Module 1: Foundations of Microbiology – History, Chemical Principles, Microscopy, and Cell Anatomy

Scope and History of Microbiology

Introduction to Microbiology

Microbiology is the study of organisms and agents too small to be seen with the naked eye, including bacteria, viruses, fungi, protozoa, and some multicellular parasites. The field is essential for understanding basic life processes and has profound impacts on medicine, agriculture, ecology, and industry.

• Microbes range in size from nanometer-scale viruses to nearly visible worm eggs.

• Microbiology is divided into basic science (studying life processes) and applied science (disease, agriculture, medicine, etc.).

• Main groups: Prokaryotes (Domains Archaea and Bacteria), Eukaryotes (Domain Eukarya: protozoa, algae, fungi, animals), and Viruses (non-cellular).

Historical Milestones

• Microscopy: Invented by Janssen & Lippershey (~1600); Robert Hooke described cells (1665); Anton van Leeuwenhoek observed protozoa (1677) and bacteria (1683).

• Spontaneous Generation: Disproved by Redi (1668), Spallanzani (1768), and Pasteur (1861) through controlled experiments showing that life does not arise spontaneously from non-living matter.

• Germ Theory of Disease: Proposed that microbes cause disease. Key contributors: Bassi (fungal disease in silkworms), Pasteur (microbes in fermentation and disease), Lister (antiseptic surgery), Koch (Koch's Postulates for disease causation).

• Immunology: Jenner (smallpox vaccination, 1798), Pasteur (artificial vaccines), Metchnikoff (phagocytosis), and the development of vaccines and antitoxins.

• Discovery of Viruses: Ivanovsky (1892) and Beijerinck (1898) showed that infectious agents smaller than bacteria (viruses) could pass through filters.

• Chemotherapy: Erlich ("magic bullet" Salvarsan for syphilis), Fleming (discovery of penicillin, 1928).

Koch's Postulates

• Logical steps to identify the causative agent of a disease:

  1. The microbe must be found in all cases of the disease.

  2. It must be isolated and grown in pure culture.

  3. It must cause disease when introduced into a healthy host.

  4. It must be re-isolated from the experimentally infected host.

• Limitations: Some microbes cannot be cultured, and some diseases are not caused by a single organism.

Chemical Principles in Microbiology

Atoms, Elements, and Compounds

• Atoms are composed of protons (+), neutrons (0), and electrons (−).

• Elements consist of one type of atom; compounds are combinations of different atoms.

• Molecular (covalent) compounds involve electron sharing; ionic compounds involve electron transfer and formation of charged ions.

• Mixtures can be separated by physical means.

Molecular Compounds and Solutions

• Covalent bonds form when atoms share electrons to fill their outer shells.

• Polarity of water makes it an excellent solvent for polar and charged molecules; nonpolar molecules (like fats) do not dissolve well.

• "Like dissolves like" principle applies to solubility.

Ionic Compounds and Ions in Solution

• Ionic bonds form between ions (charged particles) created by oxidation (loss of electrons) or reduction (gain of electrons).

• Ions dissolve readily in water, important for biological fluids and staining techniques.

Chemical Reactions and Energy

• Chemical reactions rearrange atoms to form new substances.

• Exergonic reactions release energy; endergonic reactions require energy input.

pH and Biological Systems

• pH measures the concentration of hydronium ions ($[H_3O^+]$) in solution.

• Pure water: $[H_3O^+] = [OH^-]$, pH = 7 (neutral).

• Acidic: $[H_3O^+] > [OH^-]$, pH < 7; Basic: $[H_3O^+] < [OH^-]$, pH > 7.

• Buffers resist changes in pH, crucial for cell function and culture media.

Macromolecules

• Carbohydrates: Monosaccharides (simple sugars), disaccharides (two monosaccharides), polysaccharides (long chains; e.g., starch, glycogen, cellulose, peptidoglycan).

• Lipids: Fatty acids (saturated/unsaturated), triglycerides, phospholipids (membranes), sterols (hormones, membrane structure).

• Proteins: Polymers of amino acids; structure levels: primary (sequence), secondary (folding), tertiary (3D shape), quaternary (multiple chains).

• Nucleic Acids: DNA (double helix, A-T, G-C), RNA (single strand, A-U, G-C); composed of nucleotides (phosphate, sugar, base).

• High-Energy Molecules: ATP (adenosine triphosphate) stores and transfers energy in cells.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Classification of Life

• Three Domains: Archaea, Bacteria, Eukarya.

• Six Kingdoms: Archaebacteria, Eubacteria, Protista, Fungi, Plantae, Animalia.

• Viruses are not classified within these domains/kingdoms.

Prokaryotic Cell Morphology

• Cocci: Spherical (diplococci, streptococci, tetrads, sarcinae, staphylococci).

• Bacilli: Rod-shaped (diplobacilli, streptobacilli, coccobacilli).

• Spiral: Vibrio (curved rods), spirilla (rigid, corkscrew), spirochetes (flexible helix).

• Pleomorphic: Variable shape.

Cell Membranes

• Plasma membrane: Permeability barrier, site for enzymes, energy storage (proton gradients).

• Eukaryotes: Phospholipid bilayer with sterols, embedded proteins, glycocalyx.

• Prokaryotes: Eubacteria use ester-linked lipids; Archaea use ether-linked isoprene chains (monolayer).

Transport Across Membranes

• Passive transport: Simple diffusion (small, nonpolar molecules), facilitated diffusion (via specific proteins).

• Active transport: Requires energy (ATP or proton gradients) to move substances against concentration gradients.

• Osmosis: Movement of water across membranes; isotonic (no net movement), hypotonic (water leaves cell), hypertonic (water enters cell).

Bacterial Cell Walls

• Peptidoglycan: Chains of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptides.

• Gram-positive: Thick peptidoglycan layer (up to 40 layers).

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

• Archaea: Cell walls with N-acetyltalosaminuronic acid; resistant to penicillin.

• Acid-fast bacteria: Thick, lipid-rich walls (e.g., Mycobacterium).

• Wall-less forms: Mycoplasma, Ureoplasma (contain sterols in membrane).

Outer Membrane and Periplasm (Gram-negative Bacteria)

• Outer membrane: Contains LPS (lipid A is toxic, core polysaccharide, O-antigen), porins for molecule passage.

• Periplasm: Space between outer and plasma membranes; contains enzymes, chemoreceptors, binding proteins.

Surface Structures

• Flagella: Motility structures; "runs" (straight movement) and "tumbles" (direction change); chemotaxis, phototaxis, aerotaxis, magnetotaxis.

• Axial filaments: Spirochete movement.

• Glycocalyx/Capsule: Polysaccharide/protein layer for protection, adherence, biofilm formation.

• Fimbriae: Short, numerous, for attachment.

• Pili: Longer, fewer; attachment and conjugation (DNA transfer).

Internal Structures

• Nucleoid: Region containing circular DNA, supercoiled.

• Plasmids: Small, circular DNA molecules (nonessential genes).

• Ribosomes: 70S in prokaryotes (site of protein synthesis).

• Inclusions: Storage granules (glycogen, poly-β-hydroxybutyrate, sulfur granules).

• Endospores: Highly resistant, dormant structures formed under stress; germinate when conditions improve.

Microscopy

Size Ranges and Measurement Units

• Base unit: meter (m); common units: millimeter (mm), micrometer (µm), nanometer (nm), angstrom (Å).

• Human eye resolution: ~100 µm; eukaryotic cells: ~50 µm; bacteria: 1–10 µm; viruses: <1 µm.

Compound Light Microscope Structure

• Key parts: ocular lens, objective lens, arm, base, stage, stage clip, focus knobs, iris diaphragm, condenser, light source.

• Magnification up to 1500x; maximum resolution ~0.2 µm.

• Resolution depends on wavelength ($\lambda$) and numerical aperture (NA).

Light Interactions and Aberrations

• Light can be reflected, transmitted, refracted, or absorbed.

• Chromatic aberration: Different colors focus at different points.

• Spherical aberration: Edge rays focus differently than central rays.

Microscopy Techniques

• Brightfield: Standard illumination.

• Darkfield: Bright specimen on dark background; good for small features.

• Phase-contrast: Enhances contrast in unstained, living cells.

• Differential interference (Nomarski): 3D-like images.

• Fluorescence: Detects emitted light from excited chemicals.

• Confocal and two-photon microscopy: Improved resolution and depth.

• Super-resolution microscopy: Exceeds traditional light limits.

Electron and Probe Microscopy

• Transmission Electron Microscopy (TEM): Thin sections, high magnification (10,000–100,000x), internal structures.

• Scanning Electron Microscopy (SEM): Surface imaging, 3D appearance.

• Scanning Tunneling and Atomic Force Microscopy: Surface topography at atomic resolution.

Measuring Microbes

• Ocular micrometer (in eyepiece) must be calibrated with a stage micrometer (known scale) for each objective lens.

• Once calibrated, can measure microbial size accurately.

Table: Comparison of Prokaryotic and Eukaryotic Cell Structures

Feature	Prokaryotes	Eukaryotes
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Cell Wall	Peptidoglycan (Bacteria), various (Archaea)	Cellulose (plants), chitin (fungi), absent in animals
Membrane Lipids	Ester-linked (Bacteria), ether-linked (Archaea)	Ester-linked, contain sterols
Ribosomes	70S	80S (cytoplasm), 70S (mitochondria/chloroplasts)
Organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Flagella	Simple, flagellin-based	Complex, microtubule-based

Additional info:

• Some details (e.g., specific laboratory skills, advanced microscopy types) were expanded for clarity and completeness.

• For more on macromolecule structures and functions, refer to standard microbiology textbooks.