Major Groups of Prokaryotic and Eukaryotic Microorganisms: Classification, Structure, and Diversity

Major Groups of Prokaryotic and Eukaryotic Microorganisms

Overview of Microbial Diversity

Microorganisms are classified into three major domains: Bacteria, Archaea, and Eukarya. This classification is based on genetic, structural, and metabolic differences. All three domains are believed to have arisen from a universal common ancestor, and each contains a wide variety of unicellular and multicellular organisms.

Naming and Classifying Microorganisms

Binomial Nomenclature and Taxonomy

The system of naming organisms, known as binomial nomenclature, was established by Carolus Linnaeus. Each organism is given a two-part Latin name: the genus (capitalized) and the species (lowercase). For example, Escherichia coli is named after its discoverer and its habitat (the colon). Names can be honorary, descriptive, or both.

• Genus: First part, capitalized, underlined or italicized

• Species: Second part, lowercase, underlined or italicized

• Example: Staphylococcus aureus (clusters of round, gold-colored colonies)

Prokaryotic Microorganisms: Bacteria

General Characteristics

Bacteria are prokaryotic, unicellular organisms that lack a nucleus. They are ubiquitous and display a variety of shapes and arrangements. Most bacteria are either spherical (cocci) or rod-shaped (bacilli), but some are spiral-shaped.

• Coccus: Single spherical cell

• Cocci: Multiple spherical cells

• Bacillus: Single rod-shaped cell

• Bacilli: Multiple rod-shaped cells

• Spiral forms: Vibrio, spirillum, spirochete

Bacterial Arrangements

Bacteria can form characteristic arrangements based on their patterns of division:

• Diplococci: Pairs of cocci

• Streptococci: Chains of cocci

• Tetrads: Groups of four cocci

• Sarcinae: Cubic configuration of eight cocci

• Staphylococci: Irregular clusters of cocci

Rod-shaped bacteria (bacilli) also have characteristic arrangements:

• Single bacillus: One rod

• Diplobacilli: Pairs of rods

• Streptobacilli: Chains of rods

• Coccobacillus: Short, oval rods

Bacterial Cell Walls

Bacteria are classified by the structure of their cell walls, which contain peptidoglycan. The main types are:

• Gram-positive: Thick peptidoglycan layer, stains purple in Gram stain, contains teichoic acids.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink/red.

• Acid-fast: Waxy cell wall with mycolic acids, characteristic of Mycobacterium species.

Bacterial Metabolism and Reproduction

Bacteria exhibit diverse metabolic strategies:

• Heterotrophic: Obtain energy from organic compounds

• Autotrophic: Use light (photosynthetic, e.g., with chlorophyll) or inorganic compounds

• Reproduction: Mainly asexual via binary fission

Only about 1% of bacterial species have been discovered, and less than 1% are pathogenic to humans.

Prokaryotic Microorganisms: Archaea

General Characteristics

Archaea are prokaryotic, unicellular organisms with cell walls containing pseudopeptidoglycan. They are often found in extreme environments, such as high temperature, high salinity, or acidic conditions.

• Metabolism: Can be heterotrophic or autotrophic (e.g., using bacteriorhodopsin)

• Reproduction: Asexual, primarily by binary fission

• Pathogenicity: No known archaeal pathogens

Archaea display a variety of shapes, including rods, spheres, irregular forms, and box-shaped cells (e.g., Haloquadratum).

Archaeal Diversity

• Crenarchaeota: Mostly hyperthermophiles, thrive in sulfur-rich environments (e.g., volcanoes). Examples: Sulfolobus, Thermoproteus, Pyrolobus.

• Euryarchaeota: Includes hyperthermophiles, halophiles, methanogens, and acidophiles. Examples: Thermoplasma, Halobacterium, Methanobacterium.

Eukaryotic Microorganisms: Fungi

General Characteristics

Fungi are eukaryotic organisms that can be unicellular (yeasts) or multicellular (molds, mushrooms). Their cell walls contain chitin. Fungi are important decomposers and can reproduce both asexually and sexually.

• Yeasts: Unicellular, reproduce by budding

• Molds: Multicellular, composed of hyphae forming a mycelium

Fungal Diversity

• Zygomycetes: "Bread molds"; mostly non-pathogenic except Rhizopus (can cause disease in immunocompromised individuals)

• Ascomycetes: "Sac fungi"; includes molds and yeasts. Some produce toxins (e.g., Aspergillus), others are beneficial (e.g., Penicillium for antibiotics, Saccharomyces for brewing).

• Basidiomycetes: "Club fungi"; includes mushrooms, some edible, others toxic or hallucinogenic.

Eukaryotic Microorganisms: Protozoa

General Characteristics

Protozoa are eukaryotic, unicellular organisms with variable shapes. Most lack cell walls and are motile. They are primarily heterotrophic and reproduce both asexually and sexually.

• Amoeba: Move by pseudopodia (e.g., Naegleria fowleri)

• Diplomonads: Flagellated, lack mitochondria, parasitic (e.g., Giardia lamblia)

• Parabasalids: Flagellated, lack mitochondria, parasitic (e.g., Trichomonas vaginalis)

• Kinetoplastids: Flagellated, contain kinetoplast, some are parasitic (e.g., Trypanosoma species)

• Euglenids: Flagellated, contain chloroplasts and eyespot, mostly non-pathogenic

• Ciliates: Move via cilia, have two nuclei, mostly non-pathogenic (e.g., Paramecium)

• Apicomplexans: Non-motile, important pathogens (e.g., malaria, toxoplasmosis)

Eukaryotic Microorganisms: Algae

General Characteristics

Algae are eukaryotic organisms that can be unicellular or multicellular. Their cell walls contain cellulose and pectin. Algae are primarily autotrophic, containing chloroplasts for photosynthesis, and reproduce both asexually and sexually.

• Red algae: Source of agar

• Brown algae: Also known as kelp, produce alginate

• Green algae: Diverse group, closely related to plants

• Dinoflagellates: Can cause red tides and paralytic shellfish poisoning

• Diatoms: Silica cell walls, important in aquatic ecosystems

Eukaryotic Microorganisms: Protists

Slime Molds and Oomycetes

Algae and protozoa are both classified as protists. Some unusual protists include:

• Slime molds: Found on decaying matter, start as amoeba-like cells, aggregate to form sporangia with spores when starved.

• Oomycetes (water molds): Produce hyphae, reproduce with spores, similar to fungi but more closely related to brown algae.

Summary Table: Major Groups of Microorganisms

Group	Cell Type	Cell Wall	Metabolism	Reproduction	Examples
Bacteria	Prokaryotic	Peptidoglycan	Heterotrophic/Autotrophic	Asexual (binary fission)	Escherichia coli, Staphylococcus aureus
Archaea	Prokaryotic	Pseudopeptidoglycan	Heterotrophic/Autotrophic	Asexual (binary fission)	Halobacterium, Thermoplasma
Fungi	Eukaryotic	Chitin	Heterotrophic	Asexual/Sexual	Aspergillus, Saccharomyces
Protozoa	Eukaryotic	None (most)	Heterotrophic	Asexual/Sexual	Giardia lamblia, Trypanosoma
Algae	Eukaryotic	Cellulose, pectin	Autotrophic	Asexual/Sexual	Red algae, brown algae, green algae