Core Concepts in Microbiology: Study Guide Based on Lecture Topics

Lecture 2: Foundations of Microbiology

Important Microbiologists and Their Contributions

Microbiology has been shaped by the work of many scientists whose discoveries laid the foundation for the field.

• Louis Pasteur: Disproved spontaneous generation, developed pasteurization, and contributed to vaccine development.

• Robert Koch: Established Koch's postulates, linking specific microbes to specific diseases.

• Other notable figures: Antonie van Leeuwenhoek (first observations of microbes), Joseph Lister (antiseptic surgery), Alexander Fleming (discovery of penicillin).

Example: Koch's work with Bacillus anthracis demonstrated that a specific bacterium causes anthrax.

Germ Theory of Disease

The germ theory states that many diseases are caused by microorganisms. This was a revolutionary idea that replaced earlier beliefs in miasma or 'bad air' as the cause of disease.

• Key Point: Microorganisms (bacteria, viruses, fungi, protozoa) can invade hosts and cause disease.

• Application: Led to improved hygiene, sterilization, and the development of antibiotics.

Koch's Postulates

Koch's postulates are a set of criteria used to establish a causative relationship between a microbe and a disease.

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

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

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

• 4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Microscopy

Microscopy is essential for visualizing microorganisms, which are too small to be seen with the naked eye.

• Types of Main Problems: Problems may include calculating magnification, resolution, and interpreting images (equations not required here).

• Types of Microscopes:

  • Light Microscope: Uses visible light to observe specimens; suitable for most bacteria and larger microbes.

  • Electron Microscope: Uses electron beams for much higher resolution; allows visualization of viruses and subcellular structures.

• Stains and Staining: Staining increases contrast to make cells and structures visible. Common stains include Gram stain, acid-fast stain, and endospore stain.

Lectures 3-6: Cell Structure, Classification, and Diversity

Prokaryotes vs. Eukaryotes

Cells are classified as prokaryotic or eukaryotic based on their structural features.

• Prokaryotes: Lack a nucleus and membrane-bound organelles; include Bacteria and Archaea.

• Eukaryotes: Have a nucleus and membrane-bound organelles; include fungi, protozoa, algae, and animals.

• Comparison Table:

Feature	Prokaryotes	Eukaryotes
Nucleus	No	Yes
Organelles	No	Yes
Cell Wall	Usually (peptidoglycan in Bacteria)	Sometimes (cellulose in plants, chitin in fungi)
Size	1-10 μm	10-100 μm

Timeline of Microbial Life

Microbial life has existed for billions of years, with prokaryotes appearing first, followed by eukaryotes.

• Primitive Cells: Likely simple, anaerobic, and heterotrophic.

• Evolution: Photosynthetic and oxygen-producing microbes (cyanobacteria) changed Earth's atmosphere.

Macromolecules in Cells

Cells are composed of four major types of macromolecules, each with specific functions.

• Proteins: Enzymes, structural components, transporters.

• Nucleic Acids: DNA and RNA; store and transmit genetic information.

• Carbohydrates: Energy storage, structural support.

• Lipids: Membrane structure, energy storage.

Phylogeny vs. Taxonomy

Phylogeny is the evolutionary history and relationships among organisms, while taxonomy is the classification and naming of organisms.

• Phylogenetic trees show evolutionary relationships based on genetic or morphological data.

• Taxonomic ranks: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Gram Stain

The Gram stain differentiates bacteria based on cell wall structure.

• Gram-positive: Thick peptidoglycan layer; stains purple.

• Gram-negative: Thin peptidoglycan layer and outer membrane; stains pink/red.

• Steps: Crystal violet, iodine, alcohol decolorization, safranin counterstain.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a dynamic combination of lipids and proteins.

• Membranes are flexible and proteins can move laterally within the lipid bilayer.

• Integral proteins span the membrane; peripheral proteins are attached to the surface.

Bacterial Cell Structure

Bacterial cells have various internal and external structures that contribute to their function and survival.

• Cell Wall: Provides shape and protection; made of peptidoglycan in bacteria.

• Capsule: Protective outer layer; aids in evasion of host defenses.

• Flagella: Used for motility; arrangement can help identify species.

• Pili/Fimbriae: Attachment to surfaces and other cells.

• Endospores: Highly resistant structures formed by some bacteria for survival in harsh conditions.

Bacterial Cell Shapes and Arrangements

Bacteria exhibit a variety of shapes and arrangements, which can be used for identification.

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spirilla: Spiral-shaped

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-)

Extremophiles

Extremophiles are organisms that thrive in extreme environments.

• Thermophiles: High temperature

• Halophiles: High salt concentration

• Acidophiles: Low pH

• Barophiles: High pressure

Domains of Life: Bacteria, Archaea, Eukarya

All life is classified into three domains based on genetic and biochemical differences.

• Bacteria: Prokaryotic, diverse metabolic pathways.

• Archaea: Prokaryotic, often extremophiles, unique membrane lipids.

• Eukarya: Eukaryotic, includes plants, animals, fungi, and protists.

Endosymbiotic Theory

This theory proposes that eukaryotic organelles such as mitochondria and chloroplasts originated from symbiotic prokaryotes.

• Supported by similarities in DNA, ribosomes, and reproduction between organelles and bacteria.

Fungal and Algal Biology

Fungi and algae are important groups of eukaryotic microbes.

• Fungi: Decomposers, some cause disease (e.g., white nose syndrome in bats).

• Algae: Photosynthetic, produce oxygen, form the base of aquatic food webs.

Viruses

Viruses are acellular infectious agents that require host cells to replicate.

• Basic structure: nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).

• Some have an envelope derived from host membranes.

Bacteriophages

Bacteriophages are viruses that infect bacteria. They have different life cycles (lytic and lysogenic) and play roles in gene transfer and bacterial population control.

Lecture 7: Culturing Bacteria and Microbial Nutrition

Culturing Techniques and Media

Microbiologists use various types of media to grow and study microbes.

• Solid/liquid/semisolid media: Used for different experimental purposes.

• Defined (synthetic) media: Exact chemical composition is known.

• Complex media: Contains unknown components (e.g., extracts, peptones).

• Differential media: Distinguishes between organisms based on metabolic traits.

• Selective media: Inhibits growth of some microbes while allowing others to grow.

Culturable vs. Unculturable Microbes

Not all microbes can be grown in the laboratory. Culturable microbes can be isolated and studied on artificial media, while unculturable microbes require alternative techniques such as metagenomics.

• Metagenomics: The study of genetic material recovered directly from environmental samples.

Trophic Categories and Nutritional Types

Microbes are classified based on their energy and carbon sources.

Category	Energy Source	Carbon Source	Example
Photoautotroph	Light	CO2	Cyanobacteria
Chemoautotroph	Inorganic chemicals	CO2	Nitrifying bacteria
Photoheterotroph	Light	Organic compounds	Some purple non-sulfur bacteria
Chemoheterotroph	Organic compounds	Organic compounds	Most bacteria, fungi, animals

Example: A cyanobacterium that performs photosynthesis is a photoautotroph.