SL

Bacterial Cell Architecture and Functions

Cell Structure and Function Notes

Acid-Fast Staining and Mycobacterium

  • Carbofusion: A stain designed to penetrate the waxy cell layer of certain bacteria.
  • Acid-Fast Organisms: Organisms that retain the carbofusion stain after decolorization with acid-alcohol, appearing pink. This indicates the presence of a waxy outer cell wall.
  • Nonacid-Fast Organisms: Organisms that initially stain pink with carbofusion but are decolorized by acid-alcohol, appearing clear. They then take up a counterstain (e.g., methylene blue), appearing blue.
  • Result Interpretation: If a slide shows pink organisms (acid-fast) and blue organisms (nonacid-fast), the pink organisms are identified as Mycobacterium species due to their characteristic waxy outer cell wall.
  • Waxy Cell Wall Implications (for Mycobacterium):
    • Slow Division: The waxy nature of the cell wall is believed to contribute to a very long division time for these organisms.
    • Chronic Diseases: This slow division contributes to the chronic nature of diseases like tuberculosis, which is caused by Mycobacterium tuberculosis.
    • Antibiotic Resistance: The waxy cell wall is difficult to penetrate, making Mycobacterium infections harder to treat with conventional antibiotics.
    • Comparison: These organisms take much longer to divide than typical fast-growing bacteria like E. coli.

Bacterial Cell Wall

  • Distinguishing Feature: Animal cells (eukaryotes) do not possess a cell wall, making it a crucial target for many antibiotics.
  • Antibiotic Targets:
    • Penicillins: Prevent the construction of the bacterial cell wall in new, actively growing cells. Without a properly formed cell wall, the plasma membrane is unable to withstand internal osmotic pressure and the cell bursts (lysis).
    • Cephalosporins: Another class of antibiotics that target cell wall synthesis.
    • Vancomycin: Also targets bacterial cell wall synthesis.
  • Lysozyme: An enzyme found naturally in human tears and saliva.
    • Mechanism: It breaks the \beta-1,4 glycosidic linkages in the peptidoglycan cell wall.
    • Effect: This weakens the cell wall, leading to cell bursting and death.
  • Gram-Positive vs. Gram-Negative Cell Walls:
    • Gram-Positive: Possess a very thick layer of peptidoglycan. They are generally more susceptible to penicillins, cephalosporins, and lysozyme because these agents have a direct and abundant target.
    • Gram-Negative: Have an outer membrane and a very thin layer of peptidoglycan. The outer membrane can protect the peptidoglycan layer, making these organisms generally less susceptible to cell wall-targeting antibiotics and lysozyme (though other factors also play a role).

Plasma Membrane

  • Structure:
    • Components: Composed of a phospholipid bilayer and proteins (similar to eukaryotic cells).
    • No Sterols: Unlike eukaryotic membranes, bacterial plasma membranes do not contain sterols (e.g., cholesterol), which contributes to their fluidity.
    • Proteins:
      • Peripheral Proteins: Located on the inner or outer surface of the membrane.
      • Integral Membrane Proteins: Span the entire lipid bilayer, often involved in transport processes.
      • Glycoproteins and Glycolipids: Carbohydrate components attached to membrane proteins and lipids, respectively.
  • Functions:
    • Selectively Permeable Barrier: Controls the movement of substances into and out of the cell (a universal function shared with eukaryotes).
    • Energy Production: The bacterial plasma membrane is the site for the breakdown of nutrients and the production of energy, specifically the Electron Transport Chain, where ATP is synthesized. This contrasts with eukaryotes, where this occurs in mitochondria.
    • DNA Attachment: The bacterial DNA (nucleoid) is typically attached to the plasma membrane at one point.
    • Flagella Base: Serves as the anchoring point for the base of bacterial flagella.
    • Spore Formation: Involved in the formation of endospores in spore-forming organisms.
    • Peptidoglycan Synthesis: Crucial for constructing the cell wall.
  • Antibiotic Targets: Fewer antibiotics target the plasma membrane due to its structural and functional similarities with eukaryotic cell membranes, which could lead to host toxicity.
    • Examples: Polymyxin B (often in topical agents like Neosporin), Bacitracin.

Membrane Transport

  • Overview: Mechanisms by which substances move across the plasma membrane.
  • I. Passive Transport: Movement of substances from a region of high concentration to low concentration (down the concentration gradient), requiring no cellular energy.
    • A. Simple Diffusion:
      • Mechanism: Small molecules move directly through the plasma membrane without the aid of transport proteins.
      • Control: The cell has no control over simple diffusion.
      • Factors: Rate is affected by temperature (higher tempo, faster diffusion), molecule size (smaller, faster), and gradient steepness (steeper, faster).
      • Examples: Water, carbon dioxide ( \text{CO}2 ), oxygen ( \text{O}2 ), and some small lipid-soluble molecules.
    • B. Facilitated Diffusion:
      • Mechanism: Molecules cross the membrane with the help of integral transport proteins (channels or carrier proteins).
      • Channels: Less specific, allow molecules of a certain size or charge to pass through.
      • Carrier Proteins: More specific, bind to a particular molecule, change shape, and release the molecule on the other side of the membrane. They 'carry' the molecule.
      • Rarity in Bacteria: Not very common in bacterial cells.
      • Reason: Bacteria typically maintain high intracellular concentrations of essential nutrients (e.g., glucose, amino acids) to hoard resources, which eliminates the concentration gradient needed for facilitated diffusion to function inward.
      • Examples: Glucose, ions, amino acids.
    • C. Osmosis (Special Case of Diffusion):
      • Focus: Refers exclusively to the movement of water molecules across a selectively permeable membrane.
      • Purpose: To equalize or control the solute concentration across the membrane.
      • Terms:
        • Solvent: Water.
        • Solute: Dissolved substances (e.g., glucose, ions).
      • Osmotic Pressure: The force driving water movement across the membrane. Bacterial cells often have high internal osmotic pressure due to high solute concentrations.
      • Conditions for Osmosis: Membrane must be permeable to water but impermeable to the solute, and a solute gradient must exist.
      • Effects of Environmental Tonicity on Cells:
        • Hypertonic Solution:
          • Description: High solute concentration and low water concentration outside the cell compared to inside.
          • Effect: Water flows out of the cell to equalize solute concentrations. The cell shrivels (crenation), looking like a raisin.
        • Hypotonic Solution:
          • Description: Low solute concentration and high water concentration outside the cell compared to inside. Most bacteria exist in this state.
          • Effect: Water flows into the cell to equalize solute concentrations. The cell swells and can potentially burst (lysate).
          • Cell Wall's Role: In bacteria, the rigid cell wall prevents the plasma membrane from bursting, protecting the cell in hypotonic environments.
          • Antibiotic Connection: Antibiotics that compromise the cell wall (e.g., penicillins) eliminate this protection, causing the cell to burst in its typical hypotonic environment.
        • Isotonic Solution:
          • Description: Solute concentration is equal inside and outside the cell. (e.g., human blood is isotonic).
          • Effect: Water moves freely across the membrane, but there is no net movement, and the cell maintains its normal shape.
  • II. Active Transport: Movement of substances against the concentration gradient (from low to high concentration), requiring cellular energy.
    • Prevalence in Bacteria: Most nutrients are transported into prokaryotes via active transport.
    • Reason: Allows bacteria to hoard essential resources inside the cell, even when external concentrations are low.
    • Requirements:
      • Carrier Protein: Highly specific proteins are required to bind and move specific molecules.
      • Energy (ATP): Hydrolysis of ATP ( \text{ATP} \to \text{ADP} + \text{Pi} ) provides the energy to drive transport against the gradient.
  • III. Group Translocation: A unique type of transport found exclusively in prokaryotes.
    • Mechanism: The transported molecule is chemically altered (modified) during its passage across the membrane.
    • No Concentration Gradient: Because the molecule is changed upon entry, an internal concentration gradient cannot be established, allowing continuous inward transport.
    • Energy Source: Requires energy from another high-energy compound, phosphoenolpyruvate (PEP), rather than ATP.
    • Primary Function: Prokaryotes primarily use group translocation to transport sugars.
    • Phosphotransferase System (PTS): The common system for sugar transport.
      • Process: Glucose, for example, binds to a specific membrane enzyme (e.g., glucose permease) within the PTS system. As it's transported into the cell, PEP donates a phosphate group, converting glucose into glucose-6-phosphate.
      • Benefit: Glucose-6-phosphate is already the second step in cellular respiration, so it's immediately ready for metabolic pathways, making this transport highly efficient.
      • Specificity: Different permeases exist for different sugars (e.g., fructose permease for fructose, mannitol permease for mannitol).

Cytoplasm and Cytosol

  • Cytoplasm: Refers to the entire contents within the plasma membrane of the cell.
    • Composition: Primarily water, along with proteins (enzymes), carbohydrates, lipids, inorganic ions, and amino acids.
  • Cytosol: The liquid portion of the cytoplasm where many metabolic reactions occur.
    • Technical Distinction: Technically, cytoplasm includes the cytosol plus all intracellular structures (e.g., DNA, inclusion bodies), as bacteria lack membrane-bound organelles.

Major Internal Structures of Bacterial Cells

  • I. Nucleoid:
    • Description: A region within the cytoplasm, not a membrane-bound organelle, where the bacterial genetic material is concentrated.
    • DNA Structure: Contains a single, long, continuous, circular, double-stranded piece of DNA (the bacterial chromosome).
      • Exceptions: Some organisms may have two chromosomes or linear chromosomes.
    • Attachment: Typically attached to the cell membrane at one point.
    • Compaction: Unlike eukaryotes which use histones, prokaryotes use histone-like proteins to compactly coil and fold their large chromosome into supercoils and domains, allowing it to fit within the small cell volume (occupying about 50 \% of cell space).
    • Replication Efficiency: E. coli can divide every 30 minutes, even though it takes about 40 minutes to replicate its entire genome. This is achieved through overlapping replication cycles, where new rounds of replication begin before the previous round is complete.
  • II. Plasmids (Extrachromosomal DNA):
    • Description: Small, circular, double-stranded pieces of DNA found in the cytoplasm, distinct from the main bacterial chromosome.
    • Replication: Replicate independently of the bacterial chromosome.
    • Copy Number: Can exist in low copy numbers (1-2 per cell) or high copy numbers (hundreds per cell).
    • Advantageous Genes: Plasmids carry genes that confer a selective advantage to the bacterium, but are not essential for basic survival.
      • Examples:
        • Antibiotic Resistance Genes: Enable the bacterium to survive in the presence of specific antibiotics (e.g., ampicillin resistance).
        • Toxin Production Genes: Allow production of virulence factors.
        • Toxic Metal Resistance Genes: Enable growth in environments with toxic metals.
    • Maintenance: Plasmids are energetically costly to maintain and replicate. Bacteria will typically lose plasmids if the advantageous genes they carry are no longer needed for survival in a particular environment.
    • Transfer: Plasmids can be transferred horizontally between bacteria, often via a pilus, contributing to the spread of traits like antibiotic resistance.
  • III. Ribosomes:
    • Function: Essential for protein synthesis (translation).
    • Abundance: Tens of thousands of ribosomes per bacterial cell.
    • Structure: Composed of two subunits (small and large), each made of proteins and ribosomal RNA (rRNA).
    • Distinguishing Feature (Prokaryotic vs. Eukaryotic):
      • Prokaryotic Ribosomes: Are smaller ( 70S ), composed of a 30S small subunit and a 50S large subunit, and are less dense than eukaryotic ribosomes.
      • Eukaryotic Ribosomes: Are larger ( 80S ), composed of a 40S small subunit and a 60S large subunit.
    • Antibiotic Target: The structural differences between prokaryotic and eukaryotic ribosomes make bacterial ribosomes excellent targets for selective antibiotics (e.g., macrolides, erythromycin, tetracycline, chloramphenicol), which inhibit bacterial protein synthesis without significantly harming host cells.
    • Location: Found freely floating in the cytoplasm or attached to the plasma membrane.
  • IV. Primitive Cytoskeleton:
    • Description: While once thought to lack a cytoskeleton, bacterial cells possess a primitive cytoskeleton composed of proteins, analogous to eukaryotic cytoskeletal elements.
    • Functions: Plays crucial roles in maintaining cell shape, facilitating cell division, and supporting cell wall integrity.
    • Bacterial Cytoskeletal Proteins:
      • FtsZ:
        • Homolog: Similar to eukaryotic tubulin (which forms microtubules).
        • Function: Forms a constricting ring at the mid-cell, essential for cell division.
      • MreB:
        • Homolog: Homologous to eukaryotic actin (which forms microfilaments).
        • Function: Found in rod-shaped and spiral bacteria, forming a helical structure beneath the plasma membrane that is essential for maintaining cell shape.
      • Crescentin:
        • Homolog: Homologous to eukaryotic lamin and keratin (intermediate filaments).
        • Function: Found in spiral-shaped bacteria with a single curve (comma-shaped). It causes and maintains the characteristic bend in these organisms.
  • V. Spores (Endospores):
    • Description: Highly resistant, dormant structures produced by certain Gram-positive bacteria (e.g., Bacillus and Clostridium species) as a survival mechanism.
    • Triggers: Formed in response to unfavorable environmental conditions (e.g., low nutrients, desiccation, extreme temperatures, radiation, chemical disinfectants).
    • Life Cycle:
      • Vegetative Cell: Actively growing and dividing.
      • Sporulation: When conditions become unfavorable, a vegetative cell differentiates into an endospore.
      • Dormancy: The endospore remains dormant, potentially for thousands of years, until conditions improve.
      • Germination: When conditions become favorable again, the endospore germinates back into an active vegetative cell.
    • Structure:
      • DNA Replication: The cell replicates its DNA.
      • Spore Septum: A membrane wall forms around one copy of the newly replicated DNA.
      • Cortex: A thick layer, primarily composed of peptidoglycan, forms around the core.
      • Dipicolinic Acid & Calcium Ions: Accumulate within the core and provide significant protection to the DNA.
      • Spore Coat: Several layers of protein form a highly protective outer casing.
      • Release: The mature endospore is often released from the parent cell.
    • Characteristics:
      • Very dehydrated, with minimal metabolic activity.
      • Extremely resistant to heat, cold, radiation (UV), desiccation, and chemical disinfectants.
      • Cannot be stained by ordinary methods; appear as clear circles within stained cells.
    • Sterilization: The only reliable method to kill endospores is autoclaving (high pressure and high temperature).
    • Historical Significance: The resistance of spores played a role in the historical debate on spontaneous generation, as boiling broths might kill vegetative cells but not spores, leading to later microbial growth.
    • Human Diseases Caused by Spore-Formers:
      • Anthrax: Caused by Bacillus anthracis. Spores inhaled into the lungs can germinate and cause severe infection (e.g., 2001 bioterrorism attacks).
      • Botulism: Caused by Clostridium botulinum. Spores in food (e.g., contaminated honey) can germinate and produce toxins, leading to infantile botulism in babies whose digestive tracts are not fully colonized.
      • Gas Gangrene: Caused by Clostridium perfringens.
      • Tetanus: Caused by Clostridium tetani.
  • VI. Inclusion Bodies:
    • Description: Non-membrane-bound structures found in the cytoplasm, serving as storage depots for various substances.
    • Purpose: Allow the organism to store excess nutrients or other important compounds for later use.
    • Types (Examples):
      • Metachromatic Granules (Volutin): Store phosphate. Phosphate is crucial for ATP, nucleic acids, and plasma membranes. Their presence can be a diagnostic feature (e.g., in Corynebacterium diphtheriae, the causative agent of diphtheria).
      • Polysaccharide Granules: Store glycogen and starch, providing a readily available source of glucose for energy.
      • Lipid Inclusions: Store poly-ß-hydroxybutyric acid, another form of energy reserve.
      • Sulfur Granules: Store elemental sulfur in certain bacteria.
      • Gas Vacuoles: Found in aquatic bacteria. These are hollow cavities that allow the organism to adjust its buoyancy, controlling its position in the water column.
      • Magnetosomes: Inclusion bodies containing iron oxide (magnetite). They allow certain aquatic bacteria to orient themselves along the Earth's magnetic field lines, directing their movement.

Exam Preparation Points

  • Gram Stain: Thoroughly understand each step of the Gram stain procedure and be able to predict the outcome (color) for Gram-positive and Gram-negative cells under various hypothetical scenarios (e.g., using water instead of alcohol).
  • Bacterial Structure Labeling: Be prepared to label a diagram of a bacterial cell and provide the function of each labeled structure (often a significant portion of the exam grade).
  • Macromolecules: Review general knowledge of biological macromolecules (proteins, carbohydrates, lipids, nucleic acids) and their basic building blocks.
  • Comparison Tables: Utilize comparison tables for different transport mechanisms (diffusion, facilitated diffusion, active transport, group translocation) and for prokaryotic vs. eukaryotic cell structures.
  • Case Studies: When writing case studies or short answers:
    • Underline or Italicize Organism Names: Always underline or italicize binomial scientific names (e.g., E. coli, Vibrio cholerae) every time they are written.
    • Paragraph Form: Present answers in coherent paragraph form, not just as bullet points.
    • References: Always include proper references for any information used. Give credit to sources.