Microbiology Foundations: History, Cell Structure, Microscopy, and Metabolism

Microbiology and Its Historical Foundations

Impact of Microbiology on Human History

Microbiology, the study of microorganisms, has profoundly shaped human society through its influence on health, science, and technology. Understanding its historical context is essential for appreciating modern advances in disease control and biotechnology.

• Smallpox Epidemic (1837) and the Mandan Nation: The Variola virus caused a devastating smallpox outbreak among the Mandan and other Indigenous peoples, highlighting the catastrophic effects of infectious diseases on populations lacking prior immunity.

• Consequences: High mortality, social disruption, and long-term demographic changes.

• Significance: Demonstrates the role of microbes in shaping history and public health.

Key Figures in Microbiology

• Louis Pasteur: French chemist and microbiologist who disproved spontaneous generation, developed pasteurization, and pioneered vaccines (rabies, anthrax). He established the germ theory of disease and emphasized the role of microbes in fermentation and disease.

• Robert Koch: German physician who developed pure-culture techniques, identified causative agents of diseases (e.g., Bacillus anthracis for anthrax, Mycobacterium tuberculosis for tuberculosis, Vibrio cholerae for cholera), and formulated Koch's postulates to link specific microbes to specific diseases.

Comparison: Pasteur vs. Koch

Pasteur	Koch
Fermentation, vaccines, germ theory	Specific microbes cause specific diseases
Studied microbes broadly	Focused on bacterial pathogens
Developed pasteurization, vaccines	Developed pure-culture techniques, Koch's postulates
Rejected spontaneous generation	Rejected spontaneous generation

• Agreement: Both supported the germ theory and rejected spontaneous generation.

• Difference: Pasteur focused on microbial processes and prevention; Koch emphasized isolating and identifying pathogens.

Aseptic Surgery and Antimicrobial Therapy

• Joseph Lister: Applied germ theory to surgery, using carbolic acid (phenol) to disinfect wounds and instruments, greatly reducing surgical infections.

• Aseptic Technique: Includes sterilization, hand hygiene, sterile gloves, and environmental controls.

• Antimicrobial Therapy: Use of substances to kill or inhibit microbes. Key milestones include Ehrlich's Salvarsan (syphilis), Fleming's penicillin, Domagk's sulfonamides, and Waksman's streptomycin.

21st Century Health Challenges

• Antimicrobial resistance

• Emerging/re-emerging infectious diseases

• Global pandemics

• Climate change and disease distribution

• Healthcare-associated infections

• Vaccine hesitancy and access

• Zoonotic diseases

• Aging and immunocompromised populations

• Microbiome-related health questions

• Need for new antimicrobial drugs

Three Domains of Life

Classification and Major Characteristics

All cellular life is classified into three domains based on fundamental differences in cell structure and genetics.

Feature	Bacteria	Archaea	Eukarya
Cell type	Prokaryotic	Prokaryotic	Eukaryotic
Nucleus	No	No	Yes
Chromosome	Usually circular	Usually circular	Linear
Histones	Generally absent	Present in many	Present
Ribosomes	70S	70S	80S cytoplasmic
Membrane lipids	Ester-linked	Ether-linked	Ester-linked/branched hydrocarbons
Cell wall	Peptidoglycan	S-layer/pseudomurein/etc.	Usually no peptidoglycan
Organelles	Generally absent	Generally absent	Many membrane-bound organelles

• Prokaryote: Cell lacking a membrane-bound nucleus (Bacteria, Archaea).

• Eukaryote: Cell with a membrane-bound nucleus and organelles (Eukarya).

Microscopy and Cell Visualization

Principles of Microscopy

• Magnification: How much larger an object appears.

• Resolution: Ability to distinguish two closely spaced objects as separate; more important than magnification for detail.

Types of Light Microscopy

• Brightfield Microscopy: Visible light passes through stained specimens; best for cell shape, arrangement, and Gram staining. Typical resolution: ~0.2 μm.

• Phase-Contrast Microscopy: Enhances contrast in living, unstained cells by converting phase differences into brightness differences. Ideal for observing motility and internal structures.

• Darkfield Microscopy: Only scattered light enters the objective; specimens appear bright against a black background. Useful for thin bacteria and motility.

• Fluorescence Microscopy: Uses fluorophores that absorb and emit light at different wavelengths. Allows visualization of specific molecules, cells, or structures. Includes immunofluorescence and FISH.

Staining Techniques

• Gram Stain: Differentiates bacteria by cell envelope structure.

  • Steps: Crystal violet → Iodine → Alcohol/acetone decolorizer → Safranin counterstain

  • Gram-positive: Thick peptidoglycan, retains crystal violet (purple)

  • Gram-negative: Thin peptidoglycan, outer membrane, loses crystal violet, takes up safranin (pink/red)

• Negative Stain: Stains background, not cells; useful for capsules and natural morphology.

• Endospore Stain: Uses malachite green and heat to identify bacterial endospores.

Advanced Microscopy

• Transmission Electron Microscopy (TEM): Electrons pass through thin specimens; reveals internal ultrastructure. Resolution: ~0.1–1 nm.

• Scanning Electron Microscopy (SEM): Electrons scan the surface; provides 3D surface images. Resolution: ~1–10 nm.

• X-ray Crystallography: Determines atomic structure of macromolecules. Resolution: ~0.1 nm.

Microscopy Comparison Table

Microscopy Type	Resolution	Best Visualized Structures
Brightfield	~0.2 μm	Cell shape, stained cells
Phase Contrast	~0.2 μm	Live cells, internal structures
Darkfield	~0.2 μm	Motile cells
Fluorescence	~0.2 μm	Specific molecules, tagged cells
TEM	~0.2 nm	Internal ultrastructure
SEM	~1–10 nm	Surface topology
X-ray Crystallography	~0.1 nm	Macromolecular structure

Prokaryotic and Eukaryotic Cell Structure

Bacterial Cell Envelope

• Plasma Membrane: Phospholipid bilayer with embedded proteins; selectively permeable.

• Peptidoglycan: Mesh-like polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptides; provides rigidity.

• Gram-Positive Envelope: Thick peptidoglycan, teichoic acids, lipoteichoic acids.

• Gram-Negative Envelope: Thin peptidoglycan, periplasmic space, outer membrane with lipopolysaccharide (LPS), porins.

LPS Structure

• Lipid A: Endotoxin, embedded in outer membrane.

• Core polysaccharide and O antigen: Extend outward; O antigen is variable and important for immune evasion.

Bacterial Motility

• Flagella: Helical filaments powered by proton motive force; enable swimming.

• Periplasmic Flagella (Spirochetes): Internal bundles causing corkscrew motion.

• Gliding/Twitching Motility: Surface movement via pili or secreted slime.

Bacterial Genetics and Internal Structures

• Chromosome: Usually single, circular, high gene density, no histones.

• Plasmids: Small, circular DNA; often carry antibiotic resistance genes.

• Ribosomes: 70S (30S + 50S subunits); site of protein synthesis.

• Inclusions: Storage granules (e.g., polyhydroxybutyrate, polyphosphate, sulfur, magnetite).

• Thylakoids: Internal membranes for photosynthesis in cyanobacteria.

Archaeal Cell Structure

• Membrane Lipids: Ether-linked phytanyl chains (diether bilayer or tetraether monolayer) for stability.

• Cell Wall: Pseudomurein or S-layer (protein lattice); no peptidoglycan.

• Chromosomes: Usually circular, often with histones.

• Archaella: Motility structures powered by ATP.

Eukaryotic Cell Structure

• Nucleus: Membrane-bound compartment with linear chromosomes and histones.

• Endomembrane System: Includes rough ER, Golgi apparatus, vesicles; involved in protein secretion.

• Organelles: Mitochondria (energy), lysosomes (digestion), chloroplasts (photosynthesis).

• Cytoskeleton: Microtubules, microfilaments, intermediate filaments for structure and movement.

• Cilia and Flagella: Complex, ATP-powered motility structures.

Microbial Metabolism and Energy

Energy, Enzymes, and Thermodynamics

Microbial metabolism is governed by the principles of energy transformation and enzyme catalysis.

• First Law of Thermodynamics: Energy cannot be created or destroyed.

• Second Law of Thermodynamics: Entropy (disorder) increases in spontaneous processes.

• Enthalpy (H): Total heat content.

• Entropy (S): Measure of disorder.

• Free Energy (G): Energy available to do work.

• Exergonic Reaction: Releases energy; spontaneous ($\Delta G < 0$).

• Endergonic Reaction: Requires energy input; non-spontaneous ($\Delta G > 0$).

• Activation Energy: Energy required to initiate a reaction.

• Catalyst/Enzyme: Increases reaction rate by lowering activation energy; not consumed in the reaction.

Enzyme Activity

• Optimal at specific pH and temperature; extremes can denature enzymes.

• Enzymes do not change $\Delta G$ or reaction equilibrium.

Redox Reactions and Electron Carriers

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox Reaction: Coupled oxidation-reduction process.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain.

• Reduction potential (E°): Tendency to accept electrons.

• Standard free-energy change: $\Delta G^\circ = -nF\Delta E^\circ$

• NAD+ / NADH and FAD / FADH2: Electron carriers in respiration.

Cellular Respiration

Glycolysis

• Converts glucose to pyruvate in the cytoplasm.

• Products per glucose: 2 pyruvate, 2 NADH, net 2 ATP.

• Substrate-level phosphorylation: Direct ATP formation by transfer of phosphate to ADP.

• Fermentation: Regenerates NAD+ in the absence of oxygen (e.g., lactic acid, ethanol).

Citric Acid Cycle (Krebs/TCA Cycle)

• Oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP/GTP.

• Location: Cytoplasm (prokaryotes), mitochondrial matrix (eukaryotes).

• Major products per acetyl-CoA: 2 CO2, 3 NADH, 1 FADH2, 1 ATP/GTP.

Electron Transport Chain (ETC) and Oxidative Phosphorylation

• Membrane-associated electron carriers transfer electrons, generating a proton gradient.

• Proton motive force: Drives ATP synthesis via ATP synthase.

• Terminal electron acceptor: Oxygen in aerobic respiration; alternatives in anaerobic respiration.

Mitochondrial ETC Complexes

Complex	Function	Proton Pumping
I (NADH dehydrogenase)	Receives electrons from NADH	Yes
II (Succinate dehydrogenase)	Receives electrons from FADH2	No
III (Cytochrome bc1)	Transfers electrons to cytochrome c	Yes
IV (Cytochrome c oxidase)	Transfers electrons to O2	Yes
ATP synthase	Uses proton flow to make ATP	N/A

Nutritional Categories and Element Acquisition

• Chemoheterotroph: Energy and carbon from organic compounds.

• Chemoautotroph: Energy from inorganic compounds, carbon from CO2.

• Photoautotroph: Energy from light, carbon from CO2.

• Photoheterotroph: Energy from light, carbon from organic compounds.

CHONPS Elements

• C: Carbon

• H: Hydrogen

• O: Oxygen

• N: Nitrogen

• P: Phosphorus

• S: Sulfur

Microbes acquire these elements from various sources depending on their metabolic type. Selective media can be designed to exploit these requirements.

Anaerobic Respiration and Chemolithotrophy

• Anaerobic Respiration: Uses electron acceptors other than oxygen (e.g., nitrate, sulfate).

• Denitrification: NO3- → NO2- → NO → N2O → N2

• Sulfate Respiration: SO42- as acceptor; produces H2S.

• Chemolithotrophy: Energy from oxidation of inorganic compounds (e.g., H2, NH3, Fe2+, H2S).

• Free Energy Calculation: $\Delta G^\circ = -nF\Delta E^\circ$

Ammonia Oxidation (Nitrification)

• Example: Nitrosomonas oxidizes NH3 to NO2- via ammonia monooxygenase and hydroxylamine oxidoreductase.

Phototrophy and Autotrophy

• Phototroph: Captures light energy for cellular processes.

• Oxygenic Photosynthesis: Uses water as electron donor, produces O2 (e.g., cyanobacteria, chloroplasts).

• Anoxygenic Photosynthesis: Uses other electron donors (e.g., H2S), does not produce O2.

Electron Transport in Cyanobacteria and Chloroplasts

• Light excites Photosystem II (PSII), which splits water; electrons flow through plastoquinone, cytochrome bf, plastocyanin, Photosystem I (PSI), ferredoxin, and ferredoxin–NADP reductase (FNR) to make NADPH.

• Proton gradient across thylakoid membrane drives ATP synthase.

• Cyclic electron flow around PSI can generate extra ATP without NADPH or O2 production.

Autotrophy and Carbon Fixation

• Autotroph: Uses CO2 as carbon source.

• Calvin Cycle: Fixes CO2 into organic molecules; key enzyme is RuBisCO.

• Reductive Citric Acid Cycle: Fixes CO2 using reverse steps of Krebs cycle.

Key Terms and Definitions

• Microbiology: Study of microorganisms (bacteria, archaea, fungi, protozoa, algae, viruses).

• Germ Theory: Specific microorganisms cause specific diseases.

• Koch's Postulates: Criteria to establish a causal relationship between a microbe and a disease.

• Pasteurization: Controlled heating to reduce harmful microbes in food/beverages.

• Aseptic Surgery: Practices to prevent microbial contamination during surgery.

• Antimicrobial Therapy: Use of agents to treat microbial infections.

• Antibiotic: Substance produced by microbes that inhibits/kills other microbes.