Microbial Life: Prokaryotes and Protists

Prokaryotes

Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and organelles. They are among the most ancient and diverse forms of life, occupying a wide range of habitats.

• Cell Shapes: Prokaryotes exhibit three main shapes:

  • Cocci – Spherical-shaped bacteria (e.g., Streptococcus).

  • Bacilli – Rod-shaped bacteria (e.g., Escherichia coli).

  • Spirochetes – Spiral-shaped bacteria (e.g., Treponema pallidum).

• Gram-Positive vs. Gram-Negative Bacteria:

  • Gram-Positive: Thick peptidoglycan cell wall; stains purple in Gram stain.

  • Gram-Negative: Thin peptidoglycan layer and an outer membrane; stains pink/red.

• Exotoxins vs. Endotoxins:

  • Exotoxins: Toxic proteins secreted by bacteria; can cause disease even if bacteria are not present (e.g., botulinum toxin).

  • Endotoxins: Components of the outer membrane of Gram-negative bacteria; released when bacteria die and cell walls break down (e.g., lipopolysaccharide).

• Structural Components of a Bacterial Cell:

  • Cell Wall: Provides shape and protection; composed of peptidoglycan in bacteria.

  • Plasma Membrane: Controls movement of substances in and out of the cell.

  • Flagella: Used for movement.

  • Pili: Hair-like structures for attachment and conjugation.

  • Nucleoid: Region containing the bacterial chromosome (DNA).

• Biofilms: Complex communities of microorganisms attached to surfaces and embedded in a self-produced matrix. Biofilms can form on medical devices, teeth (plaque), and natural environments.

• Prokaryotic Modes of Nutrition:

  • Autotrophs: Obtain carbon from CO2 (e.g., cyanobacteria).

  • Heterotrophs: Obtain carbon from organic compounds (e.g., most bacteria).

• Proteobacteria: A major group of Gram-negative bacteria, including many important pathogens (e.g., Escherichia coli, Salmonella), nitrogen-fixing bacteria, and others.

• Gut Microbiota: The community of microorganisms living in the digestive tracts of animals. Examples include:

  • Helicobacter pylori – Associated with stomach ulcers.

  • Ghrelin Output: Gut bacteria can influence the production of ghrelin, a hormone that stimulates appetite.

Extremophiles – Archaea

Archaea are prokaryotes distinct from bacteria, often found in extreme environments. They have unique membrane lipids and genetic features.

• Halophiles: Thrive in highly salty environments (e.g., salt lakes).

• Thermophiles: Thrive in very hot environments (e.g., hot springs, hydrothermal vents).

Eukaryotes: Protists

Protists are a diverse group of mostly unicellular eukaryotic organisms. They are not plants, animals, or fungi, but share some characteristics with each group.

• Modes of Nutrition:

  • Autotrophic Protists: Photosynthetic (e.g., algae).

  • Heterotrophic Protists: Ingest food (e.g., amoebas, paramecia).

  • Mixotrophic Protists: Can switch between autotrophy and heterotrophy (e.g., Euglena).

• Four Supergroups of Protists:

  • SAR Group: Includes Stramenopiles (e.g., diatoms, brown algae), Alveolates (e.g., ciliates, dinoflagellates), and Rhizarians (e.g., foraminiferans).

  • Excavata: Includes organisms with a feeding groove (e.g., Giardia, Euglena).

  • Unikonta: Includes amoebas, slime molds, and the ancestors of fungi and animals.

  • Archaeplastida: Includes red algae, green algae, and land plants.

Table: Comparison of Prokaryotes and Eukaryotes

Example: Escherichia coli is a rod-shaped, Gram-negative bacterium commonly found in the intestines of humans and animals. Euglena is a mixotrophic protist capable of both photosynthesis and ingestion of food particles.

Additional info: The four supergroups classification is based on molecular and morphological evidence. Many protists are important ecologically as primary producers, decomposers, or pathogens.