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

Scope and History of Microbiology

Definition and Importance of 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.

• Microbes impact human society through their roles in disease, agriculture, medicine, drug production, food, ecology, and general health.

• Microbiology is divided into basic science (understanding life processes) and applied science (practical uses in health, industry, and environment).

Major Groups of Microorganisms

• Prokaryotes: Domains Archaea and Bacteria (Eubacteria)

• Eukaryotes: Domain Eukarya (includes protozoa, algae, fungi, and some animals such as worms and arthropods)

• Non-cellular life: Viruses

Historical Milestones

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

• Louis Pasteur demonstrated that microbes cause fermentation and spoilage, and developed pasteurization.

Spontaneous Generation vs. Biogenesis

• Spontaneous generation: The belief that life arises from nonliving matter (e.g., maggots from meat).

• Francesco Redi disproved spontaneous generation for larger organisms (1668); John Needham and Lazzaro Spallanzani debated microbial generation; Pasteur's swan-neck flask experiments (1861) finally disproved spontaneous generation for microbes.

Germ Theory of Disease

• Proposed that specific microbes cause specific diseases.

• Key contributors: Agostino Bassi (fungal disease in silkworms), Pasteur (protozoan disease in silkworms), Joseph Lister (antiseptic surgery), Robert Koch (Koch's postulates for identifying disease agents).

Koch's Postulates

1. The suspected pathogen must be present in all cases of the disease and absent from healthy organisms.

2. The pathogen must be isolated and grown in pure culture.

3. The cultured pathogen must cause disease when introduced into a healthy host.

4. The same pathogen 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.

Discovery of Viruses and Immunology

• Dmitri Ivanovsky (1892) discovered viruses as agents passing through filters that retain bacteria.

• Edward Jenner developed vaccination (1798); Pasteur created artificial vaccines; Metchnikoff studied immunity; Behring developed antitoxins.

Birth of Chemotherapy

• Paul Ehrlich developed Salvarsan for syphilis ("magic bullet" concept).

• Alexander Fleming discovered penicillin (1928).

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 two or more elements.

• Molecular (covalent) compounds involve electron sharing; ionic compounds involve electron transfer and formation of 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 (e.g., fats) do not dissolve well in water.

• Biological molecules (sugars, proteins) often dissolve in water, forming solutions essential for cellular processes.

Ionic Compounds and Ions in Solution

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

• Ions dissolve readily in water and are important in biological fluids (e.g., blood, cytoplasm).

• Colored ions are used in staining techniques in microbiology.

Chemical Reactions and Energy

• Chemical reactions rearrange atoms to form new substances, involving bond breaking and formation.

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

pH and Its Biological Importance

• 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 and are important in culture media.

• pH affects protein structure and function, impacting cell viability.

Macromolecules

Carbohydrates

• Monosaccharides: Simple sugars (e.g., glucose), energy sources, and building blocks.

• Disaccharides: Two monosaccharides (e.g., sucrose = glucose + fructose).

• Polysaccharides: Long chains (e.g., starch, glycogen, cellulose, chitin, peptidoglycan).

Lipids

• Energy storage, membrane structure, and signaling molecules.

• Types: Fatty acids (saturated/unsaturated), triglycerides, phospholipids, sterols.

Proteins

• Polymers of amino acids (polypeptides) with four levels of structure:

  • Primary: Amino acid sequence

  • Secondary: Local folding (α-helix, β-sheet)

  • Tertiary: 3D shape due to side chain interactions

  • Quaternary: Multiple polypeptide chains

Nucleic Acids

• DNA: Double helix, deoxyribose sugar, bases A, G, C, T

• RNA: Single strand, ribose sugar, bases A, G, C, U

• Composed of nucleotides (phosphate, pentose sugar, nitrogenous base)

High-Energy Molecules

• ATP (adenosine triphosphate): Main energy currency in cells

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Classification: Domains and Kingdoms

• Archaea (Kingdom Archaebacteria)

• Bacteria (Kingdom Eubacteria)

• Eukarya (Kingdoms Protista, Fungi, Plantae, Animalia)

• Viruses are classified separately (not cellular life).

Prokaryotic Cell Morphology

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

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

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

• Some bacteria are pleomorphic (variable shape).

Cell Structures and Functions

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

• Prokaryotic membranes: Eubacteria use ester-linked fatty acids; Archaea use ether-linked isoprene chains.

• Eukaryotic membranes: Phospholipid bilayer with sterols and glycoproteins.

Transport Across Membranes

• Passive transport (simple and facilitated diffusion): Down concentration gradient, no energy required.

• Active transport: Against gradient, requires energy (e.g., ATP, proton motive force).

• Osmosis: Movement of water across membranes; driven by solute concentration differences (osmotic pressure).

Osmotic Conditions

Condition	Definition	Effect on Cell
Isotonic	Equal solute concentration inside and outside	No net water movement
Hypotonic	Lower solute outside than inside	Water enters cell; risk of lysis
Hypertonic	Higher solute outside than inside	Water leaves cell; risk of crenation

Bacterial Cell Walls

• Composed mainly of peptidoglycan (NAG and NAM sugars cross-linked by peptides).

• Gram-positive: Thick peptidoglycan layer (up to 40 layers), teichoic acids present.

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

• Archaea: Cell walls with N-acetyltalosaminuronic acid (not peptidoglycan), resistant to penicillin.

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

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

Gram-negative Outer Membrane and Periplasm

• Outer membrane: Contains LPS (lipid A is toxic, acts as endotoxin), porins for molecule passage.

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

Motility and Surface Structures

• Flagella: Used for movement; structure includes basal body, hook, and filament. Types: monotrichous (single), lophotrichous (tuft), peritrichous (all over).

• Runs and tumbles: Movement patterns influenced by chemotaxis (movement toward/away from chemicals).

• Axial filaments: Found in spirochetes, enable corkscrew motion.

• Glycocalyx/capsule: Protective, aids in adherence and biofilm formation.

• Fimbriae: Short, numerous, for attachment.

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

Internal Structures

• Nucleoid: Region containing circular DNA chromosome, often supercoiled.

• Plasmids: Small, circular DNA molecules with nonessential genes.

• Ribosomes: 70S type in prokaryotes, site of protein synthesis.

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

Endospores

• Highly resistant, dormant structures formed in response to stress (e.g., nutrient deprivation).

• Formation involves DNA replication, membrane invagination, cortex and coat formation, and release.

• Germination occurs 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: 20–300 nm.

Compound Light Microscope Structure

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

• Oil immersion increases resolution by matching refractive index of glass and oil.

Resolution and Magnification

• Resolution: Ability to distinguish two points as separate; limited by wavelength of light ($\lambda$).

• Maximum useful magnification: ~1500x; maximum resolution: ~0.2 µm.

• Numerical aperture: Indicates resolving power; higher values resolve finer details.

Light Interactions and Aberrations

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

• Chromatic aberration: Different colors focus at different points.

• Spherical aberration: Rays from lens edge focus differently than central rays.

Types of Light Microscopy

• Brightfield: Standard illumination.

• Darkfield: Specimen appears bright against dark background; useful for small features (e.g., flagella).

• Phase-contrast: Enhances contrast in unstained, living cells by amplifying refractive index differences.

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

• Fluorescence: Detects emitted light from fluorescent dyes.

• Confocal: Uses lasers for optical sectioning and 3D reconstruction.

• Two-photon: Infrared excitation for deeper tissue imaging.

• Super-resolution: Techniques surpassing traditional light microscopy 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, specimens coated with metal.

• 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, the ocular micrometer can be used to measure microbial size accurately.

Additional info:

• For more details on the structure and function of eukaryotic organelles, refer to standard microbiology textbooks.

• Examples of high-energy molecules include GTP, CTP, and NADH, in addition to ATP.