Comprehensive Microbiology Study Guide: History, Cellular Biology, Metabolism, Pathogens, Viruses, and Prions

Fundamentals and Historical Foundations of Microbiology

• Definition and Scope of Microbiology:

  • Microbiology is the scientific study of microscopic organisms that are too small to be observed without magnification.
  • Microorganisms (or microbes) inhabit virtually every ecological niche where adequate moisture and nutrients exist.
  • While medical microbiology focuses heavily on pathogens—microorganisms capable of causing infectious disease—only approximately $1\%$ of all identified microbes are pathogenic.
  • The vast majority of microorganisms are non-pathogenic, ecologically vital, and commercially beneficial:
  • Microbes serve essential industrial roles in food production, including the manufacture of cheese, bread, and pickles.
  • Environmental microbes drive critical biogeochemical nutrient cycling and organic matter decomposition.
  • Microorganisms constitute the foundational base of aquatic and terrestrial food webs.

• Early Hypotheses of Disease and Immunity in Antiquity:

  • Hippocrates ($460\text{--}370\text{ BC}$): Rejected the prevailing supernatural explanations of illness, positing instead that diseases arise from natural causes originating within the patient or the patient's surrounding physical environment.
  • Thucydides ($460\text{--}395\text{ BC}$): Documented during the Athenian plague that survivors did not become reinfected even when nursing actively ill patients, establishing the earliest recorded concept of acquired immunity.
  • Marcus Terentius Varro ($116\text{--}27\text{ BC}$): Formulated an early conceptualization of germ theory in his text Res Rusticae (On Farming, published in $36\text{ BC}$), warning against locating estates near swamps because minute invisible creatures (animalia minuta) float in the air, enter the body through the mouth and nose, and cause severe diseases.

• Development of Microscopy and Discovery of Cells:

  • Robert Hooke ($1665$): Credited with the discovery of cells using a compound microscope. In his seminal publication Micrographia, he described the microscopic structure of cork tissue as resembling a "honey-comb" composed of "small Boxes or Bladders of Air," terming each individual chamber a "cell."

• Anton van Leeuwenhoek ($1673$): Regarded as the "Father of Microbiology." A skilled Dutch glassmaker, he fabricated simple handheld microscopes with extraordinary resolving power, enabling him to be the first human to observe living, single-celled microorganisms (termed "animalcules"). He reported his observations in letters to the Royal Society of London.

• The Spontaneous Generation vs. Biogenesis Debate:
  • Spontaneous Generation: An ancient doctrine dating back to Aristotle ($384\text{--}322\text{ BC}$) postulating that living organisms can emerge spontaneously from nonliving matter (e.g., frogs arising from mud banks during Nile flooding, or mice appearing in grain stored in thatched barns).
  • Francesco Redi ($1668$): Formulated controlled experiments to test the spontaneous generation of maggots from meat.
  • Experiment 1: Placed raw meat in open jars versus tightly sealed cork-stopped jars. Maggots developed exclusively in open jars. Critics argued that the cork seal prevented the entry of fresh air, which was supposedly necessary for a "vital life force."
  • Experiment 2: Placed meat in jars covered with fine gauze mesh. Fresh air could circulate freely, but flies were excluded. Maggots did not form on the meat, disproving the spontaneous generation of flies.

• John Needham ($1745$): Briefly boiled plant and animal broths, poured them into flasks, and sealed them. Microbial growth appeared after several days, leading him to argue erroneously that microbes arose spontaneously.
• Lazzaro Spallanzani ($1765$): Modified Needham's protocol by sealing broths directly inside glass flasks prior to extended boiling. Sealed flasks remained clear indefinitely unless subsequently cracked and exposed to air, suggesting that microbes entered from airborne contamination rather than spontaneous generation. Critics continued to argue that sealing air out invalidated the experiment.
• Louis Pasteur ($1861$): Definitively disproved spontaneous generation and won the Paris Academy of Sciences prize.
  • Showed that air filtered through cotton contained trapped microbes.
  • Engineered unique "swan-necked" (S-curved) flasks that allowed free exchange of ambient air while trapping airborne dust and microbes in the lower U-bend.
  • Boiled broth in swan-necked flasks remained sterile indefinitely; breaking off the curved neck permitted airborne particles to drop into the broth, triggering rapid microbial contamination.

• Formulation of Modern Cell Theory:

  • Theodor Schwann and Matthias Schleiden ($1830\text{s}$): Recognized foundational cellular homologies across plant and animal tissues, concluding that all living things are composed of cells.
  • Rudolf Virchow and Robert Remak ($1850\text{s}$): Established the principle of biogenesis—that living cells arise exclusively from preexisting cells through cellular division.
  • Core Tenets of Modern Cell Theory:
  1. All cells arise exclusively from preexisting cells (the principle of biogenesis).
  2. Cells are the fundamental structural and functional units of all living organisms.

• The Germ Theory of Disease and Antiseptic Technique:

  • Miasma Theory: The historical belief that infectious diseases (including the Black Death) arose from noxious vapors or "miasmas" emanating from decomposing organic matter and cesspools.
  • Germ Theory of Disease: The foundational principle that microscopic organisms are the causative agents of infectious diseases.
  • Ignaz Semmelweis ($1840\text{s}$): Identified that puerperal (childbed) fever mortality was significantly higher in maternity clinics staffed by medical students ($10\%\text{--}20\%$) than in midwife-led wards ($1\%$). Medical students performed autopsies and conducted pelvic examinations without intervening hygiene. Implementing mandatory chlorinated lime handwashing reduced physician-ward mortality down to $1\%$.
  • Joseph Lister ($1860\text{s}$): Addressed the approximately $50\%$ mortality rate from postsurgical gangrene and wound sepsis by introducing carbolic acid (phenol) sprays as surgical antiseptics and demanding rigid handwashing and instrument sterilization.
  • Robert Koch ($1876$): Provided the first direct experimental proof of bacterial pathogenesis by isolating Bacillus anthracis from cattle dying of anthrax, culturing it in pure form, injecting it into healthy hosts to reproduce the fatal disease, and re-isolating the identical organism. This established Koch's Postulates ("one microbe, one disease").

• Pioneering Chemotherapy and Immunization:

  • Variolation: Historical practice originating in $10\text{th}$-century China involving deliberate inoculation of susceptible individuals with scabs or fluid from smallpox pustules to confer protection, despite inherent risks of triggering fatal systemic outbreaks.
  • Edward Jenner ($1796$): Observed that milkmaids infected with cowpox remained resistant to lethal smallpox. Inoculated an $8$-year-old boy with cowpox pus and subsequently challenged him with smallpox matter; the boy was immune. Coined the term "vaccination" (from the Latin vacca, meaning cow).

• Paul Ehrlich ($1909$): Pioneered antimicrobial chemotherapy in search of a "magic bullet"—a selective chemical agent that destroys pathogens without host toxicity. Screened over $600$ synthetic arsenic derivatives to identify Compound 606 (Salvarsan), which successfully targeted Treponema pallidum (the causative agent of syphilis).
• Alexander Fleming ($1928$): Discovered the first natural antibiotic when a culture plate of Staphylococcus became contaminated with Penicillium chrysogenum mold, creating a clear zone of bacterial inhibition. The active secreted compound was isolated as penicillin.

Microbial Taxonomy, Nomenclature, and Broad Classification

• Taxonomic Hierarchy and the Tree of Life:
  • Taxonomy is the biological science governing the classification, description, identification, and naming of organisms.
  • Developed by Carolus Linnaeus in the $1700\text{s}$, modern classification organizes life into nested, hierarchical categories:
  • Domain (the broadest division: Bacteria, Archaea, Eukarya)
  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species (the unique individual taxonomic identifier)

• Binomial Nomenclature Guidelines:

  • Every organism is designated by a two-word scientific name comprising the capitalized Genus and the lowercase species epithet.
  • Both names must be italicized in print or underlined when handwritten (e.g., Ursus americanus, Homo sapiens, Escherichia coli, Saccharomyces cerevisiae).
  • Following initial full mention, the genus name may be abbreviated to its capitalized initial followed by a period (e.g., E. coli, S. cerevisiae).

• Species Concept in Microbiology:

  • In eukaryotes, a species is typically defined as a population of interbreeding organisms capable of producing viable, fertile offspring.
  • Because prokaryotes reproduce asexually and frequently engage in horizontal gene transfer, a prokaryotic species is defined operationally as a population of cells with highly similar phenotypic and genetic characteristics.
  • Microorganisms within a single species can exhibit significant sub-classification into strains (genetic variants that are nearly identical genomically but display divergent attributes, such as harmless commensal versus enterohemorrhagic pathogenic E. coli).

• Distinguishing Features Across the Seven Major Microbial Groups:

  • Bacteria:
  • Cellular classification: Unicellular prokaryotes (lack membrane-bound nuclei).
  • Cell wall composition: Contains peptidoglycan.
  • Motility: Many are motile via prokaryotic flagella.
  • Metabolism: Diverse; encompasses photosynthetic (oxygen-producing or anoxygenic) and non-photosynthetic heterotrophs and autotrophs.
  • Pathogenicity: Many are harmless or beneficial; select species are major human and animal pathogens.
  • Archaea:
  • Cellular classification: Unicellular prokaryotes.
  • Cell wall composition: Pseudopeptidoglycan, polysaccharides, or protein coats; never contains true peptidoglycan.
  • Motility: Motile species utilize specialized archaeal flagella.
  • Metabolism: Diverse (chemoautotrophic, heterotrophic, non-photosynthetic or photosynthetic); frequently adapted to extreme habitats.
  • Pathogenicity: Zero known pathogenic species.
  • Fungi:
  • Cellular classification: Eukaryotes; unicellular forms are yeasts, while multicellular filamentous forms are molds.
  • Cell wall composition: Rigid walls constructed of chitin.
  • Motility: Entirely non-motile.
  • Metabolism: Strictly non-photosynthetic chemoheterotrophs utilizing absorptive nutrition.
  • Pathogenicity: Many species act as opportunistic or primary pathogens causing mycoses.
  • Protozoa:
  • Cellular classification: Unicellular eukaryotic microorganisms.
  • Cell wall composition: Lack cell walls entirely.
  • Motility: Motile via cilia, flagella, or pseudopodia (amoeboid movement).
  • Metabolism: Free-living or parasitic; predominantly chemoheterotrophic, though select species are photosynthetic.
  • Pathogenicity: Multiple medically significant parasitic pathogens.
  • Algae:
  • Cellular classification: Unicellular or multicellular eukaryotes.
  • Cell wall composition: Cellulose-rich cell walls.
  • Motility: Motile varieties possess flagella or cilia.
  • Metabolism: Strictly photosynthetic autotrophs; responsible for generating approximately $50\%$ of global atmospheric oxygen and serving as a major carbon sink.
  • Pathogenicity: Non-pathogenic via direct tissue invasion; disease states are mediated strictly through the ingestion of algal toxins.
  • Helminths:
  • Cellular classification: Multicellular eukaryotic animals (roundworms and flatworms).
  • Cell wall composition: Animal cells lacking cell walls.
  • Motility: Motile via integrated muscular contraction.
  • Metabolism: Strictly non-photosynthetic heterotrophs.
  • Pathogenicity: Included in microbiology because diagnostic stages involve microscopic eggs and larvae; all parasitic helminths function as obligate pathogens.
  • Viruses:
  • Cellular classification: Acellular infectious agents; excluded from the tree of life.
  • Cell wall composition: None (possess a protein capsid, with or without a lipid envelope).
  • Motility: Entirely non-motile.
  • Metabolism: Metabolically inert outside host cells; lack cellular machinery and energy-generating systems.
  • Pathogenicity: Obligate intracellular parasites; all viral agents are pathogenic to their specific host cells.

Molecular Biology Foundations and the Central Dogma

• The Central Dogma of Molecular Biology:

  • Defines the directional flow of genetic information inside biological systems: $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$.
  • Information can be replicated from nucleic acid to nucleic acid, or transferred from nucleic acid to protein, but cannot be transferred backwards from protein to protein or protein to nucleic acid.

• DNA Replication:

  • High-fidelity duplication of genetic blueprints preceding cell division.
  • Double-stranded antiparallel DNA relies on strict complementary base pairing: adenine ($\text{A}$) pairs exclusively with thymine ($\text{T}$) via two hydrogen bonds, while cytosine ($\text{C}$) pairs with guanine ($\text{G}$) via three hydrogen bonds.
  • During replication, DNA helicase separates the parental strands, and DNA polymerase synthesizes new complementary daughter strands using the unwound parental strands as templates.

• RNA Transcription:

  • Enzymatic synthesis of messenger RNA (mRNA) from a specific DNA gene sequence.
  • RNA polymerase binds to a gene promoter, unwinds the localized DNA region, and synthesizes a single-stranded RNA transcript complementary to the template strand (incorporating uracil, $\text{U}$, in place of thymine).
  • mRNA transports the encoded polypeptide blueprint out of the chromosome to cellular ribosomes.

• Protein Translation:

  • Synthesis of a linear polypeptide chain from the mRNA nucleotide sequence, executed by ribosomes.
  • Ribosomes decode mRNA in discrete, three-nucleotide units termed codons.
  • Transfer RNA (tRNA) molecules matching specific anticodons deliver corresponding amino acids to the ribosome.
  • Peptide bonds covalently link successive amino acids into elongating polypeptide chains, which fold into secondary, tertiary, and quaternary protein conformations.
  • Multiple ribosomes simultaneously translate a single mRNA molecule (forming polyribosomes) to amplify protein output.

• Primary Classes and Functions of Cellular Proteins:

  • Structural support: Impart mechanical stability to tissues and cell envelopes (e.g., bacterial peptidoglycan cross-peptides, eukaryotic cytoskeleton).
  • Immunological protection: Mediate immune recognition and neutralization of foreign invaders (e.g., antibodies, complement system proteins).
  • Transport: Mediate translocation of solutes across biological membranes (e.g., multidrug efflux pumps contributing to antibiotic resistance).
  • Cell signaling: Coordinate intercellular and intracellular responses to external environmental cues (e.g., cytokines activating immune cells).
  • Catalysis (Enzymes): Accelerate chemical reactions necessary for metabolism and cellular virulence (e.g., metabolic synthases, bacterial toxins).

Cellular Architecture: Prokaryotes vs. Eukaryotes

• Universal Cellular Components:
  • All living cellular entities possess:
  1. Cytoplasm: Gel-like aqueous cytosol containing dissolved ions, enzymes, and organic metabolites.
  2. Plasma membrane: Phospholipid bilayer that encloses the internal cellular compartment.
  3. Chromosomes: Genetic material encoding structural and metabolic blueprints.
  4. Ribosomes: Protein-RNA complexes responsible for cellular protein synthesis.

• Size Comparison:

  • Prokaryotic cells are typically smaller, measuring approximately $0.5\text{--}1\,\mu\text{m}$ in diameter.
  • Eukaryotic cells are significantly larger and structurally more complex, measuring approximately $5\text{--}20\,\mu\text{m}$ in diameter.

• Prokaryotic Internal Structures:

  • Nucleoid: Non-membrane-bound cytoplasmic region containing the bacterial chromosome. Prokaryotic genomes typically consist of a single, circular, haploid DNA molecule packaged with nucleoid-associated proteins (NAPs) that organize and condense the chromosome.
  • Plasmids: Small, circular, double-stranded extrachromosomal DNA elements replicating independently of the bacterial chromosome. Plasmids often carry non-essential genes conferring advantageous environmental traits, such as antibiotic resistance, metabolic enzymes, or virulence factors.
  • Ribosomes ($70\text{S}$): Protein-synthesizing complexes situated exclusively within the cytoplasm, composed of a small $30\text{S}$ subunit and a large $50\text{S}$ subunit (combined sedimentation coefficient of $70\text{S}$).
  • Inclusions: Cytoplasmic nutrient storage bodies formed by prokaryotes living in resource-fluctuating environments. Store carbon and energy reserves in the form of polymers such as glycogen or starches, which the cell accesses during starvation.
  • Endospores:
  • Specialized dormant survival structures formed by select Gram-positive bacteria (e.g., Bacillus, Clostridium) under environmental stress or starvation.
  • Endospores protect the bacterial genome inside a dehydrated, keratin-like coat resistant to boiling, desiccation, chemical disinfectants, and ionizing radiation.
  • Can persist in a metabolically inactive state for thousands of years.
  • Upon return of favorable conditions, endospores undergo germination to return to an active, growing vegetative state.

• Prokaryotic Cell Envelope:
  • Plasma Membrane: Phospholipid bilayer exhibiting selective permeability. Crucially, prokaryotic plasma membranes lack sterols (such as cholesterol), which eukaryotes utilize to modulate membrane fluidity.
  • Bacterial Cell Wall & Peptidoglycan (Murein):
  • Prevents osmotic lysis and protects bacteria from environmental mechanical stressors.
  • Consists of an alternating meshwork of two amino sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
  • Polysaccharide glycan strands of alternating NAG-NAM units are cross-linked via tetrapeptide side chains and pentapeptide bridges.

• Gram-Positive Cell Wall:
  • Features a thick, multilayered peptidoglycan shell situated external to the plasma membrane.
  • Embedded with teichoic acids (polyalcohol phosphate chains that traverse the peptidoglycan matrix), which enhance tensile strength, regulate cation transport, and facilitate bacterial adhesion to host tissues.
• Gram-Negative Cell Wall:
  • Contains a thin peptidoglycan layer located within the periplasmic space between the inner plasma membrane and an asymmetric outer membrane.
  • The outer membrane's outer leaflet contains lipopolysaccharide (LPS), an endotoxin composed of Lipid A (endotoxin core), core polysaccharide, and an O-antigen chain, which can induce severe inflammatory reactions and septic shock.

• Glycocalyx (Capsules and Slime Layers):

  • Viscous, gelatinous extracellular polymeric coating.
  • Capsule: Organized, densely woven polysaccharide or protein matrix firmly anchored to the outer cell wall. Prevents desiccation and provides antiphagocytic protection against host immune clearance.
  • Slime Layer: Loosely associated, unorganized matrix composed of polysaccharides, glycoproteins, or glycolipids that easily detaches and aids in surface adherence and biofilm initiation.

• Prokaryotic Filamentous Appendages:

  • Fimbriae: Numerous short, thin, bristle-like protein projections that mediate attachment to environmental surfaces and host cells, serving as primary colonization factors.
  • Pili: Longer, less numerous protein appendages. The specialized F-pilus (conjugation or sex pilus) directly binds a recipient bacterium to transfer plasmid or chromosomal DNA.
  • Flagella: Rigid, helical structures composed of flagellin protein subunits attached via a hook to a basal body (motor apparatus) anchored across the cell envelope.

• Bacterial Motility and Chemotaxis:

  • Bacterial flagella spin like rotary propellers to mediate movement:
  • Running: Counter-clockwise flagellar rotation bundles filaments together, driving the cell forward in a smooth, linear trajectory.
  • Tumbling: Clockwise flagellar rotation unravels and splays filaments apart, causing the cell to tumble randomly in place, altering its spatial orientation.
  • Taxis: Directed movement along an environmental gradient (chemotaxis = chemical; phototaxis = light; thermotaxis = temperature). Moving toward a stimulus is positive taxis; moving away is negative taxis.
  • In an attractant gradient, runs are prolonged while tumbles are suppressed, biasing net translocation toward the attractant.

• Prokaryotic Reproduction and Horizontal Gene Transfer (HGT):
  • Bacteria reproduce exclusively asexually via binary fission, transferring identical genomic copies to progeny through vertical gene transfer.
  • To overcome the evolutionary limitation of low asexual genetic variation, bacteria employ Horizontal Gene Transfer (HGT):
  1. Transformation: Direct uptake of exogenous, naked DNA fragments or intact plasmids released into the extracellular environment by lysed donor cells.
  2. Transduction: Accidental transfer of host bacterial chromosomal fragments packaged aberrantly inside a bacteriophage coat during viral lytic cycles.
  3. Conjugation: Direct physical transfer of plasmid or chromosomal DNA through a conjugation pilus connecting donor and recipient cells.

• Eukaryotic Cell Specialization:
  • Nucleus: Houses the genome in multiple linear chromosomes packaged around basic histone proteins to form chromatin, enveloped by a double-membrane nuclear envelope with nuclear pore complexes.
  • Ribosomes ($80\text{S}$): Cytoplasmic and rough endoplasmic reticulum-bound ribosomes have an $80\text{S}$ sedimentation rate (composed of a $40\text{S}$ and a $60\text{S}$ subunit). Semiautonomous organelles (mitochondria and chloroplasts) contain evolutionary remnant $70\text{S}$ ribosomes reflecting their endosymbiotic bacterial origins.
  • Endomembrane System: Internal network of membranous tubules and cisternae (rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, endosomes, and transport vesicles) that coordinates intracellular protein sorting, lipid synthesis, and vesicle transport.
  • Cytoskeleton: Complex, dynamic structural matrix consisting of microfilaments, intermediate filaments, and microtubules that supports amoeboid motion, vesicle transport, mitosis, and phagocytosis.
  • Plasma Membrane: Contains sterols (such as cholesterol) that regulate fluid dynamics and membrane stability across fluctuating temperatures.
  • Extracellular Matrix (ECM): Secreted sticky meshwork of glycoproteins and proteoglycans in animal and protozoan cells that supports tissue structure and facilitates cell-to-cell adhesion in the absence of a rigid cell wall.
  • Locomotor Appendages: Eukaryotic flagella and cilia are membrane-bound projections containing an axoneme ($9+2$ microtubule doublet arrangement). Motor dynein arms drive sliding between doublet microtubules, producing flexible, wave-like sinusoidal bending rather than rotary motion.

Microbial Metabolism and Energy Generation

• Metabolic Principles: Anabolism and Catabolism:

  • Metabolism represents the totality of biochemical reactions occurring within a living organism.
  • Anabolism: Endergonic biosynthetic reactions that combine simple precursor molecules into complex macromolecules, consuming cellular energy ($\text{ATP}$).
  • Catabolism: Exergonic degradation reactions that break down complex organic polymers into simpler products, releasing stored chemical energy that is harnessed as $\text{ATP}$.
  • Mechanisms of $\text{ATP}$ Generation:
  • Substrate-level phosphorylation: Direct enzymatic transfer of a high-energy phosphate group from an intermediate metabolic substrate to $\text{ADP}$.
  • Oxidative phosphorylation: Chemiosmotic synthesis of $\text{ATP}$ driven by proton gradients generated from electron transport chain redox reactions.

• Redox Reactions and Electron Carriers:

  • Oxidation: The removal of electrons from an electron donor.
  • Reduction: The addition of electrons to an electron acceptor.
  • Oxidation and reduction always occur simultaneously in coupled redox pairs.
  • Primary cellular coenzymes that capture high-energy electrons from catabolic intermediates:
  • Nicotinamide adenine dinucleotide ($\text{NAD}^+$ reduced to $\text{NADH}$)
  • Flavin adenine dinucleotide ($\text{FAD}$ reduced to $\text{FADH}_2$)
  • Nicotinamide adenine dinucleotide phosphate ($\text{NADP}^+$ reduced to $\text{NADPH}$, used primarily in anabolic biosynthesis)

• Glycolysis (Embden-Meyerhof-Parnas Pathway):

  • The universal, oxygen-independent catabolic pathway for glucose, taking place in the cytoplasm of both prokaryotic and eukaryotic cells.
  • Converts one molecule of glucose ($6\text{C}$) into two molecules of pyruvate ($3\text{C}$).
  • Net production per glucose:
  • $2\,\text{ATP}$ (via substrate-level phosphorylation)
  • $2\,\text{NADH}$

• Alternative Pathways for Glucose Catabolism in Prokaryotes:

  • Pentose-Phosphate Pathway (PPP):
  • Diverts glucose to synthesize ribose-5-phosphate ($5\text{C}$), a prerequisite building block for nucleotides, nucleic acids ($\text{DNA}$, $\text{RNA}$), and select amino acids.
  • Yields $2\,\text{NADPH}$ per glucose.
  • Produces $0\,\text{ATP}$; serves an anabolic/biosynthetic role rather than an energy-generating function.
  • Entner-Doudoroff Pathway (ED):
  • Alternative catabolic route converting one glucose into two pyruvates.
  • Yields $1\,\text{ATP}$ (substrate-level phosphorylation), $1\,\text{NADH}$, and $1\,\text{NADPH}$.
  • Less energetically efficient than standard glycolysis ($1\,\text{ATP}$ vs. $2\,\text{ATP}$), making it less prevalent.

• Cellular Respiration:

  • A catabolic sequence completing the total breakdown of pyruvate into $\text{CO}_2$ through four coordinated stages:
  1. Pyruvate Oxidation: Pyruvate ($3\text{C}$) is decarboxylated into acetyl-CoA ($2\text{C}$), releasing one $\text{CO}_2$ and reducing one $\text{NAD}^+$ to $\text{NADH}$. Occurs in the prokaryotic cytoplasm and the eukaryotic mitochondrial matrix.
  2. Krebs (Citric Acid) Cycle: Acetyl-CoA transfers its acetyl group to oxaloacetate, initiating a cyclical series of reactions that yield $2\,\text{CO}_2$, $1\,\text{ATP}$ (via substrate-level phosphorylation), $3\,\text{NADH}$, and $1\,\text{FADH}_2$ per acetyl-CoA entered ($2\,\text{ATP}$, $6\,\text{NADH}$, and $2\,\text{FADH}_2$ per glucose molecule).
  3. Electron Transport Chain (ETC): Membrane-embedded protein complexes oxidize $\text{NADH}$ and $\text{FADH}_2$. Electrons flow through sequential redox steps, releasing free energy that pumps protons ($\text{H}^+$) across the membrane (prokaryotic plasma membrane or eukaryotic inner mitochondrial membrane) to establish an electrochemical proton motive force.
  4. Chemiosmosis: Protons flow down their concentration gradient through the $\text{F}_0\text{F}_1$ $\text{ATP}$ synthase complex, rotating its catalytic rotor to drive the phosphorylation of $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$.

• Aerobic vs. Anaerobic Respiration:

  • Aerobic Respiration: Utilizes molecular oxygen ($\text{O}_2$) as the final terminal electron acceptor, reducing it to water ($\text{H}_2\text{O}$).

  • Anaerobic Respiration: Utilizes an inorganic terminal electron acceptor other than oxygen, such as nitrate ($\text{NO}_3^-$), sulfate ($\text{SO}_4^{2-}$, reduced to $\text{H}_2\text{S}$, causing a characteristic rotten-egg odor), or carbonate ($\text{CO}_3^{2-}$, reduced to $\text{CH}_4$/methane).

  • Anaerobic ETCs pump fewer protons per electron pair, generating a smaller proton gradient and lower $\text{ATP}$ yields compared to aerobic respiration.

  • Fermentation:

• Occurs when cells lack the genetic capability for respiration or when terminal electron acceptors are depleted.

• Does not involve an electron transport chain or Krebs cycle and generates zero additional $\text{ATP}$.

• Purpose: Oxidizes $\text{NADH}$ back to $\text{NAD}^+$ by transferring electrons to an endogenous organic molecule (typically pyruvate), ensuring $\text{NAD}^+$ remains available for glycolysis to continue producing $2\,\text{ATP}$ per glucose.

• Lactate Fermentation: Pyruvate directly accepts electrons from $\text{NADH}$, reducing it to lactate (executed by lactic acid bacteria and oxygen-depleted human skeletal muscle).

• Alcoholic Fermentation: Pyruvate is decarboxylated into acetaldehyde and $\text{CO}_2$; acetaldehyde then accepts electrons from $\text{NADH}$, forming ethanol (executed by brewing/baking yeasts).

• ATP Yield Comparisons and Microbial Growth Rates:

  • Aerobic Respiration (Prokaryotes): Maximum theoretical yield of up to $38\,\text{ATP}$ per glucose ($2$ from glycolysis, $2$ from Krebs, and $34$ from oxidative phosphorylation).
  • Aerobic Respiration (Eukaryotes): Yield is lower (typically $36\text{--}38\,\text{ATP}$) due to energy costs of shuttling cytosolic $\text{NADH}$ into mitochondria.
  • Anaerobic Respiration: Yield varies widely by species from single digits to the low $30\text{s}$.
  • Fermentation: Strictly limited to the $2\,\text{ATP}$ generated during glycolysis.
  • Cellular growth rate is directly correlated with $\text{ATP}$ generation efficiency: aerobically respiring microbes grow fastest, while strict fermenters grow slowest.

• Lipid and Protein Catabolism:

  • Lipid Catabolism: Extracellular lipases and phospholipases hydrolyze triglycerides and phospholipids into glycerol and free fatty acids. Glycerol is converted into glyceraldehyde-3-phosphate and enters glycolysis. Fatty acids are cleaved via $\beta$-oxidation into acetyl-CoA units, which enter the Krebs cycle (requiring respiration).
  • Protein Catabolism: Extracellular proteases and peptidases cleave large proteins into free amino acids, which enter the cell. Amino acids undergo deamination to remove the amino group ($-\text{NH}_2$), which is excreted as toxic ammonia ($\text{NH}_3$, or converted to urea in humans). The remaining organic keto acids enter glycolysis or the Krebs cycle.

• Metabolic Diversity Classifications:

  • Categorized by carbon source and energy source:
  • Carbon Source:
    • Autotroph: Fixes inorganic carbon dioxide ($\text{CO}_2$) into organic carbon.
    • Heterotroph: Relies on preformed organic molecules produced by other organisms.
  • Energy Source:
    • Phototroph: Captures light energy.
    • Chemotroph: Derives energy from the oxidation of chemical compounds.
  • The Four Nutritional Categories:
  1. Photoautotrophs: Light energy + $\text{CO}_2$ carbon source (e.g., all algae, cyanobacteria, photosynthetic protozoa).
  2. Photoheterotrophs: Light energy + organic carbon source (e.g., non-sulfur purple and green bacteria).
  3. Chemoautotrophs: Inorganic chemical energy (e.g., hydrogen sulfide, sulfur, iron, ammonia) + $\text{CO}_2$ carbon source (e.g., specialized lithotrophic bacteria).
  4. Chemoheterotrophs: Organic chemical energy + organic carbon source (e.g., all helminths, all fungi, most protozoa, most bacteria, and all animals).

Eukaryotic Pathogens: Fungi, Algae, Protozoa, and Helminths

• Fungal Pathogens and Clinical Mycoses:
  • Fungi exist as unicellular budding yeasts or multicellular filamentous molds.
  • Diseases caused by fungi are called mycoses, classified clinically by tissue invasiveness:
  • Superficial mycoses: Colonize the outer epidermis, hair, and nails without digesting host keratin.
  • Cutaneous mycoses: Colonize the epidermis, hair, and nails while actively degrading keratin. Caused by dermatophytes and clinically termed tineas (named by anatomical site):
    • Tinea corporis: Ringworm affecting the body trunk.
    • Tinea pedis: Athlete's foot.
    • Tinea cruris: Jock itch.
    • Dermatophytes thrive in dark, humid environments such as public showers, occlusive footwear, and undergarments.

* *Subcutaneous mycoses:* Penetrate beneath the epidermal barrier to establish localized infections in the dermis and underlying subcutaneous tissues.
* *Systemic mycoses:* Disseminate through deep internal organ systems, typically initiating as respiratory infections via inhaled spores before hematogenous spread.

• Fungal Intoxications (Mycotoxicoses):

  • Claviceps purpurea: Infects rye and cereal grains, synthesizing ergot toxin, which causes ergotism:

    • Gangrenous ergotism: Severe peripheral vasoconstriction leading to necrosis and gangrene of the extremities.
    • Convulsive ergotism: Neurotoxic targeting of the central nervous system, inducing mania, hallucinations, spasms, and seizures.

  • Aspergillus: Produces aflatoxin via ingestion of contaminated nuts/grains or inhalation; aflatoxin acts as a mutagen and potent hepatic carcinogen, driving hepatocellular carcinoma.

  • Algal Pathogens and Intoxications:

• Algae do not cause disease via tissue destruction, but through environmental intoxication during Harmful Algal Blooms (HABs) when dense populations release high concentrations of toxins that bioaccumulate across trophic levels via biomagnification.

• Diatoms: Unicellular aquatic plankton with silica frustules. Certain species generate the neurotoxin domoic acid. Filter-feeding shellfish accumulate domoic acid, which causes domoic acid toxicosis in consumers, leading to gastroenteritis, permanent short-term memory loss, neurological deficits, and mass mortality in marine birds and baleen whales.

• Dinoflagellates: Unicellular flagellated plankton responsible for "red tides" during population blooms. Synthesize saxitoxins, which induce paralytic shellfish poisoning (PSP) in humans consuming contaminated mollusks. Symptoms include rapid-onset perioral numbness, nausea, vomiting, peripheral paralysis, and death via asphyxiation from respiratory muscle failure.

  • Protozoan Pathogens and Complex Life Cycles:

• Protozoa are unicellular eukaryotes lacking cell walls.

• Exist as motile, feeding, and reproducing trophozoites, or undergo encystment into encapsulated, durable, dormant cysts that survive harsh environmental transitions.

• Definitive Host: The primary host organism in which a parasite reaches sexual maturity and undergoes sexual reproduction.

• Intermediate Host: A secondary host in which a parasite undergoes asexual reproduction or developmental larval stages.

• Entamoeba histolytica:

  • Causative agent of amoebiasis (amoebic dysentery).
  • Infects up to $500\text{ million}$ individuals globally, causing upwards of $100{,}000$ annual fatalities.
  • Humans serve as the definitive and sole host.
  • Life cycle: Fecal-oral ingestion of cysts from contaminated water/food $\rightarrow$ excystment into trophozoites in the intestine $\rightarrow$ mucosal invasion, ulceration, bloody diarrhea $\rightarrow$ encystment $\rightarrow$ shedding of infectious cysts in feces.

• Toxoplasma gondii:

  • Causative agent of toxoplasmosis.

  • Definitive host: Domestic and wild cats (the only host supporting sexual reproduction).

  • Intermediate hosts: Rodents, birds, and other animals.

  • Life cycle: Cats shed environmental cysts in feces $\rightarrow$ ingested by rodents/birds $\rightarrow$ encystment into tachyzoites in host muscle and neural tissue (infected rodents lose their innate fear of cat odors) $\rightarrow$ cats consume prey, completing the sexual cycle.

  • Human impact: Humans acquire infections via accidental ingestion of cat feces or raw cyst-containing meat. While healthy adults are largely asymptomatic, tachyzoites cross the placenta in pregnant women, causing congenital toxoplasmosis, leading to miscarriage, stillbirth, microcephaly, hydrocephalus, mental retardation, deafness, or blindness.

  • Helminthic Parasites (Parasitic Worms):

• Multicellular invertebrate animals with specialized adaptations for obligate parasitism:

  1. Reduced or absent digestive systems (absorb pre-digested nutrients across their tegument).
  2. Reduced nervous systems (stable host environments require minimal sensory tracking).
  3. Reduced locomotor capacity (no requirement to hunt or flee predators).
  4. Enormously complex, amplified reproductive systems (some species release over $200{,}000$ eggs daily to facilitate transmission).

• Platyhelminthes (Flatworms):

  • Predominantly monoecious (hermaphroditic; contain both functional male and female reproductive systems within a single worm).
  • Flukes (Trematodes): Nonsegmented flatworms with oral suckers for tissue attachment.
    • Schistosoma spp.: Blood flukes causing schistosomiasis. Definitive host: humans; intermediate host: freshwater snails. Snail-derived cercariae larvae penetrate human skin in water $\rightarrow$ enter bloodstream $\rightarrow$ mature and mate $\rightarrow$ spined eggs penetrate bladder and bowel, causing hematuria, chronic hepatic cirrhosis, and abdominal pain $\rightarrow$ eggs shed in urine/feces.
  • Tapeworms (Cestodes): Segmented intestinal flatworms comprising an anterior scolex (anchoring head equipped with suckers and hooks) and a linear chain of repeating proglottid segments that mature, fill with eggs, and detach.
    • Taenia solium: Pork tapeworm. Humans are definitive hosts; pigs are intermediate hosts. Ingesting undercooked cyst-infected pork yields adult intestinal tapeworms (taeniasis). Ingesting T. solium eggs leads to cysticercosis, where larvae encyst in muscles and the brain (neurocysticercosis), which is the leading cause of adult-onset acquired epilepsy in the developing world.

• Nematodes (Roundworms):
  • Cylindrical, unsegmented bodies possessing a complete, functional digestive tract.
  • Dioecious (separate biological sexes requiring male and female worms for reproduction).
  • Enterobius vermicularis: Pinworm causing enterobiasis. Humans are the sole host. Gravid adult females migrate out of the anus nocturnally to deposit eggs on the perianal fold, inducing pruritus; scratching facilitates fecal-oral reinfection and fomite spread.
  • Ascaris lumbricoides: Large intestinal roundworm causing ascariasis. Ingested eggs hatch in the small intestine $\rightarrow$ larvae penetrate intestinal vessels $\rightarrow$ enter pulmonary circulation $\rightarrow$ break into alveoli $\rightarrow$ crawl up bronchial tree and pharynx $\rightarrow$ swallowed back into small intestine $\rightarrow$ develop into large adults, which can cause severe nutritional depletion and fatal mechanical intestinal obstructions.

Acellular Pathogens: Viruses and Prions

• Defining Characteristics of Viruses:

  • Acellular (non-cellular), submicroscopic infectious entities lacking internal metabolic machinery.
  • Obligate intracellular parasites that replicate exclusively within specific living host cells.
  • Genomes consist of either $\text{DNA}$ or $\text{RNA}$, never both simultaneously.
  • Genomes are enclosed within a protein coat (capsid), which may be naked or enveloped in a host-derived phospholipid membrane.
  • Dependent on host cell ribosomes, enzymes, and translation factors to synthesize new virions.

• Virion Architecture and Morphology:

  • Individual viral particles are termed virions.
  • Capsids are constructed from repeating protein subunits called capsomeres.
  • Structural categories:
  • Naked (Nonenveloped) viruses: Composed solely of a nucleic acid core enclosed by a protein capsid.
  • Enveloped viruses: Capsid is enclosed by a host-derived phospholipid bilayer and viral matrix proteins acquired during budding.
  • Spikes (Peplomers): Glycoprotein extensions protruding from the capsid or envelope that bind host cell surface receptors to determine host range and tissue tropism.
  • Capsid symmetries:
  • Helical: Rod-shaped or cylindrical capsids where capsomeres wind around a central axis (e.g., Tobacco mosaic virus, Ebola virus).
  • Polyhedral (Icosahedral): Symmetrical multi-sided capsids forming a $20$-faced icosahedron with $12$ vertices (e.g., Poliovirus, Rhinovirus, Adenovirus).
  • Complex: Possess intricate, asymmetrical structural components (e.g., Poxviruses; T-even bacteriophages with an icosahedral head, helical sheath, and baseplate with tail fibers and pins).

• Viral Taxonomy:

  • Governed by genome composition, replication strategy, capsid morphology, envelope presence, and host range.
  • Taxonomic suffixes:
  • Family names end in -viridae (e.g., Retroviridae, Herpesviridae).
  • Genus names end in -virus (e.g., Lentivirus, Simplexvirus).
  • Species designations use descriptive English common names (e.g., Human immunodeficiency virus, Herpes simplex virus).
  • Subspecies variants are designated by letters and numbers (e.g., HIV-1, HIV-2, Hepatitis A, Hepatitis C).

• Replication Cycle of Animal Viruses:

  1. Attachment: Spikes or surface capsomeres bind specifically to host membrane receptors and glycoproteins, establishing viral tropism.
  2. Penetration:
  • Receptor-mediated endocytosis: Standard entry route for nonenveloped viruses and select enveloped viruses, internalizing the virion inside an endocytic vesicle.
  • Membrane fusion: Used by enveloped viruses; the viral envelope fuses directly with the host plasma membrane, releasing the naked nucleocapsid into the cytosol.
  1. Uncoating: Enzymatic breakdown of the capsid (mediated by host lysosomal enzymes, viral uncoating enzymes, or acidic pH changes within the endosome), releasing the viral genome into the cytoplasm.
  2. Biosynthesis:
  • DNA Viruses: Enter the host nucleus; host DNA polymerase replicates the viral DNA, and host RNA polymerase transcribes viral genes into mRNA, which host ribosomes translate into viral proteins in the cytoplasm.
  • RNA Viruses: Replicate exclusively in the cytoplasm using viral RNA-dependent RNA polymerase (RdRP):
    • Positive-sense ($\text{+ssRNA}$): Function directly as mRNA and can be immediately translated by host ribosomes.
    • Negative-sense ($\text{-ssRNA}$): Cannot be translated directly; must be transcribed by packaged RdRP into a complementary $\text{+ssRNA}$ strand before translation.
  • Retroviruses: Contain positive-sense single-stranded RNA ($\text{+ssRNA}$). Viral reverse transcriptase converts viral RNA into double-stranded complementary DNA ($\text{dsDNA}$). Viral integrase transports this DNA into the nucleus and inserts it into a host chromosome to form a permanent provirus. Host RNA polymerase then transcribes the provirus into viral mRNAs and genomic RNAs.
  1. Assembly: Capsomeres self-assemble around newly replicated viral genomes in the host cytoplasm or nucleus to form mature capsids.
  2. Release:
  • Cell Lysis: Common for nonenveloped viruses; the host plasma membrane ruptures, releasing virions and killing the cell.
  • Exocytosis: Nonenveloped capsids exit within host secretory vesicles without immediate lysis.
  • Budding: Enveloped viruses acquire their lipid bilayer by pushing through the host plasma membrane, keeping the host cell intact for continued virion release.

• Clinical Patterns of Viral Infections:

  • Acute Infection: Characterized by rapid viral proliferation, acute clinical symptoms, and either swift immune clearance of the virus or death of the host (e.g., Influenza virus, Measles virus).
  • Persistent Infection: The virus is not cleared and persists in host tissues indefinitely:
  • Latent Infection: The virus enters a dormant state (latency) following an initial acute infection, with replication halted and no detectable symptoms. Cellular stressors can trigger reactivation, initiating a new round of active viral replication and symptoms (e.g., Herpes simplex virus, Varicella-zoster virus).
  • Chronic Infection: Follows an acute episode with persistent, continuous, low-level viral replication, causing gradual, progressive tissue damage and escalating clinical disease over months or years (e.g., Hepatitis B virus, HIV).

• Viral Oncogenesis (Mechanisms of Viral Cancer):

  • Oncogenic viruses transform normal cells into malignant neoplasms via several mechanisms:
  1. Direct introduction of viral oncogenes or transactivation of host proto-oncogenes.
  2. Insertional mutagenesis: Random integration of proviral DNA into host chromosomes disrupts critical tumor suppressor genes or activates proto-oncogenes.
  3. Chronic immune suppression impairs host immune surveillance, preventing the recognition and clearance of transformed malignant cells.

• Prions (Proteinaceous Infectious Particles):

  • Acellular infectious agents composed entirely of misfolded protein, lacking any nucleic acid genome.
  • Mechanism of disease:
  • Normal cellular prion protein ($\text{PrP}^\text{C}$), found primarily on neuronal membranes, is predominantly folded into $\alpha$-helices.
  • The infectious, rogue prion conformation ($\text{PrP}^\text{Sc}$) is enriched in $\beta$-pleated sheets.
  • When $\text{PrP}^\text{Sc}$ interacts with native $\text{PrP}^\text{C}$, it acts as a template that forces the normal protein to refold into the pathogenic $\text{PrP}^\text{Sc}$ state.
  • Aggregated $\text{PrP}^\text{Sc}$ forms insoluble amyloid fibrils and plaques, killing neurons and leaving microscopic sponge-like vacuoles throughout the brain parenchyma (transmissible spongiform encephalopathies, TSEs).

• Prion Diseases and Transmission:
  • Human TSEs: Creutzfeldt-Jakob disease (CJD), Kuru (historically transmitted via funerary cannibalism among the Fore people of Papua New Guinea), and Fatal Familial Insomnia (FFI).
  • Animal TSEs: Bovine spongiform encephalopathy (BSE or "mad cow disease"), Scrapie (in sheep and goats), and Chronic Wasting Disease (CWD, in deer and elk).
  • Transmission routes: Ingestion of contaminated neural or muscle tissues, genetic inheritance of mutant prion genes, or iatrogenic exposure via contaminated surgical instruments, blood transfusions, or tissue grafts.
  • Clinical Course: Untreatable, uniformly fatal neurodegenerative decline characterized by progressive ataxia, motor dysfunction, loss of speech, dementia, and death within months to a few years.
  • Extreme Resistance: Prions resist standard sterilization, autoclaving, proteases, formalin fixation, heat, and ionizing radiation.