Prokaryote Flashcards
Compare / contrast eukaryotic v prokaryotic cells
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Nucleus:
- Eukaryotic: Has a true membrane-bound nucleus
- Prokaryotic: No nucleus; DNA located in a nucleoid region
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Organelles:
- Eukaryotic: Membrane-bound organelles (e.g., mitochondria, ER)
- Prokaryotic: No membrane-bound organelles
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DNA:
- Eukaryotic: Linear chromosomes, associated with histones
- Prokaryotic: Single circular chromosome, no histones (in most)
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Ribosomes:
- Eukaryotic: 80S (60S + 40S)
- Prokaryotic: 70S (50S + 30S)
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Cell Size:
- Eukaryotic: Larger (10–100 µm)
- Prokaryotic: Smaller (0.1–5 µm)
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Examples:
- Eukaryotic: Animals, plants, fungi, protists
- Prokaryotic: Bacteria, archaea
Describe general bacterial cell structure
- Cell membrane: Phospholipid bilayer; regulates transport and metabolic functions (e.g., respiration in prokaryotes)
- Cell wall: Provides structural support and shape; composed of peptidoglycan (thick in Gram-positive, thin in Gram-negative)
- Cytoplasm: Gel-like matrix with enzymes, nutrients, ions, ribosomes, and DNA
- Ribosomes: 70S (50S + 30S); site of protein synthesis
- Nucleoid: Irregular region containing circular, double-stranded DNA chromosome; not membrane-bound
- Plasmids: Small, circular DNA molecules; often carry advantageous genes (e.g., antibiotic resistance, virulence factors)
- Inclusions: Reserve deposits that store nutrients like glycogen, sulfur, or polyphosphate for later use
- Capsule (if present): Well-organized glycocalyx (polysaccharide or polypeptide); aids in adhesion, immune evasion, and desiccation resistance
- Fimbriae: Short, thin, hair-like appendages made of pilin protein; mediate adhesion to surfaces and cells
- Pili: Longer, fewer structures also composed of pilin; specialized pili (e.g., sex pili) used in DNA conjugation and motility (Type IV pili)
- Flagella: Long whip-like structures made of flagellin; provide motility by rotating like a propeller
- Endospores (in some Gram-positives): Dormant, highly resistant structures for survival in harsh environments
Bacteria do not have histones, but they use histone-like proteins to organize their DNA. Histones are present in archaea and eukaryotes, not in typical bacterial chromosomes.
Define Svedberg unit
Svedberg unit (S) is a measure of the sedimentation rate of a particle during ultracentrifugation, reflecting how fast it settles under centrifugal force. It depends on both size and density of the particle, not just mass, so it’s not additive
Bacterial vs eukaryotic ribosome
Note - because of size difference bacterial ribosomes are often a target of antibiotics
Define Bacterial endospore
- A highly resistant, dormant structure produced by certain Gram-positive bacteria (e.g., Bacillus, Clostridium) under stress conditions (e.g., starvation)
🌀 Sporulation (formation)
- Triggered by nutrient depletion or environmental stress
- Involves asymmetric cell division and multilayered development
- Takes ~6–8 hours
🌱 Germination (reactivation)
- Triggered by return to favorable conditions (e.g., nutrients, moisture)
- Endospore absorbs water, breaks coat, and resumes metabolism as a vegetative cell
🧬 Structural Layers (from center outward):
1. Core
- Contains DNA, ribosomes, dipicolinic acid, calcium
- Dehydrated; metabolically inactive
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Inner membrane
- Highly impermeable barrier
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Germ cell wall
- Becomes the cell wall of the vegetative cell upon germination
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Cortex
- Modified peptidoglycan layer that helps dehydrate and protect the core
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Outer membrane
- Surrounds the cortex
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Spore coat
- Protein-rich; provides chemical and enzymatic resistance
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Exosporium (in some species)
- Thin, outermost layer with glycoproteins; not present in all spores
🛡️ Resistance
- Extremely resistant to heat, desiccation, radiation, disinfectants, and UV
- Killed only by autoclaving (121°C, 15–20 min under pressure)
🔁 Not a reproductive process
- One vegetative cell forms one endospore, which germinates into one new cell
🦠 Clinical Relevance
- Crucial in transmission of diseases like anthrax, tetanus, botulism, and C. difficile colitis
Layers of bacterial endospore
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Core
- Contains DNA, ribosomes, dipicolinic acid, calcium
- Metabolically inactive and highly resistant
- Derived from original cytoplasm
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Core wall (a.k.a. germ cell wall)
- Thin peptidoglycan layer
- Will become the cell wall of the vegetative cell after germination
- Sometimes referred to as “phospholipid-rich,” though it’s mainly peptidoglycan
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Cortex
- Thick layer of modified peptidoglycan
- Maintains core dehydration; crucial for heat resistance
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Spore coat (a.k.a. endospore coat)
- Multiple layers of protective proteins (keratin-like)
- Resists chemicals, enzymes, and UV radiation
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Exosporium (only in some species)
- Loose outer glycoprotein layer
- May appear sticky or “gooey” under microscopy
- Involved in adhesion and immune evasion
How are endospores visualized
Endospore stain
- complex stain
- endospores appear green
- vegetative cells appear red
Bacteria cytoskeltal proteins
Although bacteria lack a true cytoskeleton like eukaryotes, they contain structural proteins that perform similar functions:
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FtsZ
- Homolog of eukaryotic tubulin
- Forms a contractile Z-ring at the future site of cell division
- Recruits other proteins to form the divisome
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MreB
- Homolog of eukaryotic actin
- Forms helical bands beneath the membrane
- Maintains rod shape by directing cell wall synthesis
- Absent in cocci
MreB protein
- Bacterial cytoskeletal protein that is a homolog of eukaryotic actin
- Forms dynamic helical bands just under the cell membrane
- Guides enzymes involved in peptidoglycan synthesis, helping maintain rod shape in bacteria
- Coordinates cell wall growth during elongation
- Essential for cytoskeletal organization in rod-shaped and spiral bacteria
🧬 Key facts:
- Found in rod-shaped bacteria (e.g., E. coli, Bacillus subtilis)
- Absent in cocci, which lack this protein and do not elongate
- Works with other shape-determining proteins (e.g., RodZ)
FtsZ protein
- A bacterial cytoskeletal protein and homolog of eukaryotic tubulin
- Forms a contractile ring (Z-ring) at the future site of cell division (mid-cell)
- Acts as a scaffold for assembling the divisome, the protein complex responsible for cytokinesis
- Recruits enzymes that synthesize new cell wall material during septum formation
🧬 Key Facts:
- Found in nearly all bacteria and many archaea
- Polymerizes in a GTP-dependent manner to form filaments
- The Z-ring constricts to divide the cell into two daughter cells
🔧 Function Summary:
> FtsZ coordinates septum formation and orchestrates bacterial cell division, making it functionally equivalent to the mitotic spindle in eukaryotes.
Describe Gram-positive Bacteria
- Bacteria with a thick peptidoglycan cell wall (20–80 nm)
- Retain crystal violet stain, appearing purple in Gram stain
🧱 Cell Wall Composition:
- Peptidoglycan: Thick and multilayered; provides structural support
- Teichoic acids: Polymers of glycerol or ribitol phosphate
- Negatively charged → contribute to cell surface charge and cation (e.g., Mg²⁺) binding
- Two types:
- Wall teichoic acid (WTA): Attached to peptidoglycan
- Lipoteichoic acid (LTA): Anchored in the cytoplasmic membrane
⚠️ Not all Gram-positive bacteria have both types:
- Some may lack LTA or WTA depending on species and environmental conditions
- Streptococcus pneumoniae, for example, lacks lipoteichoic acid but contains wall teichoic acid
🧬 Other Features:
- More sensitive to antibiotics targeting cell wall (e.g., β-lactams)
- Can form endospores (e.g., Bacillus, Clostridium)
- Common genera: Staphylococcus, Streptococcus, Bacillus, Listeria
Describe Gram-negative Bacteria
- Bacteria with a thin peptidoglycan layer and an outer membrane
- Do not retain crystal violet → appear pink/red after Gram staining due to safranin counterstain
🧱 Cell Wall Composition:
- Outer membrane:
- Contains lipopolysaccharide (LPS), also called endotoxin
- Acts as a barrier to many antibiotics and detergents
- Peptidoglycan: Thin layer (1–3 layers), located in the periplasmic space
- Periplasm: Gel-like space between outer and inner membranes; contains enzymes, transport proteins, and peptidoglycan
- Inner (cytoplasmic) membrane: Phospholipid bilayer with embedded proteins
🧬 LPS Structure:
- Lipid A: Toxic component; triggers strong immune response
- Core polysaccharide
- O antigen: Variable; used in serotyping
✅ Other Features:
- Porins in outer membrane allow passage of small molecules
- Often more resistant to antibiotics due to outer membrane
- No teichoic acids
- Do not form endospores
- Common genera: Escherichia, Salmonella, Neisseria, Pseudomonas
Define peptidoglycan
- A rigid, mesh-like polymer that forms the bacterial cell wall
- Provides structural support and resists osmotic lysis
🧬 Composition:
- Repeating disaccharide units:
- N-acetylglucosamine (NAG)
- N-acetylmuramic acid (NAM)
- Attached to NAM: short tetrapeptide chains
- Cross-linking of tetrapeptides provides wall strength
🧱 Distribution:
- Thick layer in Gram-positive bacteria
- Thin layer in Gram-negative, located in the periplasm
🎯 Clinical relevance:
- Target of many antibiotics (e.g., penicillin, cephalosporins, vancomycin)
- Disruption weakens the wall → leads to cell lysis
> Even though Gram-negative bacteria have less peptidoglycan, they still require it, and its inhibition can be lethal if the antibiotic reaches it.
Function of cell wall in isotonic, hypertonic, and hypotonic solution
The cell wall, particularly peptidoglycan, helps bacteria withstand osmotic pressure changes in their environment.
🌊 Hypotonic solution (low solute outside; water enters cell):
- Water rushes into the cell due to osmosis
- Risk: Cell swelling and bursting
- Role of cell wall:
- Prevents osmotic lysis
- Provides rigid support to counter internal pressure
💧 Isotonic solution (equal solute concentration):
- Water movement is balanced in and out
- Cell wall not essential for survival here
- Cell remains stable even without it (e.g., protoplasts can survive briefly)
🧂 Hypertonic solution (high solute outside; water leaves cell):
- Water flows out of the cell
- Result: Cytoplasmic shrinkage (plasmolysis)
- Cell wall cannot prevent water loss
- Cell survives but may become metabolically inactive
Lipopolysaccharide definition, structure, consequence
🧬 Definition:
- A large, amphipathic molecule found in the outer membrane of Gram-negative bacteria
- Commonly called an endotoxin, though not all endotoxins are LPS (rare exceptions exist)
- The term “endotoxin” is often used synonymously with LPS in clinical contexts
🔬 Structure (from membrane outward):
1. Lipid A
- Hydrophobic anchor that embeds LPS in the outer membrane
- Made of glucosamine disaccharide backbone with multiple fatty acid tails
- Responsible for toxicity: triggers strong immune responses
2. Core polysaccharide
- Connects lipid A to O antigen
- Contains unusual sugars (e.g., KDO)
- Relatively conserved among species
3. O antigen (O side chain)
- Long, repeating polysaccharide
- Highly variable → used for serotyping (e.g., E. coli O157:H7)
- Contributes to immune evasion
💥 Consequences:
- Recognized by TLR4 on host immune cells → triggers cytokine release
- Can cause fever, inflammation, vasodilation, hypotension, and endotoxic shock
- Released when bacteria die or lyse
Summary:
> LPS is a major component of Gram-negative outer membranes and is the most common endotoxin. Its lipid A portion is toxic and activates strong immune responses.
Describe Acid-fast bacteria
- Bacteria with a Gram-positive cell wall structure but do not stain well with Gram stain
- Appear Gram-negative or variable due to their waxy cell wall
- Require a special staining technique (acid-fast stain) to be visualized properly
🧱 Cell Wall Features:
- Contain peptidoglycan like Gram-positive bacteria
- Surrounded by a thick layer of mycolic acids (long-chain fatty acids)
Resistince
🧱 Thick, waxy cell wall → Forms a strong permeability barrier
❌ Resists phagosome-lysosome fusion → Enables intracellular survival in macrophages
🧪 Resistant to chemical damage → Survives oxidative stress and reactive oxygen/nitrogen species
💊 Limits antibiotic entry → Especially hydrophilic antibiotics and cell-wall–targeting drugs
🧫 Prevents desiccation → Enhances survival in dry environments
🔬 Requires acid-fast staining → Resists decolorization by alcohol in Gram stain
🧬 Clinical Examples:
- Mycobacterium tuberculosis
- Mycobacterium leprae
Summary:
> Acid-fast bacteria have a Gram-positive structure, but their mycolic acid-rich walls prevent them from staining correctly, requiring acid-fast staining.
Flagella
Flagella
- Long, whip-like appendages used for bacterial motility
- Rotate like a propeller to propel the bacterium through liquid environments
- Present in both Gram-positive and Gram-negative bacteria
🧬 Structure (3 main parts):
1. Filament
- Composed of flagellin protein
- Extends into the external environment
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Hook
- Connects the filament to the basal body
- Acts as a flexible joint
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Basal body
- Anchors flagellum into the cell envelope
- Functions as a rotary motor powered by a proton gradient (H⁺)
⚙️ Energy:
- Movement is driven by proton motive force (in some species, Na⁺)
🧭 Types of flagellar arrangements:
- Monotrichous – Single flagellum at one pole
- Lophotrichous – Tuft of flagella at one pole
- Amphitrichous – One or more flagella at both poles
- Peritrichous – Flagella all over the surface
- Atrichous – No flagella (non-motile)
> Flagella are protein-based structures used for motility, powered by a proton gradient, and vary in arrangement across species.
Endoflagella
- Also called axial filaments
- Specialized flagella found in spirochetes
- Provide corkscrew-like motility that allows movement through viscous environments (e.g., mucus)
🧬 Structure:
- Similar to external flagella (contain flagellin)
- Located within the periplasmic space, between the outer membrane and peptidoglycan layer
- Wrapped around the cell body, anchored at one end
- Rotation causes the entire cell to twist and move
spirochetes are classified as Gram-negative based on structure, not staining behavior
🧬 Examples (all spirochetes):
- Treponema pallidum (syphilis)
- Borrelia burgdorferi (Lyme disease)
- Leptospira interrogans (leptospirosis)
Pili - list four types
Pili are hair-like appendages made of pilin protein, found on many Gram-negative and some Gram-positive bacteria. They vary in structure and function:
ALL pili:
- “attach” to things
- are composed of pilin proteins
- extend from the bacterial surface,
- Fimbriae (attachment pilus / type I pilus)
- conjugation pilus
- competence pilus
- type IV pilus
Fimbriae and pili are structurally similar and, because differentiation between the two is problematic, these terms are often used interchangeably.
Fimbriae = short, numerous, bristle-like projections Pili = longer, fewer appendages Both are made of protein, aid in attachment, and the distinction is not always strict
Function characteristic Fimbriae Pili
- Short, numerous, bristle-like projections
- Made of pilin protein
- Function: Adhesion to host cells, tissues, and abiotic surfaces
- Important in colonization and biofilm formation
- Found mainly in Gram-negative, some Gram-positive bacteria
- Unlike other Pili does not change in length*
- Often referred to as Type I pili
Sometimes Fimbriae are not classified as pili
fimbria -> singular, fimbriae -> plural
Function characteristic Conjugation Pilus
- Also called a sex pilus
- A long, hollow, protein-based appendage made of pilin
- Found primarily in Gram-negative bacteria
- Encoded by genes on the F (fertility) plasmid
🔁 Function:
- Establishes direct contact between two bacterial cells
- Facilitates horizontal gene transfer via conjugation
- Transfers plasmid DNA (e.g., antibiotic resistance genes) from donor (F⁺) to recipient (F⁻) cell
- Only the donor cell expresses the conjugation pilus
🧬 Structural notes:
- One or a few per cell (unlike fimbriae, which are numerous)
- Pilus retracts to bring cells together, forming a conjugation bridge
Competence Pilus
- A specialized type of pilus used in transformation, a process where bacteria take up free DNA from their environment
- Helps bind and transport DNA across the cell envelope
🧬 Function:
- Enables natural transformation, a form of horizontal gene transfer
- Imported DNA may integrate into the chromosome via homologous recombination
- Contributes to genetic diversity, virulence, and antibiotic resistance
Type IV pilus
- A versatile, dynamic pilus made of pilin protein
- Found in both Gram-negative and some Gram-positive bacteria
⚙️ Key Function:
1. Twitching motility
- Pilus extends, attaches to a surface, and retracts to pull the bacterium forward
- Requires ATP-powered motor proteins
Describe bacterial Flagella
- Long, whip-like appendages made of flagellin
- Provide motility by rotating like a propeller
- Found in both Gram-positive and Gram-negative bacteria
- Allow movement toward nutrients (chemotaxis) or away from harmful stimuli
🧬 Structure (3 main parts):
1. Filament
- Long, helical structure made of flagellin protein
- Extends into the environment
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Hook
- Curved segment that connects the filament to the basal body
- Acts as a flexible joint
-
Basal body
- Anchors the flagellum to the cell wall and membrane
- Serves as a rotary motor, powered by proton motive force (H⁺ flow) into cell
⚙️ Energy Source:
- Movement driven by proton gradient (some species use Na⁺)
🧭 Flagellar Arrangements:
- Monotrichous – One flagellum at one pole
- Lophotrichous – Tuft at one pole
- Amphitrichous – One or more at each pole
- Peritrichous – Flagella all over the surface
- Atrichous – No flagella (non-motile)
