Microbiology and Cell Biology Study Flashcards

Fundamentals of Microbiology and the Microbial World

• Microorganisms (or microbes) are organisms that are generally too small to be seen without a microscope, though certain fungi and parasites can be seen with the naked eye.

• Most microorganisms are harmless or beneficial; they form the foundation of global food webs, drive nutrient cycling, and are used industrially to produce biofuels, medicines, and fermented foods.

• The human body is inhabited by trillions of microbes; for instance, the human skin and gut host vast microbial communities essential for health, nutrient digestion, and defense against pathogens.

• Microbial bioremediation utilizes microorganisms to clean up environmental pollutants; for example, following the Deepwater Horizon oil spill in the Gulf of Mexico in 2010, populations of the naturally occurring marine bacterium Alcanivorax borkumensis expanded dramatically to consume crude oil.

Historical Foundations of Microbiology and Early Concepts of Disease

• Prehistoric and ancient populations utilized microbial processes for thousands of years prior to the invention of microscopes.

• Archaeological evidence from a Neolithic village in China reveals that fermented beverages composed of rice, honey, and fruit were produced as early as $7000\text{ BC}$.

• Fermentation is an anaerobic metabolic process wherein microorganisms (yeast, bacteria, or mold) convert carbohydrates (sugars) into alcohol, organic acids, and gases such as carbon dioxide ($CO_2$).

• Yeast (Saccharomyces cerevisiae) metabolizes carbohydrates in flour to produce $CO_2$ gas, causing bread dough to expand and rise.

• Ötzi the Iceman, a $5300$ -year-old mummy discovered frozen in the Ötztal Alps on the Austrian-Italian border in 1991, preserved evidence of early disease treatments:

  • Infe cted with eggs of the parasite Trichuris trichiura (whipworm), causing abdominal pain and anemia.

  • Infected with Borrelia burgdorferi, the bacterium responsible for Lyme disease.

  • Carried fruit bodies of the fungus Piptoporus betulinus bound to his gear; this fungus exhibits laxative and antibiotic properties.

  • Covered in skin incisions filled with herbs and burned, indicating early therapeutic tattooing practices.

• Biblical references document the practice of quarantining individuals with communicable diseases such as leprosy.

• Ancient Greek medicine attributed infectious diseases to bad air or foul odors emanating from decaying organic matter, termed the miasma hypothesis or "miasmatic odors."

• Ancient Roman sanitation infrastructure was built around miasma avoidance; Roman engineers constructed aqueducts for clean water and built the Cloaca Maxima ("Greatest Sewer") to channel urban waste into the River Tiber, reducing waterborne epidemics.

• Key historical figures in early medical and biological thought:

  • Hippocrates ($460\text{--}370\text{ BC}$): The "Father of Western Medicine," rejected supernatural explanations for illness, asserting natural etiology from patients or environments; authored texts in the Hippocratic Corpus and created the Hippocratic Oath.

  • Thucydides ($460\text{--}395\text{ BC}$): The "Father of Scientific History," advocated evidence-based cause-and-effect analysis; observed during the Athenian plague ($430\text{--}410\text{ BC}$) that survivors developed adaptive immunity and could care for actively infected individuals without re-infection.

  • Marcus Terentius Varro ($116\text{--}27\text{ BC}$): In Res Rusticae (On Farming, $36\text{ BC}$), warned against building near swamps because minute invisible creatures (animalia minuta) float in air, enter through the mouth and nose, and cause severe disease.

  • Antonie van Leeuwenhoek ($1632\text{--}1723$): A Dutch cloth merchant who developed single-lens microscopes with exceptional magnifying power; in 1675, became the first person to observe live single-celled organisms, naming them "animalcules" or "wee little beasties" in rainwater.

  • Golden Age of Microbiology ($1857\text{--}1914$):

  • Louis Pasteur ($1822\text{--}1895$): French chemist who proved microbial strains have unique metabolic traits; demonstrated fermentation is caused by microbes; invented pasteurization to destroy spoilage organisms; developed vaccines for rabies and anthrax.

  • Robert Koch ($1843\text{--}1910$): German physician who established experimental criteria (Koch's Postulates) connecting a specific isolated pathogen to a specific disease; identified the causative agents of anthrax (Bacillus anthracis), cholera (Vibrio cholerae), and tuberculosis (Mycobacterium tuberculosis).

Taxonomy, Nomenclature, and Phylogeny

• Taxonomy is the formal science of classifying, describing, identifying, and naming living organisms based on shared structural, functional, and genetic traits.

• Carolus Linnaeus ($1701\text{--}1778$): Published Systema Naturae in 1735, introducing a standardized hierarchical classification system.

  • Original Linnaean hierarchy: Kingdom, Class, Order, Family, Genus, Species.

  • Divided the natural world into three kingdoms: Animal, Plant, and Mineral (mineral kingdom was later discarded).

• Binomial Nomenclature: A standardized two-word scientific naming system:

  • Consists of the genus name (capitalized) followed by the species name (lowercase).

  • Both names are italicized in print or underlined when handwritten.

  • Names are derived from Latin, Greek, or descriptive English honorifics.

  • Example: Escherichia coli (abbreviated $E.\,coli$ after initial usage).

  • Example: Haloquadratum walsbyi (genus describes salt habitat halo and square cell geometry quadratum; species honors discoverer Anthony Edward Walsby, 1980).

• Evolutionary Phylogenies and Trees of Life:

• Ernst Haeckel (1866): Published General Morphology of Organisms, proposing Kingdom Protista for unicellular organisms, and later Kingdom Monera for unicellular organisms lacking nuclei (bacteria).

• Robert Whittaker (1969): Proposed Kingdom Fungi; established two Superkingdoms/Empires: Empire Prokaryota (containing Kingdom Monera) and Empire Eukaryota (containing Kingdoms Protista, Fungi, Plantae, and Animalia).

• Carl Woese and George Fox (1970s): Developed a genetics-based tree of life using small subunit ribosomal RNA ($\text{ssu rRNA}$) gene sequence alignments.

• Woese and Fox discovered Archaea, which differed significantly from Bacteria and Eukarya in $\text{ssu rRNA}$ sequences.

• Established three Domains above the kingdom level: Bacteria, Archaea, and Eukarya.

• Viruses are omitted from phylogenetic trees because they are acellular and lack conserved ribosomal RNA sequences.

• Horizontal Gene Transfer (HGT): The exchange of genetic material between distinct species, prevalent among prokaryotes, creating an interconnected "web of life" rather than a simple branching tree.

  • Bacterial Identification and Reference Manuals:

• Bergey's Manual of Determinative Bacteriology and Bergey's Manual of Systematic Bacteriology (first published 1923) serve as standard references for bacterial identification and taxonomy.

• Non-visual identification methods include biochemical testing (enzymatic pathways), serological assays (antibody-antigen binding), and nucleic acid/protein sequencing.

  • Strains: Genetic variants or subtypes within a single microbial species.

• Most E.\,coli strains are harmless commensals in the human gut that synthesize vitamins and prevent pathogen colonization.

• Pathogenic strain E.\,coli $\text{O157:H7}$ produces Shiga toxin, causing severe abdominal cramps, bloody diarrhea, and renal failure; outbreaks are linked to contaminated water, raw produce, and undercooked ground beef.

Size, Scale, and Diversity of Microorganisms

• Spatial scale and visual limits in microbiology:

  • Human eye limit of resolution: approximately $100\,\mu\text{m}$.

  • Typical eukaryotic plant/animal cells: $10\text{--}100\,\mu\text{m}$.

  • Typical bacterial cells: approximately $1\,\mu\text{m}$.

  • Typical viruses: approximately $100\,\text{nm}$ (roughly $10\times$ smaller than typical bacteria).

• Metric Units of Measurement in Microbiology:

  • Meter ($\text{m}$): $1\,\text{m} = 10^0\,\text{m}$

  • Decimeter ($\text{dm}$): $0.1\,\text{m} = 10^{-1}\,\text{m}$

  • Centimeter ($\text{cm}$): $0.01\,\text{m} = 10^{-2}\,\text{m}$

  • Millimeter ($\text{mm}$): $0.001\,\text{m} = 10^{-3}\,\text{m}$

  • Micrometer ($\mu\text{m}$): $0.000001\,\text{m} = 10^{-6}\,\text{m}$

  • Nanometer ($\text{nm}$): $0.000000001\,\text{m} = 10^{-9}\,\text{m}$

• Major Groups of Microorganisms:

  • Bacteria (Domain Bacteria):

  • Unicellular prokaryotes lacking a membrane-bound nucleus.

  • Cell walls contain peptidoglycan.

  • Morphologies: spherical (coccus), rod-shaped (bacillus), curved rod (vibrio), short rod/oval (coccobacillus), rigid spiral (spirillum), flexible spiral (spirochete).

  • Metabolic diversity: oxygenic photosynthetic (cyanobacteria), anoxygenic photosynthetic (green sulfur/nonsulfur bacteria), nonphotosynthetic heterotrophs, and autotrophs.

  • Archaea (Domain Archaea):

  • Unicellular prokaryotes with distinct evolutionary lineages.

  • Cell walls lack peptidoglycan; composed of pseudopeptidoglycan, polysaccharides, glycoproteins, or protein S-layers.

  • Plasma membranes contain ether-linked, branched isoprenoid lipids (bilayers or monolayers).

  • Inhabit diverse environments, including extreme conditions (extremophiles: thermophiles, acidophiles, halophiles like those in Yellowstone's Morning Glory pool); no known human pathogens.

  • Protists (Domain Eukarya):

  • Algae: Unicellular or multicellular photosynthetic protists; cell walls composed of cellulose; rich in pigments; produce agar, carrageenan, and alginic acid; diatoms range from $2\,\mu\text{m}$ to $200\,\mu\text{m}$.

  • Protozoa: Unicellular non-photosynthetic or photosynthetic protists; motile via cilia, flagella, or pseudopodia; free-living or parasitic (e.g., Giardia lamblia causes severe diarrhea).

  • Fungi (Domain Eukarya):

  • Non-photosynthetic organisms with chitin cell walls.

  • Yeasts: Unicellular fungi ($>1000$ species); used in bread/alcohol fermentation (S. cerevisiae); opportunistic pathogens include Candida albicans (causes thrush and vaginal yeast infections).

  • Molds: Multicellular filamentous fungi; form visible colonies; decompose organic material; produce mycotoxins and antibiotics (e.g., Penicillium).

  • Helminths (Domain Eukarya):

  • Multicellular parasitic worms; adults are macroscopic, but studied in microbiology due to microscopic eggs and larvae.

  • Beef tapeworm (Taenia saginata): Adult length $4\text{--}10\,\text{m}$, eggs approximately $50\,\mu\text{m}$.

  • Guinea worm (Dracunculus medinensis): Ingested via water fleas containing larvae; causes leg ulcers upon adult emergence.

  • Viruses (Acellular):

  • Obligate intracellular particles composed of protein coats (capsids) and genetic material (either DNA or RNA, never both).

  • Inert outside living host cells; co-opt host machinery to replicate.

  • Examples: Coronaviruses (SARS, MERS), Ebolavirus.

  • Prions (Acellular):

  • Proteinaceous infectious particles lacking nucleic acids; approximately $1/10\text{th}$ the size of an average virus.

  • Cause transmissible spongiform encephalopathies (TSEs) such as Creutzfeldt-Jakob Disease (CJD).

• Microbial Subfields: Bacteriology, Mycology, Protozoology, Parasitology, Virology, Immunology, Clinical Microbiology, Environmental Microbiology, Applied Microbiology, Food Microbiology.

Physics of Light, Optics, and Lens Interactions

• Wave Properties of Light:

• Wavelength ($\lambda$): Distance between consecutive crests/peaks or troughs of a light wave.

• Amplitude: Height of a wave peak or depth of a trough.

• Frequency ($f$): Rate of vibration, or the number of wave crests passing a fixed point per unit time. High frequency corresponds to shorter wavelength and higher photon energy.

  • Interactions of Light with Matter:

• Reflection: Light wave bounces off a surface.

• Absorbance: Material captures the energy of a light wave.

• Transmission (Transmittance): Light wave travels through a material.

• Transparency: High proportion of light transmitted due to thinness or material clarity.

• Opacity: Light transmission is blocked.

• Interference: Superposition of overlapping light waves, generating constructive or destructive interference patterns.

• Diffraction: Bending or scattering of light waves as they interact with small obstacles or apertures.

• Refraction: Directional change (bending) of light waves when entering a medium with a different optical density.

  • Refractive Index ($n$):

• Measure of how much a medium slows the transmission speed of light relative to empty space.

• When light enters a medium with a higher refractive index, it bends toward the normal line (perpendicular to the interface).

• Dispersion: Separation of white light into constituent colors because refractive index varies with light frequency.

  • Principles of Lenses:

• Convex Lens: Curved outward; refracts parallel incoming light rays inward toward a focal point (image point) to produce magnification.

• Concave Lens: Curved inward; refracts parallel light rays outward away from a focal point, used to redirect light paths.

• Focal Length: Distance from the center of the lens to the focal point.

  • Electromagnetic Spectrum and Color:

• Visible light range: $400\,\text{nm}$ (violet) to $700\,\text{nm}$ (red).

• Positioned between ultraviolet (UV, higher frequency/energy) and infrared (IR, lower frequency/energy).

• Fluorescence: Phenomenon where fluorochromes absorb high-energy, short-wavelength light (UV or blue) and emit lower-energy, longer-wavelength visible light.

• Phosphorescence: Delayed emission of absorbed photons.

Microscopy Instruments, Techniques, and Care

• Key Microscopy Parameters:

  • Magnification: Ability of a lens system to enlarge the image of an object relative to its actual size.

  • Resolution: Ability to distinguish two adjacent points as separate entities. High resolution depends on shorter light wavelengths ($\lambda$) and higher numerical aperture.

  • Numerical Aperture ($\text{NA}$): Measure of a lens's capability to gather light.

  • Contrast: Intensity difference between specimen structures and the surrounding background.

• History of Microscopes:

  • Hans and Zaccharias Janssen (late 1500s/early 1600s) / Hans Lippershey: Early developers of compound optics.

  • Robert Hooke (1665): Published Micrographia, describing cork cells observed under his compound microscope.

  • Antonie van Leeuwenhoek: Built powerful single-lens simple microscopes achieving higher resolution than early compound scopes.

  • Joseph Jackson Lister (1830): Designed modern compound light microscopes with reduced chromatic and spherical aberrations.

• Light Microscopy Modalities:

  • Brightfield Microscope:

- Produces a dark image against a bright background.
- Monocular or binocular tube configurations.
- Key components: Illuminator, Rheostat (dimmer), Field diaphragm, Condenser lens (focuses light), Iris diaphragm, Mechanical stage ($x\text{-}y$ translational knobs), Specimen slide holder, Objective lenses on revolving nosepiece ($4\times, 10\times, 40

\times, 100\times$), Coarse focus knob ($4\times, 10\times$), Fine focus knob ($40\times, 100\times$), Ocular lenses ($10\times$). - Total Magnification Formula:       $\text{Total Magnification} = \text{Ocular Magnification} \times \text{Objective Magnification}$       Example: $10\times \text{ ocular} \times 40\times \text{ objective} = 400\times \text{ total magnification}$. - Immersion Oil ($100\times$ objective):

  - Immersion oil has a refractive index identical to glass ($n \approx 1.51$).
  - Eliminates light refraction and scattering at the glass-air interface, maximizing light collection, numerical aperture, and resolution.

• Darkfield Microscope:

- An opaque light stop is inserted into the condenser, blocking direct central light rays.
- Only light reflected or refracted by the specimen enters the objective lens, producing bright images of live, unstained specimens against a dark background (e.g., spirochetes such as *Treponema pallidum*).

• Phase-Contrast Microscope:

- Employs an annular stop in the condenser and a phase plate in the objective lens.
- Converts subtle phase shifts in light passing through structures of different refractive indices into intensity (amplitude) variations, enabling high-contrast, high-resolution visualization of live, unstained intracellular structures.

• Differential Interference Contrast (DIC / Nomarski Optics):

  • Uses two orthogonal polarized light beams passing through the specimen.

  • Interference produces high-contrast, three-dimensional-appearing images of live, unstained structures (e.g., fungal structures like Fonsecaea pedrosoi).

• Fluorescence Microscope:

  • Uses fluorochromes that absorb short-wavelength excitation light (UV/blue) and emit visible light.

  • Immunofluorescence techniques:

  - **Direct Immunofluorescence**: Fluorochrome-labeled primary antibody binds directly to the target antigen (e.g., detecting *Neisseria gonorrhoeae*).
  - **Indirect Immunofluorescence**: Unlabeled primary antibody binds the target antigen; fluorochrome-labeled secondary antibody binds the primary antibody (e.g., detecting *Schistosoma mansoni* larvae).
  - Common Fluorochromes: DAPI (stains DNA/nuclei blue), FITC (green-labeled antibodies for microtubules), TRITC/phalloidin (stains actin filaments red).

• Confocal Scanning Laser Microscope:

  • Uses a focused laser to scan multiple sequential $z$-planes at varying depths.

  • Pinholes eliminate out-of-focus light; computer software constructs high-resolution three-dimensional images of thick specimens (e.g., cyanobacterial biofilms).

• Two-Photon Microscope:

  • Uses low-energy, long-wavelength infrared light; two photons must hit a fluorochrome simultaneously to excite it, permitting deep tissue penetration with minimal phototoxicity.

  • Electron Microscopy (EM):

• Utilizes short-wavelength electron beams guided by magnetic lenses in a vacuum; achieves dramatically higher resolution ($<1\,\text{nm}$) and magnifications up to $100,000\times$ or more.

• Transmission Electron Microscope (TEM):

- Passes electrons through ultrathin sections ($<100\,\text{nm}$) prepared with an ultramicrotome and stained with heavy metals; captures internal cell ultrastructure.

• Scanning Electron Microscope (SEM):

- Scans an electron beam across the surface of a dried, fixative-treated, gold-sputtered specimen; detects secondary or backscattered electrons to generate highly detailed 3D surface topography (e.g., *Staphylococcus aureus* cell clusters).

• Scanning Probe Microscopy:

  • Employs atomic-scale sharp probes passed over a surface without light or electron beams.

  • Scanning Tunneling Microscope (STM): Measures electric tunneling current bias between a probe tip and conducting specimen to visualize individual surface atoms (e.g., gold atom columns).

  • Atomic Force Microscope (AFM): Measures probe tip deflection force as it scans across non-conducting biological molecules (e.g., nanocellulose strands).

• Microscope Care Best Practices:

  • Clean optical lenses exclusively with specialized lens paper before and after use.

  • Never use the coarse focusing knob when the $40\times$ or $100\times$ objective lenses are in place.

  • Apply immersion oil only to designated $100\times$ oil immersion objectives.

  • Wipe immersion oil off lenses immediately after use; store covered in a cabinet.

Specimen Preparation and Staining Protocols

• Specimen Fixation:

  • Wet Mount: Specimen suspended in a liquid drop under a coverslip; used for observing live, motile organisms.

  • Fixation: Process of attaching cells to a glass slide while killing organisms, arresting metabolism, and preserving cellular architecture.

  • Heat-Fixing: Spread a thin liquid layer of specimen (smear) on a slide, allow to air dry, and pass briefly over a microincinerator or slide warmer.

• Chemical Fixation: Applies ethanol, methanol, or formaldehyde to preserve delicate tissue structures.

  • Dye Chemistry and Staining Classifications:

• Stains consist of a chromophore (colored ion) and a counterion.

• Basic Dyes: Positive chromophore ($\text{cation}^+$); binds to negatively charged cell walls and nucleic acids. Examples: Crystal violet, methylene blue, safranin, basic fuchsin, malachite green.

• Acidic Dyes: Negative chromophore ($\text{anion}^-$); repelled by negatively charged cell structures, staining the background. Examples: Eosin, rose bengal, India ink, nigrosin.

• Positive Stain: Dye absorbed by the cell body.

• Negative Stain: Dye repelled by the cell, coloring the background.

• Simple Staining: Uses a single basic dye to emphasize uniform cellular morphology and arrangement.

• Differential Staining: Uses multiple reagents to distinguish organisms based on physical and chemical differences in cell structures.

  • Differential Staining Techniques:

• Gram Staining (Hans Christian Gram, 1884):

1. *Step 1: Primary Stain* -- Apply Crystal Violet ($1\,\text{min}$); all cells stain purple/blue.
2. *Step 2: Mordant* -- Apply Gram's Iodine ($1\,\text{min}$); forms an insoluble Crystal Violet-Iodine ($\text{CV-I}$) complex, trapping the dye inside peptidoglycan layers.
3. *Step 3: Decolorizer* -- Apply ethanol or acetone/ethanol solution ($10\text{--}30\,\text{sec}$); washes $\text{CV-I}$ out of thin Gram-negative outer membranes (cells become colorless), while thick Gram-positive peptidoglycan dehydrated pores trap $\text{CV-I}$ (cells remain purple).
4. *Step 4: Secondary Counterstain* -- Apply Safranin ($1\,\text{min}$); colors decolorized Gram-negative cells pink/red, leaving Gram-positive cells purple.

• Acid-Fast Staining (Ziehl-Neelsen and Kinyoun protocols):

  • Differentiates Gram-positive cells containing waxy mycolic acids in their cell walls (Mycobacteriaceae, e.g., Mycobacterium tuberculosis).

  • Primary stain: Carbolfuchsin (penetrates mycolic acid with heat in Ziehl-Neelsen or detergent in Kinyoun).

  • Decolorizer: Acid-alcohol solution.

  • Counterstain: Methylene blue.

  • Results: Acid-fast bacteria retain carbolfuchsin (bright red/pink); non-acid-fast cells decolorize and take up methylene blue.

• Capsule Staining:

  • Negative staining technique using India ink or Nigrosin to stain the background.

  • Basic dye or copper sulfate stains the cell body, revealing an unstained, clear halo (capsule) around encapsulated cells (e.g., Cryptococcus neoformans, Pseudomonas aeruginosa).

• Endospore Staining (Schaeffer-Fulton method):

  • Primary stain: Malachite green applied with steam heat to penetrate the tough endospore protein coat.

  • Decolorizer: Water washes malachite green from vegetative cells.

  • Counterstain: Safranin.

  • Results: Endospores appear green within or outside pink vegetative cells (e.g., Bacillus subtilis).

• Flagella Staining:

  • Applies tannic acid or potassium alum mordants to coat thin flagella, increasing their thickness so they become visible under light microscopy (e.g., Bacillus cereus).

Debunking Spontaneous Generation

• Theory of Spontaneous Generation (Abiogenesis):

  • Proposed that living organisms can arise spontaneously from nonliving matter containing pneuma ("vital heat").

  • Articulated by Aristotle ($384\text{--}322\text{ BC}$); cited sudden appearance of fish in new puddles, frogs along Nile riverbanks during floods, or mice in moldy grain.

  • Jan Baptista van Helmont (17th C): Proposed that mice generate spontaneously from dirty rags and wheat kernels left in an open jar for 21 days.

• Experimental Debunking of Abiogenesis:

  • Francesco Redi (1668):

- Placed meat in six containers: two open, two sealed with cork, two covered with fine gauze.
- Maggots developed only in open containers (where flies laid eggs directly on meat) and on top of gauze, proving maggots are fly offspring rather than products of spontaneous generation.

• John Needham (1745):

  • Briefly boiled broth infused with organic matter and sealed flasks; broth became cloudy with microbes, leading Needham to claim spontaneous generation occurred via a "life force."

  • Experimental flaw: Boiling duration was insufficient to kill heat-resistant bacterial endospores, and air leakage occurred during sealing.

• Lazzaro Spallanzani (1729--1799):

  • Repeated Needham's experiments; boiled sealed broth flasks for extended durations.

  • Sealed flasks remained completely clear; unsealed flasks grew microbes.

  • Proved air carried microbes into flasks; Needham countered that extended boiling destroyed the hypothetical "life force."

• Louis Pasteur (1858--1864):

- Designed specialized "swan-neck" flasks with long, twisted necks.
- Boiled broth to achieve sterilization; ambient air entered freely, but airborne microbial spores settled in the bends of the swan neck.
- Sterile broth remained clear indefinitely; breaking the swan neck allowed airborne microbes to enter, resulting in rapid turbidity.
- Definitively disproved spontaneous generation, winning the Alhumbert Prize in 1862 and proclaiming *"Omne vivum ex vivo"* ("Life only comes from life").

Foundations of Modern Cell Theory

• Historical Development of Cell Theory:

  • Robert Hooke (1665): Coined the term "cell" after observing dead cork cell walls resembling honeycomb boxes under a compound microscope (Micrographia).

  • Matthias Schleiden (1838): German botanist who observed plant tissues and concluded all plants are composed of cells (hypothesized cells formed by crystallization).

  • Theodor Schwann (1839): German physiologist who observed animal tissues and recognized structural similarities to plant cells, establishing that cells are the universal building blocks of life.

  • Robert Remak (1852): German neurologist and embryologist who published evidence that cells originate from preexisting cells via cell division.

  • Rudolf Virchow (1855): German pathologist who popularized "Omnis cellula a cellula" ("All cells arise from cells") in Cellular Pathology, plagiarizing Remak's work without citation.

• Tenets of Modern Cell Theory:

  1. All living organisms are composed of one or more cells.

  2. The cell is the basic structural and functional unit of living organisms.

  3. All cells arise only from pre-existing, living cells through cell division.

Modern Endosymbiotic Theory

• Historical Milestones:

  • Robert Brown (1831): First to describe the plant cell nucleus.

  • Andreas Schimper (1880s): Observed plant chloroplasts dividing independently of the nucleus.

  • Konstantin Mereschkowski (1905): Proposed that chloroplasts originated from photosynthetic endosymbiotic bacteria.

  • Ivan Wallin (1920s): Experimentally examined similarities between mitochondria, chloroplasts, and bacteria.

  • Lynn Margulis (1967, 1981): Re-evaluated endosymbiosis using microscopic, genetic, molecular, and fossil evidence in Symbiosis in Cell Evolution (1981), formalizing the modern Endosymbiotic Theory.

• Mechanisms of Endosymbiosis:

1. Infoldings of the ancestral prokaryotic plasma membrane gave rise to endomembrane components (nuclear envelope, endoplasmic reticulum).

2. An ancestral anaerobic eukaryote engulfed an aerobic respiration-capable bacterium via phagocytosis; the internal bacterium evolved into a mitochondrion.

3. In a secondary endosymbiotic event, an early eukaryote engulfed a photosynthetic cyanobacterium, which evolved into a chloroplast.

• Evidence Supporting Endosymbiotic Theory:

  • Genetics: Mitochondrial and chloroplast DNA genomes are single, circular, haploid chromosomes highly homologous to bacterial genomes.

  • Ribosomes: Organelle ribosomes are $70\text{S}$ in size (containing bacterial-like $\text{rRNA}$ and proteins), unlike eukaryotic cytoplasmic $80\text{S}$ ribosomes.

  • Reproduction: Mitochondria and chloroplasts divide independently within the cell via binary fission, mirroring bacterial division.

  • Membranes: Organelles possess double lipid membranes; inner membranes contain bacterial-like transport proteins and lipids.

Etiology of Infectious Disease and the Germ Theory

• Miasma Theory vs. Germ Theory:

  • Miasma Theory: Attributed diseases (e.g., Black Death, cholera) to toxic vapors ("bad air") emanating from decaying organic matter.

  • Germ Theory of Disease: States that microscopic organisms (pathogens) infect host bodies and cause specific diseases.

• Historical Contributors to the Germ Theory:

  • Girolamo Fracastoro (1546): In De Contagione et Contagiosis Morbis, proposed that disease is transmitted by direct contact, fomites (clothing), or air via invisible "seed-like spores."

  • Ignaz Semmelweis (1847): Hungarian obstetrician investigated puerperal fever (childbed fever) in Vienna hospital wards.

  • Observed $10\text{--}20\%$ maternal mortality in physician-staffed wards versus $1\%$ in midwife-staffed wards.

  • Linked deaths to medical students performing autopsies and subsequently conducting vaginal examinations without washing hands.

  • Mandated handwashing with chlorinated lime water; maternal mortality dropped to $1\%$. Work was rejected by contemporaries.

  • John Snow (1848): Tracked London cholera outbreaks to sewage-contaminated public water pumps (Broad Street pump), establishing the field of epidemiology.

  • Louis Pasteur (1856): Demonstrated that microbes drive beer/wine fermentation and spoilage, suggesting microbes cause human infections.

  • Joseph Lister (1867): Introduced carbolic acid (phenol) antiseptic techniques for surgical handwashing, wound care, and instrument sterilization, reducing postsurgical mortality from $50\%$

  • Robert Koch (1876--1906): Developed Koch's Postulates ("one microbe, one disease" framework) to prove specific pathogens cause specific diseases (B. anthracis, M. tuberculosis, V. cholerae).

Detailed Structure and Function of Prokaryotic Cells

• General Prokaryotic Architecture:

• Lacks a nuclear membrane and membrane-bound organelles.

• Fundamental components: Cytoplasm (aqueous gel), plasma membrane, single circular chromosome in a nucleoid, and $70\text{S}$ ribosomes.

  • Cell Morphologies and Arrangements:

• Shapes: Coccus (spherical), Bacillus (rod), Vibrio (curved rod), Coccobacillus (intermediate oval), Spirillum (rigid wave), Spirochete (flexible helix).

• Cellular Arrangements: Diplococcus (pairs), Tetrad (square of 4), Streptococcus (chains of cocci), Staphylococcus (grape-like clusters), Streptobacillus (chains of rods).

  • Tonicity, Osmotic Pressure, and Plasmolysis:

• Isotonic Medium: Solute concentration outside equals inside; no net movement of water.

• Hypertonic Medium: External solute concentration exceeds internal; water leaves the cell.

  • Cells lacking walls undergo crenation (shriveling).

  • Cells with walls undergo plasmolysis: plasma membrane contracts and detaches from the wall while the cell wall maintains external shape.

• Hypotonic Medium: Internal solute concentration exceeds external; water enters the cell.

  • Cells lacking walls undergo osmotic lysis (bursting).

  • Cell walls prevent excessive expansion, providing turgor pressure resistance.

  • Internal Prokaryotic Components:

• Nucleoid: Unbound region containing a circular, haploid chromosome; packaged by Nucleoid-Associated Proteins (NAPs) in bacteria or histone-like proteins in archaea.

• Plasmids: Small, circular, double-stranded extrachromosomal DNA molecules; carry non-essential genes conferring advantageous traits (e.g., antibiotic resistance).

• Ribosomes: $70\text{S}$ ribosomes composed of a $30\text{S}$ small subunit and a $50\text{S}$ large subunit (constructed from $\text{rRNA}$ and proteins).

• Inclusions (Nutrient and Functional Compartments):

  • Polyhydroxybutyrate (PHB): Monolayer-bound lipid droplets for carbon/energy storage; used to manufacture biodegradable bioplastics (Bacillus, Pseudomonas).

  • Volutin Granules (Metachromatic Granules): Inorganic phosphate polymers used for ATP synthesis and biofilm formation (Corynebacterium diphtheriae).

  • Sulfur Granules: Elemental sulfur reserves in sulfur bacteria (Thiobacillus).

  • Gas Vacuoles: Protein-shelled gas vesicles providing buoyancy control in water columns.

  • Magnetosomes: Inclusions of magnetic iron oxide ($Fe_3O_4$) or iron sulfide ($Fe_3S_4$) surrounded by lipid membranes, enabling magnetotaxis (Magnetospirillum magnetotacticum).

  • Carboxysomes: Protein shells filled with RuBisCO and carbonic anhydrase for inorganic carbon fixation (Anabaena, Halothiobacillus).

• Endospores:

  • Dormant, non-vegetative structures formed by Gram-positive bacteria of the genera Bacillus and Clostridium under nutrient starvation.

  • Sporulation Steps:

  1. Asymmetric cell division separates DNA forespore from mother cell.
  2. Forespore engulfed by a second membrane.
  3. Cortex of calcium and dipicolinic acid deposited between membranes.
  4. Protein spore coat deposited; mother cell lyses to release mature endospore.
- Highly resistant to extreme heat, desiccation, chemical disinfectants, and radiation; undergo germination to re-enter vegetative growth when conditions improve.

• Prokaryotic Cell Envelope Structures:

  • Plasma Membrane:

  • Bacterial/Eukaryotic: Phospholipid bilayer with ester linkages and straight-chain fatty acids (fluid mosaic model).

  • Archaeal: Ether linkages, branched isoprenoid chains, forming lipid bilayers or monolayer sheets.

  • Transport Mechanisms: Simple diffusion (passive), facilitated diffusion (channels/carriers), active transport (energy-requiring pumps), and group translocation (chemical modification during entry, e.g., phosphotransferase system adding phosphate to glucose).

  • Photosynthetic Infoldings: Thylakoids in cyanobacteria; chromatophores, lamellae, or chlorosomes in photosynthetic bacteria.

  • Cell Wall:

  • Composed of Peptidoglycan (Murein): Alternating disaccharide chains of $N$-acetylglucosamine ($\text{NAG}$) and $N$-acetylmuramic acid ($\text{NAM}$) connected by peptide cross-links.

- **Gram-Positive Cell Wall**: Thick peptidoglycan layer ($30\text{--}100\,\text{nm}$) rich in teichoic acids ($\text{TA}$) and lipoteichoic acids ($\text{LTA}$); tetrapeptides linked by pentaglycine cross-bridges. Acid-fast *Mycobacteriaceae* have an additional external waxy mycolic acid layer.
- **Gram-Negative Cell Wall**: Thin peptidoglycan layer ($\le 4\,\text{nm}$) located in the periplasmic space between inner membrane and outer membrane. Outer membrane contains murein lipoproteins and Lipopolysaccharide ($\text{LPS}$). $\text{LPS}$ is an endotoxin comprising Lipid A, core polysaccharide, and $\text{O}$ side chain ($\text{O}$ antigen, e.g., *E. coli* $\text{O157:H7}$).

- Archaeal Cell Wall: Pseudopeptidoglycan ($\text{NAT}$ replaces $\text{NAM}$), glycoproteins, or S-layers.

• Glycocalyx and Outer Layers:

  • Capsule: Organized polysaccharide/protein layer tightly bound to wall; evades phagocytosis (e.g., Streptococcus pneumoniae).

  • Slime Layer: Loose, unorganized layer; aids biofilm surface attachment and desiccation protection.

  • S-Layer: Monomolecular array of structural proteins/glycoproteins.

• Appendages:

  • Fimbriae: Short, abundant protein bristles for cell attachment.

  • Pili: Longer, sparse appendages; F pilus (sex pilus) transfers DNA between cells during bacterial conjugation.

  • Flagella: Rotating helical protein chains (flagellin) attached to a hook and basal body motor.

  - Flagellar Arrangements: Monotrichous (single polar), Amphitrichous (tufts at both ends), Lophotrichous (tuft at one end), Peritrichous (covering entire cell surface).
  - Flagellar Motility:

    - Counterclockwise rotation = **Run** (flagella bundle together, propelling cell forward).
    - Clockwise rotation = **Tumble** (flagella splay apart, reorienting cell).
    - Chemotaxis: Chemical attractants increase run duration and decrease tumble frequency.

Detailed Structure and Function of Eukaryotic Cells

• General Eukaryotic Architecture:

• Defined by a membrane-bound nucleus containing linear chromosomes wrapped around histones.

• Contains membrane-bound cytoplasmic organelles and an internal cytoskeleton matrix.

  • Eukaryotic Morphologies: Spheroid, fusiform, bell-shaped, ovoid, ring-shaped, crescent, stellate, polygonal.

  • Nucleus and Nucleolus:

• Bound by a double-membrane nuclear envelope containing rosette-shaped nuclear pores and an inner nuclear lamina (intermediate filament network).

• Nucleolus: Dense, non-membrane-bound subnuclear region where ribosomal RNA ($\text{rRNA}$) is synthesized and preribosomal subunits are assembled.

• Nuclear Variations: Uninucleate, binucleate (Paramecium micronucleus/macronucleus), heterokaryotic (fungi), coenocytic (nuclear division without cytokinesis).

  • Ribosomes:

• Cytoplasmic and RER-bound ribosomes are $80\text{S}$ ($40\text{S}$ small subunit + $60\text{S}$ large subunit).

• Organellar ribosomes (inside mitochondria and chloroplasts) are $70\text{S}$.

• Cycloheximide selectively inhibits eukaryotic $80\text{S}$ ribosomes; chloramphenicol targets prokaryotic $70\text{S}$ ribosomes.

  • Endomembrane System:

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Membrane studded with $80\text{S}$ ribosomes; synthesizes membrane-bound and secretory proteins packaged into transport vesicles.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, and detoxifies cellular toxins.

• Golgi Apparatus: Stacked membranous disks (dictyosomes).

  • Receives ER vesicles at the cis face.

  • Glycosylates proteins and lipids to produce glycoproteins, glycolipids, and proteoglycans.

  • Packages processed molecules into outgoing vesicles at the trans face for exocytosis or organellar delivery.

• Lysosomes: Discovered by Christian de Duve; single-membrane vesicles containing acid hydrolase digestive enzymes to break down food, damaged organelles, and phagocytosed pathogens.

  • Non-Endomembrane Organelles:

• Peroxisomes: Discovered by Christian de Duve; form independently via peroxin protein assembly and divide by binary fission. Produce hydrogen peroxide ($H_2O_2$) during fatty acid/amino acid breakdown, utilizing catalase to neutralize $H_2O_2$ into $H_2O$ and $O_2$. Specialized variants: Glyoxysomes (produce sugar in plant/yeast cells), Glycosomes (in trypanosomes).

  • Cytoskeleton Components:

• Microfilaments:

- Intertwined actin polymer strands ($6\,\text{nm}$ diameter).
- Interact with myosin motor proteins; drive amoeboid movement (pseudopodia gel-sol actin transition), cytoplasmic streaming, cytokinesis cleavage furrow formation, and animal muscle contraction.

• Intermediate Filaments: Polymerized multi-strand cables ($10\,\text{nm}$ diameter); maintain nuclear position, construct nuclear lamina, form desmosomes (desmin) between cells, and form structural hair/nail/skin layers (keratin).

• Microtubules:

- Hollow cylinders composed of $\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$ dimers ($23\,\text{nm}$ diameter).
- Guide intracellular vesicle transport via motor proteins (dynein and kinesin), construct the mitotic spindle during cell division, and form the core framework of flagella and cilia.

• Centrosomes: Microtubule-organizing centers containing a pair of perpendicular centrioles (each composed of 9 microtubule triplets).

  • Energy Transformation Organelles:

• Mitochondria:

- Enclosed by an outer lipid membrane and an inner membrane folded into cristae.
- Inner membrane embeds electron transport chain complexes; matrix contains mitochondrial DNA, $70\text{S}$ ribosomes, and TCA cycle enzymes.

• Chloroplasts: Site of photosynthesis in plants and algae; three membrane layers (outer, inner, thylakoid system). Thylakoid membrane sacs contain chlorophyll and cluster into grana stacks suspended within the gel-like stroma.

• Mitochondria-Related Organelles: Mitosomes in Giardia lamblia (reduced double-membrane structures lacking DNA); Hydrogenosomes in Trichomonas vaginalis (generate molecular hydrogen anaerobically); Kinetoplasts in trypanosomes (single large branched mitochondrion containing concentrated kinetoplast DNA loops).

  • Plasma Membrane and Transport Mechanisms:

• Bilayer of phospholipids with embedded sterols (cholesterol) and sphingolipids.

• Endocytosis:

- *Phagocytosis* ("cell eating"): Engulfment of large particles or whole cells into membrane vacuoles.
- *Pinocytosis* ("cell drinking"): Internalization of small fluid volumes and dissolved solutes via microvesicles.
- *Receptor-Mediated Endocytosis*: Specific binding of extracellular ligands to surface receptors triggering vesicle internalization.

• External Structures:

  • Cell Wall: Plants/algae (cellulose, silica, agar, carrageenan), fungi (chitin).

  • Extracellular Matrix (ECM): Secreted carbohydrate/protein mass in animal cells; consists of proteoglycans, collagen, fibronectin, and integrins; transmits signals and serves as a pathogen docking site (e.g., Streptococcus pyogenes binding fibronectin).

  • Flagella and Cilia:

- Composed of a $9+2$ array of parallel microtubule doublets powered by dynein motor protein sliding.
- Anchored to the cell by a basal body ($9$ microtubule triplet array).
- Eukaryotic flagella are flexible, whipping structures (unlike rigid rotating bacterial flagella); cilia are shorter, more numerous structures used for cell motility, feeding, or sweeping extracellular mucus/debris.

Clinical Focus Case Studies

• Case Study 1: Suspected Meningitis and Neurological Disease (Cora)

  • Initial Presentation: Cora, a 41-year-old female lawyer, presents with severe headaches, high fever, stiff neck, confusion, and drowsiness.

  • Diagnostic Workup: Lumbar puncture (spinal tap) performed between lumbar vertebrae to extract cerebrospinal fluid (CSF). CSF evaluated for clarity, cell counts, and microbiology sediment Gram stain.

  • Initial Clinical Findings: Approximately $80\%$ of bacterial meningitis cases show organisms on CSF Gram stain; Cora's Gram stain and subsequent bacterial/fungal cultures were negative. Antibiotics initiated empirically failed to produce improvement.

  • Disease Progression: Cora develops ataxia (walking difficulty), widespread muscle rigidity, involuntary myoclonic limb jerking, and rapid cognitive decline.

  • Advanced Testing & Diagnosis: CSF assays reveal elevated levels of $14\text{-}3\text{-}3$ protein; electroencephalogram (EEG) shows triphasic periodic waves indicative of neurodegenerative destruction. Diagnosed with sporadic Creutzfeldt-Jakob Disease (CJD), a transmissible spongiform encephalopathy (TSE) caused by infectious prions.

  • Outcome: CJD is fatal ($90\%$ mortality within 1 year); treatment is palliative. Cora dies 8 months post-diagnosis; postmortem brain autopsy confirms CJD.

• Case Study 2: Unresponsive Pneumonia (Barbara)

  • Initial Presentation: Barbara, a 19-year-old college student, presents with fever, sore throat, severe fatigue, headache, and a violent unproductive cough.

  • Physical Findings: Auscultation reveals lung crackles and shortness of breath; pulse oximetry confirms hypoxemia (low blood oxygen); chest radiograph (X-ray) demonstrates a left lung shadow (fluid accumulation indicative of pneumonia).

  • Treatment Course: PA prescribes amoxicillin (a $\beta\text{-lactam}$ penicillin derivative targeting peptidoglycan synthesis). After a 7-day course, symptoms persist.

  • Clinical Deduction: Persistence indicates the pathogen lacks peptidoglycan (a virus, fungus, or cell-wall-less bacterium) or possesses amoxicillin resistance.

  • Resolution: PA prescribes azithromycin (a macrolide antibiotic targeting bacterial $50\text{S}$ ribosomal subunits). Barbara recovers completely. Causative agent identified as Mycoplasma pneumoniae, a wall-less bacterium immune to $\beta\text{-lactam}$ antibiotics.

Bioethics in Microbiological Research

• Historical Violations of Ethical Standards:

  • Guatemala Syphilis Experiments ($1946\text{--}1948$): US government-funded study in which over $1300$ Guatemalan subjects (soldiers, prisoners, psychiatric patients, sex workers) were intentionally infected with syphilis, gonorrhea, and chancroid without informed consent to test antibiotic efficacy. At least 83 subjects died as a direct result.

• Principles of Modern Biological Ethics:

  • Informed Consent: Human subjects must fully understand study procedures, potential health risks, and voluntary participation rights prior to enrolment.

  • Institutional Review Board (IRB): Independent administrative body that reviews human subject research protocols to ensure ethical compliance and safety.

  • Institutional Animal Care and Use Committee (IACUC): Committee overseeing humane treatment, care, and necessity of animal subjects in scientific studies.

  • Clinical Ethics Requirements: Maintaining patient confidentiality, accurate medical record-keeping, open communication, and respecting patient autonomy regarding treatment decisions.