Introduction to Microbiology

Prokaryotes vs. Eukaryotes

Microorganisms are classified as either prokaryotic or eukaryotic based on their cellular structure.

• Prokaryotes: Cells lacking a true nucleus and membrane-bound organelles. Their DNA is located in a nucleoid region. Examples: Bacteria, Archaea.

• Eukaryotes: Cells with a true nucleus enclosed by a nuclear membrane and various membrane-bound organelles. Examples: Fungi, Protozoa, Algae, Animals, Plants.

Cell-Based Organisms vs. Viruses

• Cell-Based Organisms: Composed of one or more cells, capable of independent metabolism and reproduction.

• Viruses: Acellular entities, consisting of genetic material (DNA or RNA) surrounded by a protein coat. They require a host cell for replication and lack independent metabolism.

Key Scientists in Microbiology

• Antoni van Leeuwenhoek: First to observe and describe microorganisms using a simple microscope.

• Francesco Redi: Disproved spontaneous generation for larger organisms with meat and maggot experiments.

• Louis Pasteur: Disproved spontaneous generation for microbes using swan-neck flask experiments; developed pasteurization.

• Robert Koch: Established Koch's postulates, linking specific microbes to specific diseases.

• Ignaz Semmelweis: Advocated handwashing to prevent puerperal fever.

• Joseph Lister: Introduced antiseptic techniques in surgery.

• Edward Jenner: Developed the first vaccine (smallpox).

• Hans Christian Gram: Developed the Gram stain for bacterial classification.

Theories of Spontaneous Generation vs. Biogenesis

• Spontaneous Generation: The belief that life arises from non-living matter.

• Biogenesis: The principle that life arises only from pre-existing life.

• Key Experiments: Redi's meat experiment (macroscopic), Pasteur's swan-neck flask (microscopic) supported biogenesis.

Germ Theory of Disease

• States that specific diseases are caused by specific microorganisms.

• Contributions from Pasteur (fermentation, spoilage) and Koch (postulates) were foundational.

Chemical Principles in Microbiology

Covalent vs. Ionic Bonds

• Covalent Bonds: Atoms share electron pairs. Example: H2O.

• Ionic Bonds: Electrons are transferred from one atom to another, creating charged ions. Example: NaCl.

Hydrogen Bonds and Water

• Hydrogen Bond: Weak attraction between a hydrogen atom (partially positive) and an electronegative atom (e.g., O, N).

• Hydrogen bonding gives water its high boiling point, surface tension, and solvent properties.

pH, Acids, Bases, and Buffers

• pH: Measure of hydrogen ion concentration;

• Acid: Substance that donates H+ ions (pH < 7).

• Base: Substance that accepts H+ ions (pH > 7).

• Buffer: Compound that stabilizes pH by absorbing or releasing H+.

Calculating pH

• Given [H+], use

• Example: If [H+] = 1 x 10-4 M,

Elements in Organic Compounds

• Major elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Sulfur (S)

Types of Lipids

• Fats (Triglycerides): Energy storage (e.g., animal fat).

• Phospholipids: Major component of cell membranes.

• Steroids: Structural (cholesterol) and signaling roles.

• Waxes: Protective coatings (e.g., on leaves, bacterial cell walls).

Phospholipid Structure and Amphipathic Nature

• Composed of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group (hydrophilic).

• Amphipathic: Molecules with both hydrophilic and hydrophobic regions, allowing formation of bilayers as barriers in membranes.

Carbohydrates

• Composed of C, H, O; general formula (CH2O)n.

• Monosaccharide: Single sugar unit (e.g., glucose).

• Disaccharide: Two monosaccharides joined (e.g., sucrose).

• Polysaccharide: Many monosaccharides (e.g., starch, cellulose).

• Isomer: Molecules with same formula but different structure (e.g., glucose and galactose).

Amino Acids and Proteins

• Amino Acid: Building block of proteins, contains amino group, carboxyl group, and side chain (R group).

• Protein Structure:

  • Primary: Sequence of amino acids

  • Secondary: Alpha helices and beta sheets

  • Tertiary: 3D folding

  • Quaternary: Multiple polypeptides

• Protein Functions: Enzymes, structural, transport, signaling, defense.

Nucleic Acids and ATP

• DNA: Double-stranded, deoxyribose sugar, stores genetic information.

• RNA: Single-stranded, ribose sugar, involved in protein synthesis.

• ATP: Adenosine triphosphate, main energy currency of the cell.

Anatomy of Bacterial Cells

Comparing Eukaryotic and Bacterial Cell Structures

• Cell Wall: Present in bacteria (peptidoglycan), some eukaryotes (cellulose, chitin).

• Organelles: Absent in bacteria; present in eukaryotes (nucleus, mitochondria, etc.).

• Glycocalyx: Outer coating for protection/adhesion; present in both, but structure varies.

• Membrane Transport: Both use selective permeability, but mechanisms differ.

• Ribosomes: 70S in bacteria, 80S in eukaryotes.

• Flagella: Simpler in bacteria; complex in eukaryotes.

Bacterial Shapes and Arrangements

• Cocci: Spherical

• Bacilli: Rod-shaped

• Spirilla: Spiral-shaped

• Arrangements: chains (strepto-), clusters (staphylo-), pairs (diplo-)

Capsule vs. Slime Layer

• Capsule: Organized, firmly attached glycocalyx; protects against phagocytosis.

• Slime Layer: Loosely attached, unorganized; aids in adherence.

Fimbriae and Pili

• Fimbriae: Short, numerous; attachment to surfaces.

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

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-Positive: Thick peptidoglycan, teichoic acids, stains purple.

• Gram-Negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink.

Gram Stain Mechanism

• Crystal violet stains all cells; iodine forms complex; alcohol decolorizes Gram-negative; safranin counterstains Gram-negative.

Unique Bacterial Genera

• Mycoplasma: Lacks cell wall.

• Mycobacterium: Waxy cell wall (mycolic acid), acid-fast.

Membrane Proteins and Permeability

• Peripheral Protein: Loosely attached to membrane surface.

• Integral Protein: Embedded within membrane.

• Selective Permeability: Only certain molecules cross membrane.

Osmosis and Tonicity

• Hypotonic: Lower solute outside; water enters cell.

• Hypertonic: Higher solute outside; water leaves cell.

• Isotonic: Equal solute; no net water movement.

• Osmotic Pressure: Force exerted by water movement.

Membrane Transport Mechanisms

• Simple Diffusion: Passive movement down concentration gradient.

• Facilitated Diffusion: Passive, via transport proteins.

• Osmosis: Diffusion of water.

• Active Transport: Requires energy (ATP).

• Group Translocation: Substance chemically modified during transport (unique to prokaryotes).

Cell Wall and Osmotic Pressure

• Cell wall prevents lysis in hypotonic environments.

Internal Structures: Eukaryotes vs. Bacteria

• Eukaryotes: Nucleus, mitochondria, ER, Golgi, etc.

• Bacteria: Nucleoid, plasmids, ribosomes, inclusions.

Plasmids

• Small, circular DNA molecules; carry non-essential genes (e.g., antibiotic resistance).

Endospores

• Dormant, resistant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Formed in response to harsh conditions (sporulation); germinate when conditions improve.

Microscopy, Staining, and Classification

Key Terms in Microscopy

• Electromagnetic Spectrum: Range of all types of electromagnetic radiation.

• Magnification: Increase in apparent size of an object.

• Resolution: Ability to distinguish two points as separate.

• Contrast: Difference in light intensity between specimen and background.

Compound Light Microscope Components

• Ocular lens, objective lenses, stage, condenser, light source, coarse/fine focus knobs.

Total Magnification

• Calculated as:

Refractive Index and Oil Immersion

• Refractive Index: Measure of how light bends as it passes through substances.

• Oil immersion (100x objective) reduces light refraction, improving resolution.

Types of Light Microscopes

• Compound Light: General use, stained specimens.

• Phase-Contrast: Enhances contrast in unstained cells.

• Fluorescence: Uses fluorescent dyes; detects specific structures.

Specimen Preparation and Staining

• Fixation, staining (basic or acidic dyes), mounting.

• Basic Dyes: Positively charged; bind to negatively charged cell components.

• Acidic Dyes: Negatively charged; stain background (negative staining).

Differential and Structural Stains

• Differential Stains: Distinguish cell types (e.g., Gram, acid-fast).

• Structural Stains: Highlight specific structures (e.g., capsule, endospore).

• Gram Stain: Differentiates Gram-positive and Gram-negative bacteria.

• Acid-Fast Stain: Identifies mycobacteria.

• Capsule Stain: Visualizes capsules.

• Endospore Stain: Detects endospores.

Electron Microscopy

• Transmission Electron Microscope (TEM): Internal structures, high resolution.

• Scanning Electron Microscope (SEM): Surface structures, 3D images.

Microbial Metabolism

Key Terms

• Metabolism: All chemical reactions in a cell.

• Catabolism: Breakdown of molecules, releases energy.

• Anabolism: Synthesis of molecules, requires energy.

• Catalyst: Substance that speeds up reactions without being consumed.

• Activation Energy: Energy required to start a reaction.

• Denaturation: Loss of protein structure/function due to environmental changes.

• Redox Reaction: Transfer of electrons (reduction = gain, oxidation = loss).

• ATP: Main energy carrier.

• Substrate Level Phosphorylation: Direct transfer of phosphate to ADP.

• Oxidative Phosphorylation: ATP synthesis via electron transport chain.

• Electron Transport Chain: Series of proteins transferring electrons to generate ATP.

• Proton Motive Force: Electrochemical gradient driving ATP synthesis.

• Chemiosmosis: Movement of protons across membrane to generate ATP.

Endergonic vs. Exergonic Reactions

• Endergonic: Require energy input.

• Exergonic: Release energy.

Cofactor vs. Coenzyme

• Cofactor: Inorganic ion required for enzyme activity (e.g., Mg2+).

• Coenzyme: Organic molecule assisting enzymes (e.g., NAD+).

Enzyme-Substrate Complex

• "Lock and key" model: Enzyme's active site fits specific substrate.

Factors Affecting Enzyme Activity

• Temperature: High/low temps can denature enzymes.

• pH: Extreme pH alters enzyme structure.

• Saturation: All active sites occupied at high substrate concentration.

• Competitive Inhibitor: Binds active site, blocks substrate.

• Noncompetitive Inhibitor: Binds elsewhere, changes enzyme shape.

Glycolysis Pathways in Prokaryotes

• Embden-Meyerhof-Parnas (EMP): Main glycolytic pathway; produces ATP and NADH.

• Entner-Doudoroff (ED): Alternative; produces ATP, NADH, and NADPH.

• Pentose Phosphate Pathway (PPP): Produces NADPH and pentoses for biosynthesis.

Aerobic Respiration Pathways

• Glycolysis: Cytoplasm (both cell types); 2 ATP, 2 NADH per glucose.

• Krebs Cycle: Mitochondrial matrix (eukaryotes), cytoplasm (prokaryotes); 2 ATP, 6 NADH, 2 FADH2 per glucose.

• Electron Transport Chain: Inner mitochondrial membrane (eukaryotes), plasma membrane (prokaryotes); ~34 ATP per glucose, requires O2.

Overall Aerobic Respiration Equation

Aerobic vs. Anaerobic Respiration

• Aerobic: Final electron acceptor is O2.

• Anaerobic: Final electron acceptor is an inorganic molecule other than O2 (e.g., NO3-).

Fermentation

• Occurs after glycolysis when no electron transport chain is available.

• Types: Lactic acid fermentation, alcoholic fermentation.

• Final electron acceptor is an organic molecule.

Catabolism vs. Anabolism

• Catabolism: Degradative, energy-yielding.

• Anabolism: Biosynthetic, energy-consuming.

Metabolic Classifications

• Photoautotroph: Light energy, CO2 carbon source.

• Photoheterotroph: Light energy, organic carbon source.

• Chemoautotroph: Inorganic chemicals, CO2 carbon source.

• Chemoheterotroph: Organic chemicals, organic carbon source.

Microbial Growth

Growth Temperature and pH Classifications

• Psychrophile: Grows best at <15°C.

• Psychrotroph: Grows at 0–30°C.

• Mesophile: Grows at 20–45°C.

• Thermophile: Grows at 45–80°C.

• Hyperthermophile: Grows at >80°C.

• Acidophile: Grows at low pH (<5.5).

• Neutrophile: Grows at pH 5.5–8.

• Alkaliphile: Grows at high pH (>8).

• Halophile: Requires high salt concentrations.

Free Radicals and Enzymes

• Free Radical: Highly reactive molecule with unpaired electron (e.g., superoxide O2-).

• Produced during aerobic metabolism; can damage cell components.

• Enzymes: Superoxide dismutase, catalase (2H2O2 → O2 + 2H2O).

Oxygen Requirements of Microorganisms

• Obligate Aerobe: Requires O2.

• Obligate Anaerobe: Killed by O2.

• Facultative Anaerobe: Grows with or without O2, prefers O2.

• Aerotolerant Anaerobe: Tolerates O2, does not use it.

• Microaerophile: Requires low O2 levels.

Metabolism Types and Oxygen

• Obligate aerobes: Aerobic respiration.

• Obligate anaerobes: Anaerobic respiration or fermentation.

• Facultative anaerobes: All three possible.

Clostridium perfringens and Oxygen

• Obligate anaerobe; cannot survive in oxygen-rich environments.

• Causes gas gangrene in deep, oxygen-poor tissues.

Biofilms and Quorum Sensing

• Biofilm: Community of microorganisms attached to a surface, embedded in extracellular matrix.

• Quorum Sensing: Cell-to-cell communication to coordinate gene expression.

• Coordinated Gene Expression: Synchronized activity in response to population density.

• Planktonic Bacteria: Free-living, not in biofilm.

Biofilm Formation Steps

• Attachment → Microcolony formation → Maturation → Dispersion.

• Biofilm bacteria are more resistant to antibiotics and immune responses than planktonic forms.

Media Types

• Chemically Defined: Exact composition known.

• Complex: Contains extracts, composition varies.

• Selective: Inhibits some microbes, allows others.

• Differential: Distinguishes microbes by appearance.

Pure Culture

• Population of cells derived from a single cell.

Growth Curves and Phases

• Logarithmic plotting used to visualize growth.

• Phases: Lag → Exponential (Log) → Stationary → Death.

Measuring Microbial Growth

• Plate Counts with Serial Dilutions: Counts viable cells.

• Filtration: Concentrates cells for counting.

• Microscopic Direct Count: Counts all cells (live and dead).

• Turbidity: Measures cloudiness; indirect estimate of growth.