Microbiology Lecture 1: The Microbial World and Microbial Fundamentals

Foundations of Microbiology and Microorganisms

• Definition of Microorganisms:

  • Organisms that are too small to be seen with the unaided eye, requiring microscopic magnification for observation.

  • Include bacteria, fungi (molds and yeasts), protozoa, microscopic algae, and viruses.

• Roles of Microbes in Human Life and Ecosystems:

  • Decomposition: Break down organic waste products, recycling elements back into the biosphere.

  • Photosynthesis: Generate oxygen gas and organic carbon compounds (e.g., cyanobacteria and microscopic algae).

  • Chemical Production: Synthesize industrial chemicals including ethanol, acetone, and various vitamins.

  • Food Fermentation: Produce foods such as vinegar, cheese, bread, yogurt, and alcoholic beverages.

  • Industrial Applications: Produce enzymes used in manufacturing, such as cellulase for textile processing.

  • Pharmaceutical Production: Produce therapeutic drugs, including antibiotics (e.g., penicillin from Penicillium fungi) and human hormones (e.g., recombinant insulin produced by modified bacteria).

  • Pathogenicity: A small fraction of microorganisms ($>5000$ known species) are pathogenic, meaning they cause infectious diseases in plants, animals, or humans.

• Microbial Applications in Textile Manufacturing (Designer Jeans):

  • Denim Fading: Utilizes enzymes from the fungus Trichoderma.

  • Cotton Production: Utilizes bacterial cellulose synthesized by Gluconacetobacter.

  • Bleaching: Employs peroxidase enzymes sourced from mushrooms.

  • Indigo Dye: Produced via bio-engineering using Escherichia coli.

• Practical Benefits of Studying Microbiology:

  • Enables massive industrial production of beneficial products.

  • Prevents food spoilage through tailored preservation techniques.

  • Prevents and treats infectious diseases.

  • Facilitates understanding of disease etiology and transmission dynamics to control or prevent epidemics.

Historical Evolution of Microbiology

• Early Cell Theory:

  • 1665 — Robert Hooke: Reported that living structural units were composed of small boxes or "cells" while observing cork slices under a microscope. This marked the foundation of the Cell Theory: the principle that all living organisms are composed of fundamental structural units called cells.

• The Debate Over Spontaneous Generation vs. Biogenesis:

  • Spontaneous Generation: The historical hypothesis that living organisms can arise spontaneously from nonliving matter, requiring a hypothetical "vital force."

  • Biogenesis: The scientific hypothesis that living cells can arise only from preexisting living cells.

  • Louis Pasteur (1861): Disproved the hypothesis of spontaneous generation using long-necked flasks bent into an S-shape (swan-neck flasks).

    • Experimental Design: Beef broth was placed in swan-neck flasks, and the necks were heated and bent into an S-shape. The broth was then boiled to kill existing organisms.

    • Observations: The S-shaped curve allowed air to pass into the flask but trapped airborne microorganisms in the bend, preventing them from reaching the nutrient broth.

    • Outcome: No microbial growth occurred in the cooled broth, even after extended periods. Sealed original vessels kept at the Pasteur Institute in Paris remain uncontaminated after more than 100 years.

    • Conclusion: Microorganisms reside in nonliving matter (air, liquids, solids), but nonliving matter itself does not generate microbial life.

• The First Golden Age of Microbiology (1857–1914):

  • Initiated by Louis Pasteur's work; marked rapid discoveries regarding the relationship between microorganisms and disease, immunity, and chemical drug treatments.

  • Pasteurization and Fermentation: Pasteur demonstrated that yeasts convert sugars to alcohol via fermentation in the absence of air, whereas bacterial contamination converts alcohol to acetic acid (vinegar), spoiling beverages. Pasteurization was developed as a method to heat liquids sufficiently to kill spoilage bacteria without ruining the product.

  • Germ Theory of Disease:

    • 1840s — Ignaz Semmelweis: Demonstrated that handwashing with chlorinated lime solutions by physicians drastically reduced the incidence of puerperal (childbed) fever in obstetrical clinics.

    • Robert Koch: Established the direct link between a specific microorganism and a specific infectious disease (etiology of disease).

• Koch's Postulates:

  1. The exact microorganism must be isolated from a diseased or dead animal host.

  2. The microorganism must be isolated and grown in a pure culture media, then structurally identified.

  3. Microorganisms from the pure culture must be inoculated into a healthy, susceptible laboratory animal host.

  4. The specific disease must be reproduced in the laboratory animal.

  5. The exact microorganism must be re-isolated from the experimentally infected host, grown in pure culture, and identified as identical to the original pathogen.

• The Second Golden Age of Microbiology (1943–1970):

  • Focused primarily on microbial genetics, molecular biology, and ultrastructure.

  • George Beadle and Edward Tatum: Demonstrated that genes direct the synthesis of specific proteins (enzymes).

  • Martha Hershey and Alfred Chase: Used bacteriophages (viruses) to confirm that DNA is the genetic material.

  • Francis Crick: Used Escherichia coli and viral models to elucidate how the DNA genetic code directs protein synthesis.

  • Development of the Electron Microscope (1940s–1950s): Provided magnification power ranging from $10\times$ up to $500000\times$ (and later up to $1000000\times$), allowing high-resolution visualization of subcellular structures, organelle morphology, and viral particles.

• The Third Golden Age of Microbiology (Present Day):

  • Global Health Impact: Infectious diseases account for approximately 15 million deaths globally every year. In the United States, bacterial infections contribute to more than 100,000 deaths annually.

  • Polymicrobial Diseases: Pathogens can act synergistically where one microbe causes multiple clinical pathologies or multiple species interact to manifest a single disease state.

  • Modern Challenges: Antimicrobial resistance among bacterial strains, emerging new infectious diseases, re-emerging historical diseases, and threat of bioterrorism.

Chemical Foundations of Microbial Life

• Atomic Structure:

  • Atom: The fundamental unit of matter that cannot be subdivided into smaller substances by ordinary chemical means.

  • Subatomic Particles:

    • Protons ($p^+$): Positively charged particles located within the atomic nucleus.

    • Neutrons ($n^0$): Uncharged (neutral) particles located within the atomic nucleus.

    • Electrons ($e^-$): Negatively charged particles orbiting the nucleus within specific electron shells corresponding to discrete energy levels.

  • Neutral Atoms: Contain equal numbers of protons and electrons.

• Chemical Bonds and Molecules:

  • Molecule: Two or more atoms held together by chemical bonds.

  • Compound: A molecule containing two or more different chemical elements bound together.

  • Ions: Charged atoms formed when an atom loses or gains electrons.

    • Cations: Positively charged ions formed by the loss of one or more electrons (e.g., $\text{Na}^+$).

    • Anions: Negatively charged ions formed by the gain of one or more electrons (e.g., $\text{Cl}^-$).

  • Ionic Bonds: Chemical attractions resulting from the electrostatic force between oppositely charged ions. For example, a sodium atom ($\text{Na}$) donates an electron to a chlorine atom ($\text{Cl}$), generating $\text{Na}^+$ and $\text{Cl}^-$, which combine to form an ionic compound:         $\text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl}$

  • Covalent Bonds: Formed when two atoms share one or more pairs of electrons. Covalent bonds are stronger and far more prevalent in biological systems than ionic bonds.

  • Hydrogen Bonds: Weak bonds formed when a hydrogen atom covalently bound to a highly electronegative atom (typically oxygen or nitrogen) experiences an electrostatic attraction to another electronegative atom (oxygen or nitrogen) in a neighboring molecule or another region of the same molecule.

• Chemical Reactions and Energetics:

  • Collision Theory: States that chemical reactions occur due to continuous collisions between atoms, ions, or molecules possessing sufficient kinetic energy.

  • Activation Energy: The minimal collision energy required to disrupt electron configurations and initiate a chemical reaction.

  • Reaction Rate: The frequency of collisions containing energy equal to or exceeding the activation energy threshold.

  • Endergonic Reactions: Chemical reactions that absorb net energy from their surroundings.

  • Exergonic Reactions: Chemical reactions that release net energy to their surroundings.

  • Anabolism: Synthesis reactions occurring within a cell that build complex molecules from simpler ones (endergonic processes).

  • Catabolism: Decomposition reactions within a cell that break down complex organic compounds into simpler ones (exergonic processes).

  • Exchange Reactions: Reactions involving both synthesis and decomposition components ($\text{AB} + \text{CD} \rightarrow \text{AD} + \text{CB}$).

  • Reversibility: Many chemical reactions are reversible under appropriate cellular conditions ($\text{A} + \text{B} \rightleftharpoons \text{AB}$).

• Inorganic Compounds and Water:

  • Inorganic Compounds: Typically small, structurally simple compounds that lack carbon-hydrogen bonds.

  • Water ($\text{H}_2\text{O}$):

    • An inorganic, polar molecule featuring an unequal charge distribution (partial negative charge near oxygen, partial positive charge near hydrogens).

    • Functions as a universal biological solvent. Polar solutes undergo dissociation in water, forming hydrated ions.

    • Hydrogen bonding between water molecules grants high heat capacity, acting as a cellular temperature buffer.

• Acids, Bases, and pH:

  • Acids: Proton ($\text{H}^+$) donors that dissociate into one or more $\text{H}^+$ ions and negative anions:         $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$

  • Bases: Hydroxide ($\text{OH}^-$) donors or proton acceptors that dissociate into $\text{OH}^-$ and positive cations:         $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$

  • pH Scale: Logarithmic measure of hydrogen ion concentration, defined from 0 to 14:         $\text{pH} = -\log_{10}[\text{H}^+]$

    • Acidic Solutions: $\text{pH} < 7$ (high $[\text{H}^+]$).

    • Neutral Solutions: $\text{pH} = 7$ ($=[\text{H}^+] = [\text{OH}^-]$).

    • Basic (Alkaline) Solutions: $\text{pH} > 7$ (low $[\text{H}^+]$, high $[\text{OH}^-]$).

• Macromolecules in Biological Systems:

  • Carbohydrates:

    • Composed of carbon, hydrogen, and oxygen with the general stoichiometric formula $(\text{CH}_2\text{O})_n$.

    • Functions: Cellular energy storage and structural components.

    • Isomers: Molecules possessing identical molecular formulas but different structural arrangements.

    • Monosaccharides: Simple monomeric sugars containing 3 to 7 carbon atoms (e.g., glucose, fructose, galactose, deoxyribose, ribose).

    • Disaccharides: Formed when two monosaccharides combine via dehydration synthesis (releasing $\text{H}_2\text{O}$); broken down into monosaccharides via hydrolysis (consuming $\text{H}_2\text{O}$).

    • Polysaccharides: Macromolecules consisting of tens to hundreds of monosaccharide units joined through dehydration reactions. Examples include starch, glycogen, dextran, and cellulose (all polymers of glucose differing in glycosidic linkage geometry and branching).

  • Lipids:

    • Primary nonpolar, hydrophobic structural components of plasma membranes; insoluble in water.

    • Simple Lipids (Triglycerides/Fats): Composed of a single glycerol molecule esterified to three fatty acid chains formed via dehydration synthesis.

      • Saturated Fatty Acids: Lack double bonds; hydrocarbon chains packed tightly together.

      • Unsaturated Fatty Acids: Contain one or More double bonds ($\text{C}=\text{C}$), producing kinks that keep chains loosely packed.

    • Complex Lipids (Phospholipids): Consist of glycerol, two nonpolar hydrophobic fatty acid tails, and a polar hydrophilic phosphate group (often bound to an organic group). Amphipathic nature causes self-assembly into a lipid bilayer structure in aqueous environments.

    • Sterols: Complex eukaryotic lipid structures with four fused carbon rings that insert between phospholipid tails to adjust membrane fluidity.

  • Proteins and Amino Acids:

    • Proteins consist of monomeric subunits called amino acids.

    • Amino Acid Structure: Central alpha-carbon attached to an amino group ($-\text{NH}_2$), a carboxyl group ($-\text{COOH}$), a hydrogen atom, and a variable side chain ($\text{R}$-group).

    • Amino acids join covalently via peptide bonds formed through dehydration reactions.

    • Levels of Protein Structure:

      • Primary Structure: The specific, linear sequence of amino acids in a polypeptide chain.

      • Secondary Structure: Localized folding into $\alpha$-helices or $\beta$-pleated sheets stabilized by hydrogen bonds along the polypeptide backbone.

      • Tertiary Structure: Overall 3D shape of a single polypeptide chain, determined by hydrophobic interactions, disulfide bridges ($-\text{S}-\text{S}-$), ionic bonds, and hydrogen bonding between $\text{R}$-groups.

      • Quaternary Structure: Structural relationship resulting from the aggregation of two or More individual folded polypeptide subunits into a functional protein complex.

  • Nucleic Acids (DNA and RNA):

    • Nucleotides consist of a pentose sugar (deoxyribose or ribose), a nitrogenous base (purine or pyrimidine), and a phosphate group.

    • Deoxyribonucleic Acid (DNA): Double-stranded helical molecule storing genetic instructions. Sugars and phosphate groups form an antiparallel backbone ($5'$ to $3'$ direction relative to the opposite strand). Nitrogenous bases form complementary hydrogen-bonded rungs:

      • Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.

      • Guanine (G) pairs with Cytosine (C) via 3 hydrogen bonds.

  • Adenosine Triphosphate (ATP):

    • The principal energy storage and transfer molecule in all living cells.

    • Composed of an adenine base, a ribose sugar, and three phosphate groups.

    • Hydrolysis of the terminal high-energy phosphate bond releases usable chemical energy:             $\text{ATP} + \text{H}_2\text{O} \rightleftharpoons \text{ADP} + \text{P}_i + \text{Energy}$

Taxonomy, Classification, and Identification of Microorganisms

• Taxonomic Nomenclature:

  • Established by Carolus Linnaeus in 1735.

  • System of binomial nomenclature gives every organism two scientific names: the Genus (capitalized) and the specific epithet (lowercase).

  • Names are italicized or underlined, Latinized, used globally, and may describe the organism's morphology, habitat, or honor a scientist.

  • Examples:

    • Escherichia coli: Honors discoverer Theodor Escherich; specific epithet denotes its primary habitat, the colon.

    • Staphylococcus aureus: Genus describes clustered (staphylo-) spherical (coccus) cells; specific epithet describes golden-colored (aureus) colonies.

  • Scientific names can be abbreviated after the first full reference ($E.\,coli$, $S.\,aureus$).

• Taxonomy vs. Phylogeny (Systematics):

  • Taxonomy: The science of classifying organisms into groups based on shared phenotypic and genotypic characteristics.

  • Systematics (Phylogeny): The study of the evolutionary history and relationships of organisms.

  • Taxonomic Hierarchy: All living organisms are grouped into hierarchical ranks: Domain $\rightarrow$ Kingdom $\rightarrow$ Phylum $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ Family $\rightarrow$ Genus $\rightarrow$ Species.

• The Three-Domain System:

  • Developed by Carl Woese in 1978, based on comparisons of nucleotide sequences in ribosomal RNA (rRNA).

  • Domain Bacteria: Prokaryotic organisms possessing peptidoglycan cell walls. Initiator amino acid in protein synthesis is formylmethionine. Sensitive to traditional antibiotics.

  • Domain Archaea: Prokaryotic organisms lacking peptidoglycan (walls contain pseudomurein or lack walls). Initiator amino acid is methionine. Inhabit extreme environments. Includes:

    • Methanogens: Strict anaerobes that produce methane gas.

    • Extreme Halophiles: Require high salt concentrations for survival.

    • Hyperthermophiles: Require extremely high environmental temperatures.

  • Domain Eukarya: All eukaryotic organisms possessing membrane-bound nuclei. Divided into four eukaryotic kingdoms: Protista, Fungi, Plantae, Animalia.

• Ribosomal RNA (rRNA) Variations Across Domains:

  • Eukaryotic cytoplasmic ribosomes are $80\text{S}$ (composed of a $60\text{S}$ subunit with $28\text{S}$, $5.8\text{S}$, and $5\text{S}$ rRNAs, plus a $40\text{S}$ subunit with $18\text{S}$ rRNA).

  • Prokaryotic ribosomes (and eukaryotic organellar ribosomes) are $70\text{S}$ (composed of a $50\text{S}$ subunit with $23\text{S}$ and $5\text{S}$ rRNAs, plus a $30\text{S}$ subunit with $16\text{S}$ rRNA).

• Endosymbiotic Theory:

  • Eukaryotic cells evolved from primitive prokaryotic ancestral cells that underwent plasma membrane invaginations to surround genetic material.

  • Mitochondria and chloroplasts originated as endosymbiotic aerobic prokaryotes and photosynthetic prokaryotes (cyanobacteria), respectively, ingested by larger precursor host cells.

• Definitions of Species Across Domains:

  • Eukaryotic Species: A group of closely related organisms that interbreed among themselves and produce fertile offspring.

  • Prokaryotic Species: A population of prokaryotic cells with high structural, biochemical, and genetic similarity.

    • Culture: Bacteria grown and maintained in laboratory media.

    • Clone: A population of identical cells derived from a single parent cell.

    • Strain: Genetically distinct variations or subgroups isolated within a clone (e.g., $E.\,coli\text{ O157:H7}$ versus non-pathogenic $E.\,coli$ laboratory strains).

• Microbial Identification Methods:

  • Biochemical Tests: Evaluate metabolic enzymatic activity (e.g., sugar fermentation patterns, oxidase test, urease production, $H_2S$ generation).

  • Rapid Identification Test Kits: Automated panel systems (e.g., API strip, Vitek-2) that perform multiple biochemical tests simultaneously to identify pathogens within hours.

  • MALDI-TOF MS (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry): Ionizes microbial proteins using laser pulses and measures flight time across an electric field detector, matching protein mass spectra against a reference database to identify microorganisms within 15 minutes.

Functional Anatomy of Prokaryotic Cells

• General Characteristics:

  • Genetic material is typically organized as a single circular chromosome, not enclosed within a nuclear membrane.

  • Lack complex, membrane-bound cellular organelles (e.g., no mitochondria, endoplasmic reticulum, or Golgi apparatus).

  • Cell walls contain peptidoglycan (in Bacteria) or pseudomurein (in Archaea).

  • Divide primarily via binary fission.

• Cell Size, Shapes, and Cellular Arrangements:

  • Average Size: Ranges from $0.2\,\mu m$ to $2.0\,\mu m$ in diameter, and $2\,\mu m$ to $8\,\mu m$ in length.

  • Morphologies (Shapes):

    • Monomorphic: Bacteria that maintain a single, consistent cellular shape.

    • Pleomorphic: Bacteria that display varied cellular shapes.

    • Bacillus: Rod-shaped cells.

    • Coccus: Spherical or oval-shaped cells.

    • Spiral: Curved or helical rods. Includes Vibrio (curved rods), Spirillum (rigid helical shape, uses external flagella), and Spirochete (flexible helical shape, uses axial filaments).

    • Unusual Shapes: Star-shaped (Stella) and rectangular (Haloarcula).

  • Arrangements of Cocci:

    • Diplococci: Pairs of spherical cells following division in one plane.

    • Streptococci: Chain-like arrangements following continuous division in one plane.

    • Tetrads: Groups of four cells resulting from division in two perpendicular planes.

    • Sarcinae: Cube-like configurations of eight cells resulting from division in three regular planes.

    • Staphylococci: Irregular, grape-like clusters resulting from division across multiple random planes.

  • Arrangements of Bacilli:

    • Diplobacilli: Pairs of rod-shaped cells.

    • Streptobacilli: Chains of rod-shaped cells.

    • Coccobacilli: Short, oval rods resembling cocci.

• Structures External to the Cell Wall:

  • Glycocalyx ("Sugar Coat"):

    • Viscous, gelatinous polymer composed of polysaccharides, polypeptides, or both, synthesized internally and secreted to the exterior surface.

    • Capsule: A neatly organized glycocalyx firmly attached to the outer cell wall. Prevents phagocytosis by host immune cells, directly enhancing microbial virulence (e.g., Streptococcus pneumoniae).

    • Slime Layer: Unorganized, loosely attached glycocalyx that easily detaches.

    • Extracellular Polymeric Substance (EPS): Glycocalyx matrix facilitating cell attachment to surfaces, forming microbial biofilms, protecting cells from dehydration, and serving as a nutrient reserve.

  • Flagella:

    • Long filamentous protein appendages that rotate to propel bacteria.

    • Subunit Composition: Constructed from helical chains of the protein flagellin.

    • Structural Parts:

      1. Filament: Outermost region extending into the environment.

      2. Hook: Flexible joint structure composed of distinct protein.

      3. Basal Body: Central rod surrounded by ring structures anchored directly into the cell wall and plasma membrane.

    • Motility Patterns: Flagellar rotation produces alternating periods of smooth directional movement ("runs") and periodic random alterations in direction ("tumbles").

    • Taxis: Movement toward a favorable stimulus (attractant) or away from a harmful stimulus (repellent), such as chemotaxis (chemical stimulus) or phototaxis (light stimulus).

    • H Antigen: Flagellar membrane protein useful for distinguishing specific bacterial serovars (e.g., Escherichia coli O157:H7, where H7 designates the specific flagellar antigen).

  • Axial Filaments (Endoflagella):

    • Bundles of fibrils anchored at one cell pole that spiral around the cell beneath an outer sheath in spirochetes (e.g., Treponema pallidum).

    • Rotation creates a helical corkscrew motion enabling traversal through viscous body fluids.

  • Fimbriae and Pili:

    • Hairlike protein appendages composed of pilin subunits.

    • Fimbriae: Short, numerous appendages allowing bacteria to adhere to biological surfaces and mucous membranes, necessary for biofilm formation and colonization.

    • Pili: Longer structures, typically present as one or two per cell. Involved in specialized motility (twitching motility and gliding motility) and conjugation (sex pili that form physical bridges to transfer plasmid DNA between bacterial cells).

• The Prokaryotic Cell Wall:

  • Prevents bacterial cells from undergoing osmotic lysis under hypotonic conditions and maintains cellular morphology.

  • Main structural framework is Peptidoglycan (Murein): A repeating disaccharide polymer consisting of alternating units of:

    • N-acetylglucosamine (NAG)

    • N-acetylmuramic acid (NAM)

  • Parallel disaccharide strands are linked by tetrapeptide side chains attached to NAM units, which are interconnected by peptide cross-bridges.

• Gram-Positive vs. Gram-Negative Cell Walls:

• Mechanism of the Gram Stain Procedure:

  1. Primary Stain (Crystal Violet): Stains both Gram-positive and Gram-negative cells dark purple.

  2. Mordant (Iodine): Forms a large, insoluble Crystal Violet-Iodine ($\text{CV-I}$) complex inside the cell matrix.

  3. Decolorizing Agent (Ethanol/Acetone):

     • Gram-Positive: Dehydrates the thick peptidoglycan layer, causing pores to close and trapping the large $\text{CV-I}$ complex inside (remains purple).

     • Gram-Negative: Dissolves lipid outer membrane and creates large holes in the thin peptidoglycan layer, allowing $\text{CV-I}$ to wash out completely (becomes colorless).

  4. Counterstain (Safranin): Imparts a pink/red color to decolorized Gram-negative cells, while Gram-positive cells remain deep purple.

• Atypical Cell Walls:

  • Acid-Fast Cell Walls: Possess a layer of waxy lipid called mycolic acid bound outside a peptidoglycan layer. Resists decolorization by acid-alcohol during staining (e.g., Mycobacterium tuberculosis, Nocardia species). Stained using carbolfuchsin dye.

  • Mycoplasmas: Naturally wall-less bacteria. Microscopic cell membrane is stabilized against osmotic rupture by sterols.

  • Archaea: Lack true peptidoglycan; possess cell walls composed of pseudomurein (lacks NAM and D-amino acids).

• The Plasma (Cytoplasmic) Membrane:

  • Phospholipid bilayer featuring embedded integral (transmembrane) proteins and surface-associated peripheral proteins.

  • Fluid Mosaic Model: The lipid bilayer is dynamic and fluid (viscosity comparable to olive oil), allowing proteins to migrate laterally while phospholipids rotate and self-seal.

  • Selective Permeability: Regulates passage of molecules into and out of the cytoplasm. Small nonpolar molecules pass freely; ions and large polar molecules are restricted.

  • Metabolic Functions: Contains enzymes responsible for cellular respiration and ATP synthesis. Photosynthetic bacteria possess specialized membrane infoldings termed chromatophores containing photosynthetic pigments.

  • Membrane Disruption: Targeted and damaged by short-chain alcohols, quaternary ammonium compound detergents, and polymyxin antibiotics, causing loss of membrane integrity and cytoplasmic leakage.

• Transport Mechanisms Across Membranes:

  • Passive Processes: Solutes move down concentration gradients (from high concentration to low concentration) without cellular ATP expenditure.

    • Simple Diffusion: Unassisted net movement of small molecules (e.g., $\text{O}_2$, $\text{CO}_2$) directly through the phospholipid bilayer until equilibrium is attained.

    • Facilitated Diffusion: Transmembrane transporter proteins (channels or carriers) facilitate the movement of ions or larger polar molecules across the membrane down their concentration gradient.

    • Osmosis: Net movement of water molecules across a selectively permeable membrane from an area of high water concentration (low solute concentration) to an area of lower water concentration (high solute concentration). Occurs via simple diffusion or through specific membrane water channels called aquaporins.

    • Osmotic Effects on Cells:

      • Isotonic Solution: Solute concentration inside the cell equals concentration outside; no net movement of water.

      • Hypotonic Solution: Solute concentration outside the cell is lower than inside; water flows into the cell. Intact bacterial cell walls prevent bursting; damaged or wall-less cells undergo osmotic lysis.

      • Hypertonic Solution: Solute concentration outside the cell is higher than inside; water exits the cytoplasm, causing cell shrinkage (plasmolysis).

  • Active Processes: Movement of substances against concentration gradients (from low concentration to high concentration), requiring active membrane transporter proteins and cellular ATP consumption.

• Internal Prokaryotic Structures:

  • Cytoplasm: The internal substance enclosed by the plasma membrane; composed of roughly 80% water along with proteins, carbohydrates, lipids, inorganic ions, and cytoskeleton fibers.

  • Nucleoid: Cytoplasmic region containing the bacterial chromosome. The chromosome is typically a single, circular, double-stranded DNA molecule carrying essential genetic information, lacking a nuclear membrane or histones.

  • Plasmids: Small, circular, extrachromosomal double-stranded DNA molecules that replicate independently of the chromosomal DNA.

    • Carry non-essential genes that confer evolutionary advantages (e.g., antibiotic resistance genes, toxin production genes, heavy metal resistance).

    • Can be gained or lost without damaging basic cell viability, and are transferred via conjugation.

  • Ribosomes: Sites of protein synthesis. Prokaryotic ribosomes are $70\text{S}$, constructed from a small $30\text{S}$ subunit and a large $50\text{S}$ subunit.

  • Inclusions (Reserve Deposits):

    • Metachromatic Granules (Volutin): Reserve deposits of inorganic phosphate used for ATP synthesis.

    • Polysaccharide Granules: Inclusions consisting of glycogen and starch energy reserves.

    • Lipid Inclusions: Poly-$\beta$-hydroxybutyric acid energy storage deposits.

    • Sulfur Granules: Deposits of elemental sulfur accumulated by sulfur-oxidizing bacteria.

    • Carboxysomes: Inclusions packed with the enzyme Ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), required for $\text{CO}_2$ fixation during photosynthesis.

    • Gas Vacuoles: Protein-covered hollow cylinders that adjust cell buoyancy in aquatic environments.

    • Magnetosomes: Inclusions of iron oxide ($\text{Fe}_3\text{O}_4$) that act as microscopic compasses to guide bacteria along geomagnetic fields while destroying cytoplasmic hydrogen peroxide ($\text{H}_2\text{O}_2$).

• Bacterial Endospores:

  • Dormant, highly resistant structures produced internally by specific Gram-positive genera (notably Bacillus and Clostridium) when essential environmental nutrients (e.g., carbon or nitrogen) become depleted.

  • Extremely resistant to high heat, desiccation, toxic chemicals, enzymatic degradation, and ultraviolet radiation.

  • Sporulation (Sporogenesis): Process of endospore formation over several hours:

    1. Newly replicated bacterial chromosome is isolated by an ingrowth of plasma membrane called a spore septum.

    2. Spore septum double-wraps around the isolated DNA and cytoplasm fragment, forming a forespore.

    3. Peptidoglycan layer deposits between the two inner membranes.

    4. A thick protein spore coat forms around the structure, conferring chemical resistance.

    5. Mother cell lyses, freeing the mature endospore into the environment.

  • Germination: Return of a dormant endospore to a metabolically active vegetative state triggered by physical or chemical damage to the spore coat in a nutrient-rich environment.

Functional Anatomy of Eukaryotic Cells

• Flagella and Cilia:

  • Structures used for cellular locomotion or moving fluids across surface membranes.

  • Constructed of complex arrays of microtubules arranged in a characteristic "$9+2$" pattern (nine outer microtubule doublets surrounding two central single microtubules), surrounded by a plasma membrane sheath.

  • Flagella: Few in number and long relative to total cell length; move in a wave-like manner.

  • Cilia: Numerous and short; display coordinated, oar-like beating movements.

• Cell Wall and Glycocalyx:

  • Cell walls are structurally simpler than prokaryotic cell walls, lacking peptidoglycan.

  • Plants and Algae: Cell walls composed primarily of cellulose.

  • Fungi: Cell walls composed primarily of chitin.

  • Yeasts: Cell walls composed of glucan and mannan polymers.

  • Animal Cells: Lack cell walls; outer plasma membrane covered by a glycocalyx layer of carbohydrates covalently bound to proteins and lipids.

• Plasma Membrane Characteristics:

  • Phospholipid bilayer containing integral and peripheral proteins.

  • Contains complex lipid sterols (e.g., cholesterol) that provide structural stability against osmotic pressure changes.

  • Contains cell-surface carbohydrates that act as cell recognition receptors.

  • Endocytosis: Active transport mechanism unique to eukaryotic cells where the plasma membrane engulfs extracellular material:

    • Phagocytosis: Pseudopods extend out and surround solid particles, bringing them into the cell within a phagocytic vacuole.

    • Pinocytosis: Plasma membrane folds inward, taking in extracellular fluid and dissolved solutes.

• Internal Eukaryotic Structures and Organelles:

  • Cytoplasm and Cytosol: Cytosol is the fluid portion of cytoplasm. Features a complex cytoskeleton composed of microfilaments, intermediate filaments, and microtubules that maintain cell shape and direct organelle movement.

  • Ribosomes: Sites of protein synthesis. Eukaryotic cytoplasmic and membrane-bound ribosomes are $80\text{S}$ (composed of a $60\text{S}$ large subunit and a $40\text{S}$ small subunit). $70\text{S}$ ribosomes are retained inside mitochondria and chloroplasts.

  • Nucleus: Double-membrane spherical structure (nuclear envelope) containing the host DNA. Features nuclear pores that regulate transport. DNA is bound to basic proteins called histones to form chromatin, which condenses into distinct chromosomes during mitosis and meiosis. Contains a dense nucleolus region for ribosomal RNA synthesis.

  • Endoplasmic Reticulum (ER): Network of flattened membrane sacs (cisternae).

    • Rough ER: Outer surface studded with $80\text{S}$ ribosomes; processes and folds synthesized proteins.

    • Smooth ER: Lacks ribosomes; synthesizes membrane phospholipids, fats, and steroid hormones, and detoxifies metabolic byproducts.

  • Golgi Complex: Consists of 3 to 20 stacked membrane cisternae. Receives protein-containing transport vesicles from the rough ER, chemically modifies proteins (forming glycoproteins and lipoproteins), and packages them into secretory vesicles for exocytosis or internal delivery.

  • Lysosomes: Membrane-bound vesicles formed by the Golgi complex containing potent digestive enzymes that hydrolyze engulfed material and worn-out cellular components.

  • Vacuoles: Membrane-bound cavities derived from Golgi or endocytosis that store nutrients, water, or metabolic waste products, and maintain cell turgor.

  • Mitochondria: Double-membrane organelles responsible for cellular respiration and ATP production. Smooth outer membrane encloses an inner membrane folded into deep ridges called cristae, surrounded by an internal fluid matrix containing enzymes, $70\text{S}$ ribosomes, and circular mitochondrial DNA. Replicate independently via binary fission.

  • Chloroplasts: Double-membrane photosynthetic organelles found in plants and algae. Contain flattened membrane sacs called thylakoids containing chlorophyll pigments, arranged in stacks called grana, surrounded by fluid stroma.

Microbial Metabolism: Principles and Enzyme Mechanics

• Catabolism and Anabolism Integration:

  • Metabolism: The sum total of all enzymatic chemical reactions occurring within a living organism.

  • Catabolism: Exergonic metabolic pathways that break down complex organic molecules into simpler compounds, releasing stored chemical energy and generating chemical building blocks. Catabolic reactions drive the phosphorylation of ADP to ATP:         $\text{ADP} + \text{P}_i + \text{Energy} \rightarrow \text{ATP}$

  • Anabolism: Endergonic metabolic pathways that consume chemical energy to construct complex cellular macromolecules from simpler precursor molecules. Driven by ATP cleavage:         $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

  • Metabolic Pathways: Sequences of enzymatically catalyzed chemical reactions occurring within cells, dictated by cellular genetic encoding.

• Enzyme Catalysis Mechanics:

  • Biological Catalysts: Enzymes speed up chemical reactions without being consumed or permanently altered.

  • Enzymes increase reaction rates by binding specific substrates and lowering the activation energy required for the reaction to proceed.

• Enzyme Components:

  • Apoenzyme: Protein portion of an enzyme; inactive by itself.

  • Cofactor: Nonprotein component required for activity (e.g., metallic ions like $\text{Fe}^{2+}$, $\text{Mg}^{2+}$, $\text{Zn}^{2+}$).

  • Coenzyme: An organic cofactor molecule, frequently derived from vitamins. Functions as an intermediate carrier of electrons or functional chemical groups.

    • Nicotinamide Adenine Dinucleotide ($\text{NAD}^+$): Electron carrier involved primarily in catabolic energy-yielding reactions.

    • Nicotinamide Adenine Dinucleotide Phosphate ($\text{NADP}^+$): Electron carrier involved primarily in anabolic biosynthetic reactions.

    • Flavin Adenine Dinucleotide ($\text{FAD}$): Electron carrier involved in oxidation-reduction cycles.

    • Coenzyme A ($\text{CoA}$): Plays a central role in decarboxylation and fatty acid catabolism.

  • Holoenzyme: The fully active, functional enzyme complex composed of an apoenzyme bound to its essential cofactor/coenzyme:         $\text{Apoenzyme (inactive)} + \text{Cofactor/Coenzyme} = \text{Holoenzyme (active)}$

• Mechanisms of Enzymatic Action:

  1. The substrate collides with and binds to a specific region on the surface of the apoenzyme known as the active site, forming an enzyme-substrate complex.

  2. The substrate is transformed via molecular rearrangement, bond cleavage, or bond formation.

  3. The transformed reaction products are released from the active site.

  4. The unaltered enzyme is free to bind new substrate molecules.

• Factors Influencing Enzyme Activity:

  • Temperature: Reaction rates increase with elevated temperature due to higher kinetic collision frequency until an optimal temperature is surpassed. Beyond this optimum (e.g., above $37^\circ\text{C}$ for human pathogens), heat disrupts hydrogen and non-covalent bonds holding tertiary structure, causing protein denaturation and inactivation.

  • pH: Enzymes exhibit peak catalytic activity at an optimal pH. Extreme alterations in $[\text{H}^+]$ disrupt ionic interactions, altering 3D structure and causing denaturation.

  • Substrate Concentration: At low substrate levels, reaction rates increase proportionally with substrate concentration. Under maximum substrate availability, all active sites are continuously filled (saturation point), reaching maximum catalytic rate ($V_{\max}$).

  • Enzyme Inhibitors:

    • Competitive Inhibitors: Molecules with chemical structures highly similar to the natural substrate that compete directly for binding to the enzyme's active site (e.g., sulfanilamide drugs competitively inhibit bacterial binding of $p$-aminobenzoic acid [PABA], preventing folic acid synthesis).

    • Noncompetitive (Allosteric) Inhibitors: Inhibitors that do not compete for the active site; instead, they bind to a distinct regulatory site on the enzyme termed the allosteric site. Binding induces a conformational change in the enzyme's active site, preventing substrate binding or catalytic activation.

    • Feedback Inhibition (Negative Feedback): A regulatory mechanism in which the final end-product of a metabolic pathway noncompetitively/allosterically inhibits the key enzyme (typically the first enzyme) operating near the start of that pathway, stopping overproduction of metabolic intermediates.

• Oxidation-Reduction (Redox) Reactions:

  • Oxidation: The chemical removal of electrons ($e^-$) from a molecule, releasing energy.

  • Reduction: The chemical gain of electrons ($e^-$) by a molecule.

  • Redox Pair: Every oxidation reaction is coupled to a simultaneous reduction reaction.

  • Dehydrogenation: In biological systems, cellular oxidations often involve the simultaneous loss of an electron and a proton ($\text{H}^+$), equivalent to the removal of a complete hydrogen atom ($\text{H}$). Organic molecules rich in hydrogen atoms (such as glucose) represent highly reduced compounds packed with potential chemical energy.

• Mechanisms of ATP Generation:

  1. Substrate-Level Phosphorylation: Direct, enzymatic transfer of a high-energy phosphate group from a phosphorylated organic metabolic intermediate directly to ADP to generate ATP.

  2. Oxidative Phosphorylation: Passage of high-energy electrons down an electron transport chain consisting of sequential membrane-bound electron carriers to a final electron acceptor. Energy released during electron transfer is harnessed to pump protons across a membrane, generating a proton gradient (chemiosmosis) that powers ATP synthesis via ATP synthase.

  3. Photophosphorylation: Occurs exclusively in photosynthetic cells. Light energy absorbed by photosynthetic pigments excites electrons, which pass through an electron transport chain to generate ATP and NADPH via chemiosmosis.

Carbohydrate Catabolism and Energy Production Pathways

• Glycolysis (Embden-Meyerhof Pathway):

  • The universal catabolic pathway for oxidizing a single six-carbon ($6\text{C}$) glucose molecule into two three-carbon ($3\text{C}$) pyruvic acid molecules. Occurs in the cytoplasm and does not require molecular oxygen ($\text{O}_2$).

  • Stage 1: Preparatory Stage (Energy Investment):

    1. Glucose is phosphorylated by ATP to Glucose 6-phosphate.

    2. Rearranged to Fructose 6-phosphate.

    3. Phosphorylated by a second ATP molecule to yield Fructose 1,6-diphosphate. (Total investment: $2\,\text{ATP}$).

    4. Fructose 1,6-diphosphate is cleaved by aldolase into two $3\text{C}$ isomers: Dihydroxyacetone phosphate (DHAP) and Glyceraldehyde 3-phosphate (GP/G3P).

    5. DHAP is converted into a second molecule of GP.

  • Stage 2: Energy-Conserving Stage (Energy Payoff):

    6. Both GP molecules are oxidized via dehydrogenation (transferring $2\,e^-$ and $1\,\text{H}^+$ per GP to $\text{NAD}^+$ to yield $2\,\text{NADH}$), while inorganic phosphate is added to yield two 1,3-diphosphoglyceric acid molecules.

    7. High-energy phosphate is transferred to ADP, generating $2\,\text{ATP}$ molecules via substrate-level phosphorylation.

    8. 3-phosphoglyceric acid rearranges to 2-phosphoglyceric acid.

    9. Loss of $\text{H}_2\text{O}$ creates a high-energy phosphate bond in Phosphoenolpyruvic acid (PEP).

    10. High-energy phosphate from PEP is transferred to ADP, generating $2\,\text{ATP}$ molecules via substrate-level phosphorylation and producing two final Pyruvic acid molecules.

  • Net Glycolysis Equation:         $\text{Glucose} + 2\,\text{ATP} + 2\,\text{ADP} + 2\,\text{P}_i + 2\,\text{NAD}^+ \rightarrow 2\,\text{Pyruvic acid} + 4\,\text{ATP} + 2\,\text{NADH} + 2\,\text{H}^+$         (Net Gain per glucose: $2\,\text{Pyruvic acid}$, $2\,\text{ATP}$, and $2\,\text{NADH}$).

• Alternative Carbohydrate Catabolic Pathways:

  • Pentose Phosphate Pathway: Operates simultaneously with glycolysis. Uses $5\text{C}$ pentose sugars to produce $\text{NADPH}$ (used in biosyntheses) and yields a net gain of $1\,\text{ATP}$. Generates critical intermediate pentoses needed for nucleic acid synthesis, glucose synthesis from $\text{CO}_2$, and amino acids.

  • Entner-Doudoroff Pathway: Yields $\text{NADPH}$, $\text{NADH}$, and $1\,\text{ATP}$ from one glucose molecule without utilizing glycolysis or the pentose phosphate pathway. Found in specific Gram-negative bacteria such as Pseudomonas, Rhizobium, and Agrobacterium.

• Aerobic Respiration:

  • Defined as an obligate energy-yielding process wherein organic substrate oxidation donates electrons to an electron transport chain, terminating at molecular oxygen ($\text{O}_2$) as the ultimate inorganic final electron acceptor.

  • Intermediate Step (Pyruvic Acid Oxidation):

    • Pyruvic acid ($3\text{C}$) produced by glycolysis enters the mitochondrion (in eukaryotes) or remains in the cytoplasm (in prokaryotes).

    • Undergoes decarboxylation (loss of one carbon as $\text{CO}_2$) and oxidation (transfer of electrons to $\text{NAD}^+$ forming $\text{NADH}$).

    • The remaining $2\text{C}$ acetyl group combines with Coenzyme A to yield Acetyl CoA:             $2\,\text{Pyruvic acid} + 2\,\text{CoA} + 2\,\text{NAD}^+ \rightarrow 2\,\text{Acetyl CoA} + 2\,\text{CO}_2 + 2\,\text{NADH}$

  • The Krebs Cycle (Citric Acid Cycle):

    • A cyclic pathway that extracts chemical energy stored within Acetyl CoA.

    1. Cycle Initiation: Enzymes remove $\text{CoA}$ and combine the $2\text{C}$ acetyl group with $4\text{C}$ oxaloacetic acid to form $6\text{C}$ citric acid.

    2. Isomerization converts citric acid to isocitric acid.

    3. Isocitric acid undergoes oxidation and decarboxylation to release $\text{CO}_2$, reduce $\text{NAD}^+$ to $\text{NADH}$, and form $5\text{C}$ $\alpha$-ketoglutaric acid.

    4. $\alpha$-Ketoglutaric acid undergoes a second oxidation and decarboxylation step, releasing $\text{CO}_2$, reducing $\text{NAD}^+$ to $\text{NADH}$, and adding $\text{CoA}$ to form $4\text{C}$ succinyl CoA.

    5. Succinyl CoA cleaves $\text{CoA}$, releasing energy that generates GTP (or ATP directly) via substrate-level phosphorylation, leaving succinic acid.

    6. Succinic acid is oxidized by $\text{FAD}$ to form fumaric acid and $\text{FADH}_2$

    7. Fumaric acid is hydrated to malic acid.

    8. Malic acid undergoes a final oxidation, generating $\text{NADH}$ and regenerating oxaloacetic acid to enter another cycle round.

    • Yield per Acetyl CoA (1 turn): $2\,\text{CO}_2$, $3\,\text{NADH}$, $1\,\text{FADH}_2$, and $1\,\text{ATP}$.

    • Yield per Glucose molecule (2 turns): $4\,\text{CO}_2$, $6\,\text{NADH}$, $2\,\text{FADH}_2$, and $2\,\text{ATP}$.

• Electron Transport Chain (ETC) and Chemiosmosis:

  • Location: Prokaryotic plasma membrane; inner mitochondrial membrane of eukaryotes.

  • Carrier Molecules: Sequence of membrane-bound electron carrier complexes that cycle between oxidized and reduced states as electrons pass down the gradient:

    • Flavoproteins: Contain flavin coenzymes (e.g., FMN).

    • Cytochromes: Proteins containing an iron-bearing heme group (e.g., $cyt\,b$, $cyt\,c_1$, $cyt\,c$, $cyt\,a$, $cyt\,a_3$).

    • Ubiquinones (Coenzyme Q): Small nonprotein soluble lipid carriers.

  • Chemiosmotic Mechanism:

    1. High-energy electrons carried by $\text{NADH}$ and $\text{FADH}_2$ are transferred to carrier complexes ($\text{NADH}$ donates to FMN; $\text{FADH}_2$ donates lower down the chain to Coenzyme Q).

    2. As electrons travel down the chain through successive redox steps, released energy powers active transport complexes (e.g., $\text{NADH}$ dehydrogenase complex, cytochrome $b-c_1$ complex, cytochrome oxidase complex) to pump protons ($\text{H}^+$) across the membrane.

    3. Protons accumulate in the periplasmic space (prokaryotes) or mitochondrial intermembrane space (eukaryotes), establishing a electrochemical gradient termed the proton motive force.

    4. Protons diffuse back down their concentration gradient through the transmembrane channel of ATP Synthase.

    5. The kinetic energy generated by returning proton flow induces rotation in ATP Synthase, driving the phosphorylation of ADP to ATP:             $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

  • Theoretical ATP Yields:

    • Each $\text{NADH}$ oxidized via the ETC yields up to $3\,\text{ATP}$ molecules.

    • Each $\text{FADH}_2$ oxidized via the ETC yields up to $2\,\text{ATP}$ molecules.

  • Final Electron Acceptor: Molecular oxygen ($\text{O}_2$) acts as the terminal electron acceptor, combining with free protons to produce water:         $\frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \rightarrow \text{H}_2\text{O}$

• Anaerobic Respiration:

  • Operates identically to aerobic respiration using glycolysis, an intermediate step, the Krebs cycle, and an electron transport chain, but utilizes an inorganic molecule other than $\text{O}_2$ as the ultimate terminal electron acceptor.

  • Yield: Generates less total ATP than aerobic respiration because the electronegativity of non-oxygen acceptors is lower, resulting in a shorter electron transport chain and smaller proton gradient.

• Fermentation:

  • An anaerobic energy-yielding pathway that releases energy from the partial oxidation of organic molecules (primarily glucose).

  • Key Characteristics:

    • Does not require oxygen ($\text{O}_2$).

    • Does not utilize the Krebs cycle or an electron transport chain.

    • Uses an organic molecule synthesized inside the cell as the final terminal electron acceptor (typically pyruvic acid or a derivative).

    • Yields small amounts of ATP (only the $2\,\text{ATP}$ generated per glucose via substrate-level phosphorylation during glycolysis).

    • Primary Function: Oxidizes generated $\text{NADH}$ back to $\text{NAD}^+$, replenishing electron carriers required to maintain continuous glycolysis.

  • Major Types of Fermentation:

    • Lactic Acid Fermentation: Pyruvic acid produced by glycolysis is directly reduced by $\text{NADH}$ to form two molecules of lactic acid (e.g., Streptococcus, Lactobacillus).

    • Alcohol Fermentation: Pyruvic acid is first decarboxylated to form acetaldehyde and $\text{CO}_2$ gas. Acetaldehyde is then reduced by $\text{NADH}$ to form ethanol (e.g., Saccharomyces yeast).

    • Other Fermentative End-Products: Microbial genera produce varied end-products including propionic acid, butyric acid, acetone, butanol, isopropyl alcohol, succinic acid, acetic acid, and formic acid.

Lipid and Protein Catabolism

• Lipid Catabolism:

  • Extracellular enzymes called lipases hydrolyze lipids (triglycerides) into glycerol and fatty acid components.

  • Glycerol Processing: Glycerol is phosphorylated and converted to Dihydroxyacetone phosphate (DHAP), entering glycolysis to form pyruvic acid.

  • Fatty Acid Processing: Fatty acids undergo $\beta$-oxidation, wherein two-carbon fragments are sequentially cleaved off to form Acetyl CoA units, generating $\text{NADH}$ and $\text{FADH}_2$ in the process. Acetyl CoA feeds directly into the Krebs cycle.

• Protein Catabolism:

  • Proteins are too large to cross bacterial plasma membranes directly. Microbes secrete extracellular enzymes (proteases and peptidases) to hydrolyze complex proteins into individual amino acid monomers.

  • Before amino acids can enter catabolic pathways, they must undergo chemical modification:

    • Deamination: The removal of an amino group ($-\text{NH}_2$), releasing ammonia ($\text{NH}_3$) as a waste product.

    • Decarboxylation: The removal of a carboxyl group ($-\text{COOH}$), producing $\text{CO}_2$.

    • Desulfurization: The removal of a sulfhydryl group ($-\text{SH}$), producing $\text{H}_2\text{S}$.

  • The resulting organic acid intermediates feed into glycolysis, pyruvic acid conversion, or the Krebs cycle.

• Diagnostic Biochemical Tests Based on Metabolism:

  • Fermentation Tests: Test media contains a single carbohydrate source (e.g., mannitol or glucose), a protein nutrient base, and a pH indicator dye (e.g., phenol red). Microbes that ferment the sugar generate acidic byproducts, lowering pH and causing a distinct color change. Gas production is detected using an inverted glass Durham tube.

  • Oxidase Test: Identifies the presence of cytochrome $c$ oxidase within the electron transport chain (e.g., distinguishing positive Pseudomonas from negative enterics).

Biosynthetic and Integration Pathways (Anabolism)

• Integration of Metabolism (Amphibolic Pathways):

  • Amphibolic Pathways: Dual-function metabolic pathways that participate in both catabolic (breakdown) and anabolic (synthesis) operations.

  • Common metabolic intermediates serve as central structural hubs connecting carbohydrate, lipid, protein, and nucleic acid pathways.

• Biosynthetic Pathways:

  • Polysaccharide Biosynthesis: Synthesized from glucose intermediates. Bacteria convert Glucose 6-phosphate using ATP or UTP energy into activated sugar derivatives such as Adenosine diphosphoglucose (ADPG, precursor to bacterial glycogen) or Uridine diphosphate $N$-acetylglucosamine (UDPNaC, precursor to peptidoglycan).

  • Lipid Biosynthesis: Glycerol is synthesized from the glycolytic intermediate Dihydroxyacetone phosphate (DHAP). Fatty acid hydrocarbon chains are constructed by linking Acetyl CoA units together via condensation reactions.

  • Amino Acid Biosynthesis: Intermediate molecules extracted directly from the Krebs cycle (such as $\alpha$-ketoglutaric acid or oxaloacetic acid), the pentose phosphate pathway, or the Entner-Doudoroff pathway are converted into amino acids via amination (adding an amine group to an organic acid) or transamination (transferring an amine group from a source amino acid to an organic precursor acid).

  • Nucleic Acid Biosynthesis: Pentose sugars (ribose and deoxyribose) sourced from the pentose phosphate or Entner-Doudoroff pathways are joined to purine and pyrimidine bases (synthesized from amino acids glutamine, aspartic acid, and glycine) and phosphate groups to build nucleotides.