Bacterial Chemotaxis, Specialized Adaptations, and the Endosymbiotic Theory

Bacterial Movement and Chemotaxis

• Conceptual Overview: Unlike humans and higher animals who possess eyesight, hearing, taste, and smell to navigate their environment for necessities like food, bacteria lack these complex senses. They require a specific mechanism to move toward desirable materials (attractants) or away from undesirable ones (repellents).

• Chemotaxis Defined:

  • Etymology: "Chemo" refers to chemicals; "Taxis" refers to movement.
  • Definition: Chemotaxis is the movement of an organism toward or away from a chemical signal or stimulus.

• Bacterial vs. Eukaryotic Flagella Movement:

  • Eukaryotic Cells: Organisms such as sperm cells or unicellular protists possess flagella that can move in three dimensions and generate significant force. They often contain a crystalline rod and a structure of multiple microtubules and structural proteins.
  • Bacterial Cells: Bacterial flagella are simpler, lacking crystalline rods and microtubules. They are composed primarily of the protein flagellin. Because of this structural simplicity, bacterial flagella can only do one thing: spin.
  • The Corkscrew Motion: Bacterial locomotion is characterized by a spinning, corkscrew-like motion. Because a single flagellum cannot generate much force or move in all directions, many bacteria have multiple flagella (e.g., peritrichous) to facilitate movement in desired directions.

• Polarity and Receptor Proteins:

  • Bacilli (rod-shaped) bacteria exhibit a form of polarity with an anterior (front) and a posterior (back) side, though they are not cephalized (no actual head).
  • Receptor Proteins: These are surface proteins produced via messenger RNA from the nucleoid region, translated by ribosomes, and embedded in the outer membrane (Gram-negative) or outer protein layer (Gram-positive).
  • The Active Site: These proteins function like a "dating site," possessing an active site waiting for a suitable ligand (molecule) to attach. For example, a bacteria capable of lactose fermentation might have receptors specifically looking for lactose molecules.

• The "Swim and Tumble" Mechanism:

  • Bacteria do not move in a straight line toward a food source because they cannot see it; they follow concentration gradients.
  • Swimming: When a signal attaches to an anterior receptor, the flagella rotate to propel the cell forward.
  • Tumbling: The organism eventually stops and tumbles. This random movement allows it to reorient. If it detects a stronger signal (higher concentration), it swims again.
  • Result: This circuitous, meandering path is compared to the game Marco Polo, where a blindfolded person moves toward the sound of a voice without a direct bead on the target.

• Types of Chemotaxis:

  • Positive Chemotaxis: Movement toward an attractant (e.g., nutrients like sugar). Receptors for these are generally located on the anterior side, away from the flagellar bundle in lophotrichous bacteria.
  • Negative Chemotaxis: Movement away from a repellent (e.g., toxins, antibiotics, or secretions from competitors). Receptors for these are located on the posterior end.
  • Flagellar Reversal: While propulsion typically occurs away from the flagella, some bacteria can reverse the spin clockwise or counterclockwise to move in the opposite direction.

• Sessile vs. Motile Bacteria:

  • Motile: Possess flagella and undergo chemotaxis.
  • Sessile: Organisms that are fixed in one spot (stuck). These organisms do not waste protein production on surface receptors for locomotion because they cannot move toward food or away from predators, even if they could sense them.

Specialized Adaptive Structures in Nonpathogenic Bacteria

Bacteria have evolved for approximately $3 \times 10^9$ years, giving them a much longer evolutionary history than eukaryotes ($1.2$ to $1.3 \times 10^9$ years). This has resulted in several unique cytoplasmic and structural adaptations.

• Thylakoid Membranes (Cyanobacteria):

  • Cyanobacteria History: These are roughly $2 \times 10^9$ years old and are responsible for moving Earth from a non-oxygenated to an oxygen-rich environment via photosynthesis.
  • Structure: These are highly folded inner membranes that increase surface area for photosystems (containing chlorophyll a and b).
  • Photosynthesis: They fix carbon dioxide ($CO_2$) and produce oxygen ($O_2$) and organic energy.
  • Biofuel Potential: In the future, cyanobacteria could be used in large fields of clear containers to produce sugars that other bacteria then convert into biofuels like ethanol or various alcohols.

• Gas Vesicles:

  • Found in aquatic bacteria to maintain buoyancy.
  • They function similarly to a swim bladder in fish or vacuoles in eukaryotes.
  • By filling or emptying these vesicles with gas, bacteria can inhabit specific depths in a water column where nutrients or sunlight levels are optimal, preventing them from sinking into predator-rich zones or rising to areas where sunlight might burn their photosystems.

• Storage Granules:

  • Analogous to a "Florida garage" (where things are stored because there are no basements) or a farm's peach harvest being canned into jams.
  • Bacteria store excess materials such as carbohydrates, amino acids, sulfur, or phosphate for later use.
  • Pathogenic Connection: Some pathogens, like Vibrio cholerae, use storage granules to store exotoxins (e.g., cholera toxin) before they even infect a host, so they are ready to change the host's environment immediately upon arrival.

• Magnetosomes:

  • Structures that store nonorganic materials, usually metals (inorganics).
  • These allow bacteria to sense the Earth's magnetic field to determine "up" and "down" (magnetotaxis), moving toward higher magnetic pull closer to the Earth's core.

The Eukaryotic Cell and Endomembrane System

• Complexity vs. Efficiency:

  • Eukaryotic Cells: Compartmentalized into membrane-bound organelles (Mitochondria, Golgi, Lysosomes, etc.). Described as a "large factory" (like Ford) with many specialized stations but less efficiency due to high management layers and waste.
  • Bacterial Cells: Described as a "studio apartment" where all processes occur in one space (the cytoplasm), making them highly efficient because they are small.

• Exploitation of the Endomembrane System by Pathogens:

  • Process: Pathogens like Vibrio cholerae exploit the host's endomembrane system (Rough ER, Golgi, Lysosomes).
  • Receptor-Mediated Endocytosis: The cholera exotoxin (consisting of A and B protein subunits) attaches to receptors on host enterocytes (digestive tract cells). The host cell invaginates its membrane, forming an endosome.
  • The "Mail System" Analogy: Just as the US Postal Service ships an envelope without knowing it contains a toxin (like anthrax), the Golgi apparatus glycosylates and tags the toxin, and the Endoplasmic Reticulum (ER) moves it to the target site.
  • Consequence: The toxin triggers cyclic AMP, causing the cell to pump out ions like chlorine ($Cl^-$), sodium ($Na^+$), and potassium ($K^+$). This creates a high salt concentration outside the cell, causing massive water loss and severe dehydration through osmosis.

The Theory of Endosymbiosis

• Origin of Mitochondria and Chloroplasts:

  • The theory (notably hypothesized by Margulis) suggests that around $1.5$ to $2.0 \times 10^9$ years ago, a large Archaean cell engulfed a smaller independent bacteria via endocytosis.
  • Instead of digesting the smaller cell for energy, a symbiotic relationship formed: the smaller cell provided high-rate energy production, and the larger cell provided protection and resources.

• Evidence for Endosymbiosis:

  1. Membranes: Both mitochondria and chloroplasts (plastids) have two membranes. The outer membrane matches the host's phospholipids; the inner membrane contains peptidoglycan, a substance unique to prokaryotes.
  2. DNA: They possess a single circular chromosome, identical to bacterial DNA structure, rather than linear eukaryotic chromosomes.
  3. Ribosomes: They have their own ribosomes, which are the same size and composition as bacterial ribosomes ($70S$) rather than eukaryotic ($80S$) ones.
  4. Reproduction: They divide independently of the host cell's mitosis on their own schedule (mitochondriogenesis).
  5. Size: They are roughly the same size as free-living bacteria.

• Maternal Lineage:

  • In humans, mitochondria are inherited solely from the maternal side (the mother's egg).
  • This has allowed scientists to trace a "Mitochondrial Eve" back roughly $300,000$ years to a common female ancestor. Companies like Ancestry and 23andMe use this mitochondrial DNA sequencing to determine maternal lineage.

Questions & Discussion

• Question 1: Which of the following will be found in both prokaryotic and eukaryotic cells?

  • Options: a) Ribosomes, b) Mitochondria, c) Endoplasmic reticulum, d) Conjugation pili.
  • Correct Answer: a) Ribosomes. (Mitochondria and ER are eukaryotes only; conjugation pili are prokaryotes only).

• Question 2: Which eukaryotic cell structure is similar and closely related to the prokaryotic cell?

  • Correct Answer: Mitochondria (due to endosymbiosis). Microtubules and the Golgi apparatus do not share these similarities.

• Question 3: Which term describes a microscopic appearance of bacterial cells that appear as a chain of round-shaped cells?

  • Correct Answer: Streptococci. (The parallel septum formation creates the chain; staphylococci are random clusters; diplobacilli are rods).

• Question 4: Which of the following is a characteristic of a Gram-positive cell wall?

  • Options: a) Waxy due to mycolic acid, b) Outer membrane with lipopolysaccharides, c) Thick with several layers of peptidoglycan, d) Periplasmic space.
  • Correct Answer: c) Thick with several layers of peptidoglycan. (Mycolic acid is for acid-fast; LPS and periplasmic space are for Gram-negative).