1. Cellular Structure and Organization:
* Prokaryotes: Lack a true nucleus and membrane-bound organelles. Includes bacteria and archaea.
* Eukaryotes: Have a true nucleus and membrane-bound organelles. Includes fungi, protists, and algae.
* Viruses: Non-cellular entities that require a host cell to replicate.
2. Morphology (Shape and Size):
* Bacteria:
* Cocci: Spherical
* Bacilli: Rod-shaped
* Spirilla: Spiral-shaped
* Fungi:
* Yeasts: Single-celled, oval-shaped
* Molds: Filamentous, multicellular
* Protists: Wide range of shapes and sizes.
3. Metabolism and Nutrition:
* Photoautotrophs: Obtain energy from sunlight and carbon from CO2 (e.g., algae, cyanobacteria).
* Chemoautotrophs: Obtain energy from inorganic chemicals and carbon from CO2 (e.g., some bacteria).
* Photoheterotrophs: Obtain energy from sunlight and carbon from organic compounds (e.g., some bacteria).
* Chemoheterotrophs: Obtain energy and carbon from organic compounds (e.g., animals, fungi, most bacteria).
* Obligate aerobes: Require oxygen for respiration.
* Obligate anaerobes: Cannot survive in the presence of oxygen.
* Facultative anaerobes: Can survive with or without oxygen.
4. Genetic Characteristics:
* DNA sequence analysis: Comparing the sequence of DNA to identify evolutionary relationships.
* Ribosomal RNA (rRNA) analysis: Comparing the sequence of rRNA genes, which are highly conserved.
* Genome sequencing: Determining the entire genetic sequence of an organism.
5. Physiological and Biochemical Properties:
* Temperature optima: The temperature range at which an organism grows best.
* pH optima: The pH range at which an organism grows best.
* Oxygen requirements: Whether an organism requires oxygen, tolerates oxygen, or is poisoned by oxygen.
* Metabolic pathways: Unique biochemical reactions that an organism uses to obtain energy and synthesize nutrients.
6. Ecological Role:
* Decomposers: Break down organic matter (e.g., fungi, bacteria).
* Producers: Generate organic matter through photosynthesis (e.g., algae, cyanobacteria).
* Pathogens: Cause disease in humans, animals, or plants (e.g., some bacteria, viruses, fungi).
7. Clinical Significance:
* Virulence factors: Properties of microorganisms that contribute to their ability to cause disease.
* Antibiotic susceptibility: Whether an organism is sensitive to specific antibiotics.
Important Note: While these criteria provide a framework for classifying microorganisms, there are many exceptions and overlaps. It's essential to consider multiple factors when attempting to classify a microorganism.
