BIOL 1720 II: Bacteria and Archea

Bacterial Classification: Gram Positive vs. Gram Negative

  • One of the primary divisions of bacteria is into two categories: Gram Positive and Gram Negative.

    • Gram Positive Bacteria:

    • Characterized by simpler cell walls.

    • Possess a thick layer of peptidoglycan on the outside.

    • Gram Negative Bacteria:

    • Have thin peptidoglycan layers which are located between two membranes: inner plasma membrane and outer membrane.

    • Outer membrane provides additional protection and distinct properties.

    • Distinguishing Features: Presence of peptidoglycan differentiates all bacteria from archaea and eukaryotes.

Gram Staining Process

  • Gram Staining enables the differentiation between Gram Positive and Gram Negative bacteria based on their cell wall composition.

    • Staining Steps:

    1. Initial Dye: Bacteria are dyed with safranin (pinkish-red), which allows visibility of all bacteria.

    2. Crystal Violet Application: The introduction of crystal violet, a darker dye, which binds to the peptidoglycan layer.

      • Gram Positive bacteria turn purple due to the ability of crystal violet to adhere to their thick peptidoglycan layer.

      • Gram Negative bacteria appear pink as the crystal violet is washed away during rinsing due to their thinner peptidoglycan layer.

    3. Mechanism Insight: Crystal violet specifically binds to Gram Positive bacteria while failing to penetrate Gram Negative due to their additional outer membrane, allowing differentiation.

Importance of Understanding Bacterial Types

  • Understanding the differences between Gram Positive and Gram Negative bacteria is crucial for medical applications:

    • Pathogenicity: Gram Negative bacteria are often more pathogenic due to their structural defenses against antibiotics.

    • Antibiotic Resistance: The outer membrane of Gram Negative bacteria provides a barrier to many antibiotics, making them harder to treat.

    • Treatment Development: Distinguishing between types allows for the development of targeted antibiotics.

Protective Structures in Bacteria

  • Bacteria can employ various mechanisms for survival:

    • Endospores:

    • Not all prokaryotes form endospores, but those that do can survive harsh conditions for centuries.

    • Functionally similar to extreme hibernation, retaining viability until conditions improve.

    • Fibriae:

    • Short hair-like structures that resemble Velcro, allowing attachment to surfaces and other particles for food.

    • Pili (Sex Pili):

    • Longer, specialized structures that facilitate genetic exchange between bacteria.

    • Important for DNA sharing in prokaryotic reproduction, contributing to genetic diversity.

Modes of Motility

  • Taxis refers to directed movement in response to stimuli:

    • Phototaxis: Movement towards light, advantageous for photosynthetic organisms.

    • Chemotaxis: Movement towards chemical stimuli, beneficial for locating nutrients or avoiding toxins.

  • Motility Structures:

    • Cilia: Small, hair-like structures aiding in movement.

    • Flagella: More complex structures with motor mechanisms allowing rapid movement.

Internal Organization of Bacterial Genomes

  • Bacterial Genomes:

    • Generally small and circular, lacking membrane-bound organelles like a nucleus.

    • Contain a concentrated DNA region known as the nucleoid, which organizes the DNA without membrane.

    • Plasmids: Small circular DNA molecules separate from the chromosomal DNA, often carrying beneficial genes.

Reproduction and Genetic Exchange in Bacteria

  • Binary Fission: The method of bacterial replication where the cell splits into two identical cells, rapidly increasing populations.

  • Enhanced Genetic Variability Techniques:

    • Transformation: Uptake of extracellular or exogenous DNA from the environment.

    • Transduction: Viral-mediated DNA transfer between bacteria, often using bacteriophages during the lysogenic cycle.

    • Conjugation: Direct DNA transfer between bacteria of different species via sex pili, allowing for significant genetic exchange, including antibiotic resistance.

Nutritional Modes in Bacteria

  • Bacteria can be categorized based on their feeding habits into autotrophs (self-feeders) and heterotrophs (organism feeders):

    1. Autotrophs:

      • Photoautotrophs: Utilize light and CO2 for energy (e.g., cyanobacteria).

      • Chemoautotrophs: Extract energy from inorganic chemicals (e.g., sulfur bacteria).

    2. Heterotrophs:

      • Photoheterotrophs: Use light but require organic compounds (e.g., some aquatic microbes).

      • Chemoheterotrophs: Depend on organic compounds for energy and carbon (e.g., many bacteria, fungi, and animals).

Archaea: An Emerging Domain

  • Archaea display significant diversity and include unique groups such as extremophiles and methanogens:

    • Extremophiles: Survive in extreme conditions (e.g., high temperatures, salinity).

    • Methanogens: Produce methane, play roles in greenhouse gas dynamics; often strict anaerobes that thrive in anoxic environments.

    • Biological Importance: Archaea participate in essential environmental roles such as nutrient cycling and bioremediation of toxic compounds.

  • Interactions with Ecosystems: Archaea exhibit symbiotic relationships and vital roles in nutrient recycling, decomposition, and adaptation in varying environments.