Foundations of Microbiology: History, Cell Structure, and Function

History and Development of Microbiology

Early Years: 17th – 18th Century

The study of microbiology began with the observation and classification of microorganisms. Key figures contributed to the development of taxonomy and the understanding of microbial diversity.

• Leeuwenhoek: Observed several categories of microorganisms using early microscopes.

• Carolus Linnaeus: Developed a taxonomic system for grouping organisms.

• Bacteria/Archaea: Prokaryotic

  • No nucleus

  • Small, single cell

  • Asexual reproduction

  • Archaea: Differ in cell wall, live in extreme environments

• Fungi: Eukaryotic

  • Include protozoa (animal-like protists)

  • Single or multicellular

  • Sexual and asexual reproduction

  • Some are parasites

• Algae: Eukaryotic

  • Plant-like protists

  • Single and multicellular

  • Photosynthetic

• Small Multicellular Animals: Various forms

• Viruses: Acellular

  • Very small

  • Composed of protein + nucleic acid (+ sometimes lipid)

  • Obligate parasites: replicate only if host cell is present

Golden Age: 19th – 20th Century

Major advances in microbiology occurred, including the refutation of spontaneous generation and the development of germ theory.

• Spontaneous Generation: Louis Pasteur disproved the idea that microbes arise spontaneously using swan-neck flask experiments.

• Fermentation: Pasteur demonstrated that yeast ferments grape juice into alcohol, while bacteria produce acid.

• Industrial Microbiology: Began with the use of microbes in fermentation and other processes.

• Germ Theory of Disease: Pasteur proposed that specific diseases are caused by specific pathogens.

• Koch's Postulates:

  1. Suspected causative agent must be found in every case of the disease and absent from healthy hosts.

  2. Agent must be isolated and grown outside the host.

  3. When agent is introduced to a healthy, susceptible host, the host must get the disease.

  4. Same agent must be found in the diseased experimental host.

• Prevention of Infection:

  • Semmelweis: Hand washing

  • Lister: Antisepsis

  • Jenner: Smallpox vaccine

  • Snow: Discovered cholera transmission

Modern Age of Microbiology: 20th Century Onward

Microbiology expanded to include molecular biology, genetics, and environmental studies.

• Study of chemical reactions of life

• Genetics: How genes work (molecular biology, recombinant DNA technology, gene therapy)

• Role of microorganisms in the environment

• Defending against disease: Immunology, chemotherapy

• Challenges: Emerging/reemerging diseases, antibiotic resistance, genetically modified microbes, understanding human microbiome

Cell Structure and Function

Basic Characteristics of Life

Microorganisms exhibit fundamental characteristics of living things, including growth, reproduction, responsiveness, metabolism, and cellular structure.

• Growth: Increase in size

  • Bacteria/archaea/eukaryotes: occurs in all

  • Viruses: do not grow

• Reproduction: Production of new cells

  • Bacteria/archaea/eukaryotes: occurs in all

  • Viruses: host cell replicates the virus

• Responsiveness: Ability to react to environmental stimuli

  • Bacteria/archaea/eukaryotes: occurs in all

  • Viruses: responsiveness only in host cell

• Metabolism: Controlled chemical reactions

  • Bacteria/archaea/eukaryotes: occurs in all

  • Viruses: use host metabolism

• Cellular Structure: Membrane-bound structure capable of all above functions

  • Bacteria/archaea/eukaryotes: present in all

  • Viruses: lack cytoplasmic membrane/cellular structure

Prokaryotes vs. Eukaryotes

Cells are classified based on the presence or absence of a nucleus and internal structures.

• Prokaryotes:

  • Lack nucleus

  • Can read DNA and make protein simultaneously (faster response time)

  • Lack various internal structures bound with phospholipid membranes (e.g., mitochondria)

  • Typically 1.0 μm or smaller

  • Domains: Bacteria and Archaea

• Eukaryotes:

  • Have nucleus

  • Have internal membrane-bound organelles

  • Larger (10–100 μm)

  • More complex structure

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

Typical Prokaryotic Cell Structures

• In all: ribosome, cytoplasm, nucleoid, plasma membrane

External Structures in Bacterial Cells

• Glycocalyx: Sugar coat found in some bacteria; gelatinous, sticky substance surrounding outside of cell; composed of polysaccharides, polypeptides, or both.

• Slime Layer: Loosely attached to cell surface; sticky, water-soluble layer; facilitates attachment; prevents desiccation.

• Capsule: Firmly attached to cell surface; composed of organized repeating units of organic chemicals; facilitates attachment; may prevent bacteria from being recognized by host.

Flagella

Flagella are long structures that extend beyond the cell surface and are used for motility.

• Types:

  • Monotrichous: one flagellum

  • Amphitrichous: one on each end

  • Lophotrichous: small bundle

  • Peritrichous: all around

• Structure: Composed of filament (tail), hook, and basal body (in membrane)

• Movement: Rotates 360°

  • CW: tumble

  • CCW: run

• Function: Move in response to stimuli (taxis)

• Axial filaments: Specialized flagella

Pili (Fimbriae)

• Found in some bacteria; short, sticky, bristle-like projections made of protein; used to adhere to other bacteria and surfaces (important in biofilms).

• Conjugation pili: Longer, specialized structures for DNA transfer.

Bacterial Cell Wall

The cell wall provides structure, shape, and protection from osmotic forces. It is composed of peptidoglycan, which is unique to bacteria.

• Gram Positive: Thick layer of peptidoglycan; contains teichoic acids; appears purple in Gram staining.

• Gram Negative: Thin layer of peptidoglycan; outer membrane contains phospholipids, proteins, and lipopolysaccharide (LPS); appears red/pink in Gram staining.

  • Has periplasmic space between inner and outer membrane.

Effect of Isotonic/Hypertonic/Hypotonic Solutions on Cells

Solution Type	With Cell Wall	Without Cell Wall
Isotonic	No effect	No effect
Hypertonic	Water leaves cell; cell wall: no effect	Water leaves cell; cell shrivels
Hypotonic	Water enters cell; cell wall: no effect	Water enters cell; cell bursts

Bacterial Cytoplasmic Membrane

• Structure: Phospholipid bilayer; composed of lipids and associated proteins (integral and peripheral).

• No cholesterol.

Cytoplasm of Bacteria

• Cytosol: Liquid portion; contains DNA region (nucleoid).

• Inclusions: Found in some bacteria; may include reserve deposits of chemicals.

• Endospores: Found in 2 genera of Gram-positive bacteria (Bacillus and Clostridium); defensive strategy against unfavorable conditions; resistant to extreme conditions (heat, radiation, chemicals); survival mechanism.

  • Formation: DNA replicated → cytoplasmic membrane invaginates to make forespore → membrane grows and engulfs forespore with second membrane → vegetative cell's DNA disintegrates → cortex of peptidoglycan deposited between membranes; dipicolinic acid and Ca2+ ions accumulate in cortex → endospore coat forms around endospore → endospore matures; completion of spore coat increases resistance to heat and chemicals → endospore released from original cell.

• Nonmembranous Organelles:

  • Cytoskeleton: In all bacteria; composed of 3/4 types of protein fibers; play roles in cell division, shape, segregation of DNA molecules, movement through environment.

  • Ribosomes: In all bacteria; 70S (small 30S and large 50S subunits).

External Structure of Eukaryotes

• Some have glycocalyx: Not as organized as capsules; anchor animal cells to each other; strengthen cell surface; provide protection against dehydration; function in cell recognition/communication.

Eukaryotic Cell Wall

• Fungi, algae, and plants have cell walls composed of various polysaccharides:

  • Cellulose (plants)

  • Chitin (fungi)

  • Algal cell walls: variety of polysaccharides

Eukaryotic Cytoplasmic Membrane

• All eukaryotic cells have a fluid mosaic of phospholipids and proteins.

• Contain steroid lipids to help maintain fluidity.

• Active transport processes only found in eukaryotes: endo/exocytosis.

Cytoplasmic Contents of Eukaryotes

• Flagella: Some have; differ structurally/functionally from prokaryotic flagella; composed of tubulin arranged to form microtubules; anchored to cell by basal body; no hook; movement undulates rhythmically.

• Cilia: Some have; shorter and more numerous than flagella; coordinated beating propels cell through environment; used to move substances past surface of the cell.

• Other Non-membranous Organelles:

  • Ribosomes: Larger than prokaryotic (80S; small 40S and large 60S subunits).

  • Cytoskeleton: Extensive network of microtubules, anchors organelles, produces basic shape of cell; made up of tubulin microtubules, actin microfilaments, and intermediate filaments; play role in mitosis, cytokinesis, formation of flagella/cilia; centromere: region of cytoplasm where centrioles are found.

• Membranous Organelles:

  • Nucleus: Contains most of cell's DNA; semiliquid portion called nucleoplasm; contains chromatin; RNA synthesized in nucleoli present in nucleoplasm; surrounded by nuclear envelope.

  • ER (Endoplasmic Reticulum): Netlike arrangement of flattened, hollow tubules continuous with nuclear envelope; functions as transport system.

  • Golgi Body: Receives, processes, and packages large molecules in secretory vesicles that fuse with membrane for export from cell; composed of flattened hollow sacs surrounded by phospholipid bilayer.

  • Lysosomes, Peroxisomes, Vacuoles, Vesicles: Store and transfer chemicals within cells; may store nutrients in cell; lysosomes contain catabolic enzymes; peroxisomes contain enzymes that degrade poisonous wastes.

  • Mitochondria: Have 2 membranes; produce most of cell's ATP; interior matrix contains 70S ribosomes + circular molecule of DNA.

• Endosymbiotic Theory: Eukaryotes formed from union of small aerobic prokaryotes with larger anaerobic prokaryotes.

  • Smaller prokaryotes became internal parasites; lost ability to exist independently; larger cell became dependent on parasites for aerobic ATP production; aerobic prokaryotes evolved into mitochondria; photosynthetic prokaryotes evolved into chloroplasts.

Additional info: These notes cover foundational concepts in microbiology, including the history of the field, cell structure and function, and the differences between prokaryotic and eukaryotic cells. They are suitable for exam preparation and provide context for further study in microbial metabolism, genetics, and physiology.