasmic cell membrane: Barrier separating inside of cell from outside environment.

• Cytoplasm: Aqueous mixture of macromolecules, small organics, ions, and ribosomes.

• Ribosomes: Protein synthesizing structures.

• Cell wall: Present in some microbes, confers structural strength.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotes: Bacteria and Archaea. No organelles, no membrane-bound structures.

• Eukaryotes: Plants, animals, fungi, protozoa. Contains organelles, DNA enclosed in a membrane-bound nucleus, mitochondria and chloroplasts for ATP synthesis.

Genes, Genomes, Nucleus, and Nucleoid

• Eukaryotic DNA: Linear chromosomes within nucleus, much larger genome.

• Prokaryotic DNA: Single circular chromosome in nucleoid region, may have plasmids (extrachromosomal DNA).

Activities of Microbial Cells

Cellular Functions

• Enzymes: Protein catalysts.

• Transcription: DNA information converted to RNA.

• Translation: RNA used by ribosome to synthesize proteins.

• DNA replication: Copying genome.

• Motility: Many cells move through self-propulsion.

• Differentiation: Some microbes modify structures to form specialized cells.

• Communication: Some microbes respond to chemical signals from other microbes.

• Evolution: Genetic changes passed to offspring over time.

Cell Size and Morphology

Size and Shape

• Morphology: Cell size and shape; 1 micrometer = one millionth of a meter.

• Prokaryotes: 0.2 micrometers to 600+ micrometers in diameter.

• Eukaryotes: Typically 5 to 100 micrometers in length.

• Surface to volume ratio: Higher in smaller cells, supporting greater nutrient and waste exchange per unit cell volume.

Major Morphologies of Prokaryotic Cells

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum: Flexible spiral

• Spirochete: Rigid spiral

• Appendaged/Irregular: Asymmetrical forms

• Clustered: Diplococci, streptococci, cubes, filaments

An Introduction to Microbial Life

Three Domains of Life

• Bacteria: Prokaryotes, usually undifferentiated single cells, 0.5-10 micrometers, 80+ phyla.

• Archaea: Prokaryotes, five well-described phyla, often extremophiles.

• Eukarya: Plants, animals, fungi, protozoa; at least six kingdoms.

• Viruses: Obligate parasites, not cells, replicate only within host, DNA or RNA genome.

Microorganisms and the Biosphere

• Earth age: 4.6 billion years

• First life: Only anaerobic microbes

• Cyanobacteria: First oxygenic phototrophs (~3.6 billion years ago)

• Plants/animals: ~0.5 billion years ago

• LUCA: Last universal common ancestor; ~2x1030 microbial cells on earth

• Microbial ecology: How microbes affect plants, animals, and the ecosystem

The Impact of Microorganisms on Human Society

Beneficial and Harmful Effects

• Agents of disease: Food and waterborne, bacterial and viral pathogens

• Microorganisms as disease agents: Control of infectious diseases, vaccination, antibiotic therapy

• Microorganisms and nutrition: Agriculture depends on microbial activities (e.g., nitrogen fixation, cellulose digestion)

• Microorganisms and food: Can cause spoilage and foodborne disease; influence harvest, storage, safety

• Microorganisms and industry: Used in pharmaceuticals, brewing, biofuels, bioremediation, and biotechnology

Microscopy and the Origins of Microbiology

Light Microscopy

• Discovery: Robert Hooke (first to describe microbes), Anton van Leeuwenhoek (first to see bacteria)

• Magnification: Ability to distinguish two adjacent objects as distinct; limit of resolution ~0.2 micrometers

• Types: Bright field, phase contrast, differential interference contrast, dark field, fluorescence

Improving Contrast in Light Microscopy

• Staining: Increases contrast; dyes bind to cellular materials

• Differential stains: Gram stain divides bacteria into Gram positive (purple/violet) and Gram negative (pink)

• Phase contrast: Improves image contrast of unstained, live cells

• Dark field: Light reaches specimen from the sides; image appears bright on dark background

• Fluorescence: Specimens fluoresce under UV; useful in clinical diagnostics

Imaging in Three Dimensions

• Differential interference contrast (DIC): Uses polarized light for 3D appearance

• Confocal scanning laser microscopy (CSLM): Uses computer-focused laser for 3D imaging

Electron Microscopy

• Uses electrons instead of visible light; much greater resolving power

• Types: Transmission electron microscopy (TEM) and scanning electron microscopy (SEM)

• TEM: Visualizes structures at molecular level; can observe intact cells (cryoEM)

• SEM: Scans specimen coated with heavy metal; produces 3D image

Microbial Cultivation and the Horizon of Microbiology

Pure and Enrichment Cultures

• Pure cultures: Isolate a single type of microorganism

• Enrichment culture: Selects microbes with specific metabolic characteristics

Pasteur and Spontaneous Generation

• Louis Pasteur: Discovered that living organisms discriminate between optical isomers; proved alcoholic fermentation is biological

• Spontaneous generation: Disproved by Pasteur; life does not arise spontaneously from non-living matter

Koch, Infectious Disease, and Pure Culture

• Robert Koch: Demonstrated link between microbes and infectious diseases; developed Koch's postulates

• Koch's postulates: Criteria for establishing causative link between microbe and disease

Discovery of Microbial Diversity

• Sergei Winogradsky: Demonstrated chemolithotrophy (energy from inorganic compounds)

• Martinus Beijerinck: Developed enrichment culture technique; discovered nitrogen fixation

Molecular Basis of Life

• Controlled growth: Ability to grow bacteria rapidly under controlled conditions

• Foundations: Led to molecular biology, genetics, and biochemistry

Additional info: These notes provide a comprehensive overview of introductory microbiology, including microbial structure, diversity, microscopy, cultivation, and the impact of microbes on society and science.