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.

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

• Microbiology is important for understanding disease, agriculture, medicine, ecology, and biotechnology.

Subfields of Microbiology

• Bacteriology: Study of bacteria.

• Virology: Study of viruses.

• Mycology: Study of fungi.

• Parasitology: Study of protozoa and parasitic worms.

• Immunology: Study of immune responses to microbes.

Major Historical Contributions

• Anton van Leeuwenhoek (1670s): First to observe and describe microorganisms using simple microscopes.

• Robert Hooke (1665): Coined the term "cell" after observing cork.

• Francesco Redi (1668): Disproved spontaneous generation for larger organisms with controlled experiments using meat and gauze.

• John Needham (1745): Claimed spontaneous generation after observing microbial growth in heated, sealed flasks.

• Lazzaro Spallanzani (1768): Showed that boiling and sealing prevented microbial growth, challenging Needham's results.

• Louis Pasteur (1850s–1860s): Disproved spontaneous generation with swan-neck flask experiments; developed pasteurization; linked microbes to fermentation and spoilage.

• Joseph Lister (1860s): Introduced antiseptic techniques in surgery, reducing infections.

• Robert Koch (1876–1882): Developed Koch's postulates to link specific microbes to diseases; discovered causes of anthrax and tuberculosis.

• Dmitri Ivanovsky (1892): Discovered viruses as filterable agents causing tobacco mosaic disease.

• Edward Jenner (1798): Developed the first vaccine (smallpox) using cowpox material.

• Paul Ehrlich (early 1900s): Developed Salvarsan, the first chemotherapeutic agent for syphilis.

• Alexander Fleming (1928): Discovered penicillin, the first true antibiotic.

Koch's Postulates

• Logical steps to establish a microbe as the cause of a disease:

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

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

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

  4. The same microbe 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.

Development of Immunology and Virology

• Ancient practices (e.g., variolation in China) and Jenner's vaccination laid the foundation for immunology.

• Pasteur developed artificial vaccines (e.g., anthrax, rabies).

• Discovery of viruses as infectious agents smaller than bacteria (Ivanovsky, Beijerinck, Loeffler & Frosch).

Chemical Principles in Microbiology

Atoms, Elements, and Compounds

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

• Elements consist of only one type of atom; cannot be broken down further.

• Compounds are substances formed from two or more elements.

• Molecular (covalent) compounds: Atoms share electrons (e.g., H2O, CO2).

• Ionic compounds: Atoms transfer electrons, forming charged ions (e.g., NaCl).

• Mixtures: Physical combinations of substances that can be separated by physical means.

Water and Solutions

• Water is a polar molecule, making it an excellent solvent for polar and ionic substances.

• "Like dissolves like": Polar solutes dissolve in polar solvents; nonpolar solutes (e.g., fats) do not dissolve well in water.

• Biological molecules (sugars, proteins) often dissolve in water, facilitating cellular processes.

Ionic Compounds and Ions in Biology

• Ions are atoms or molecules with a net charge, formed by oxidation (loss of electrons) or reduction (gain of electrons).

• Ionic compounds are important in biological structures (e.g., bones, teeth) and physiological processes (e.g., nerve impulses).

• Ions in solution are crucial for cellular function and staining techniques in microbiology.

Chemical Reactions and Energy

• Chemical reactions rearrange atoms to form new substances.

• Exergonic reactions release energy (e.g., cellular respiration).

• Endergonic reactions require energy input (e.g., photosynthesis).

pH and Buffers

• 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 maintaining cellular environments.

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

Macromolecules

• Carbohydrates: Monosaccharides (simple sugars), disaccharides (two sugars), polysaccharides (complex, 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 include 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: Domains and Kingdoms

• Three Domains: Archaea, Bacteria, Eukarya.

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

• Viruses are not classified within these domains/kingdoms due to their acellular nature.

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, tightly coiled).

• 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 lipids; Archaea use ether-linked isoprene chains.

• Eukaryotic membranes: Phospholipid bilayer with embedded proteins and sterols; may have glycocalyx.

Transport Across Membranes

• Passive transport (simple diffusion): Small, uncharged, nonpolar molecules move down concentration gradients (e.g., O2, CO2).

• Facilitated diffusion: Membrane proteins help transport polar/charged molecules (e.g., amino acids, ions).

• Active transport: Uses energy (ATP, proton gradients) to move substances against concentration gradients (e.g., Na+-K+ pump).

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

Osmotic Effects on Cells

• Isotonic: No net water movement.

• Hypotonic: Water exits cell; cell shrinks (crenation).

• Hypertonic: Water enters cell; cell may burst (lysis).

Bacterial Cell Walls

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

• Gram-positive: Thick peptidoglycan layer (up to 40 layers), retains crystal violet stain.

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

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

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

• Wall-less bacteria: (e.g., Mycoplasma) have sterols in membrane for stability.

Outer Membrane and Periplasm (Gram-negative Bacteria)

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

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

Surface Structures

• Flagella: Used for motility; structure includes basal body, hook, filament; movement via "runs" and "tumbles"; chemotaxis guides movement.

• Axial filaments: Found in spirochetes; enable spiral movement.

• Glycocalyx/capsule: Polysaccharide/protein layer; protects against desiccation, phagocytosis, aids in adherence and biofilm formation.

• Fimbriae: Short, numerous; attachment to surfaces.

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

Internal Structures

• Nucleoid: Region containing circular bacterial chromosome; supercoiled for compactness.

• Plasmids: Small, circular DNA molecules; carry nonessential genes (e.g., antibiotic resistance).

• Ribosomes: Sites of protein synthesis; 70S in prokaryotes.

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

Endospores

• Highly resistant, dormant structures formed under stress (e.g., Bacillus, Clostridium).

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

• Germination occurs when conditions improve.

Microscopy

Size Ranges and Measurement Units

• Base unit: meter (m).

• Common units: millimeter (mm, 10–3 m), micrometer (µm, 10–6 m), nanometer (nm, 10–9 m), angstrom (Å, 10–10 m).

• 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, fine/coarse focus knobs, iris diaphragm, field diaphragm, condenser, condenser height knob, 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 (NA): Higher NA yields better resolution.

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; stained specimens.

• Darkfield: Specimen appears bright against dark background; useful for thin or unstained specimens.

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

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

• Fluorescence: Detects emitted light from fluorescent dyes or proteins.

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

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

• Super-resolution: Breaks diffraction limit for finer detail.

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, can measure microbial size accurately.

Laboratory Skills (Overview)

• Preparation of slant agar, deep agar, and broth subcultures.

• Colony transfer and isolation using streak or spread methods.

• Classification of colonies by form, elevation, pigmentation, and size.

• Observation of growth patterns in broth and on agar.

Table: Comparison of Gram-Positive and Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick (up to 40 layers)	Thin (1–2 layers)
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
Periplasmic Space	Usually absent or small	Present
Sensitivity to Penicillin	High	Low
Staining	Retains crystal violet (purple)	Counterstained (pink/red)

Additional info: This table summarizes key differences relevant to staining, structure, and antibiotic sensitivity.