Prokaryotes, Eukaryotes, and Microbial Diversity: Comparative Review

Prokaryotes vs Eukaryotes

Structural and Functional Differences

Prokaryotes and eukaryotes represent two fundamental cell types in microbiology, distinguished by their structural features and complexity. Understanding these differences is essential for classifying microorganisms and their roles in disease.

• Prokaryotes: Lack a true nucleus; DNA is located in a nucleoid region. No membrane-bound organelles. Cell wall contains peptidoglycan (in bacteria).

• Eukaryotes: Possess a true nucleus surrounded by a nuclear membrane. Contain membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Flagella: Prokaryotic flagella are simple, composed of flagellin; eukaryotic flagella are complex, made of microtubules (9+2 arrangement).

• Ribosomes: Prokaryotes have 70S ribosomes; eukaryotes have 80S ribosomes.

• Membrane: Both have plasma membranes, but eukaryotic membranes may contain sterols.

• Mitochondria: Present only in eukaryotes; site of aerobic respiration.

Example: Escherichia coli (prokaryote) vs. Saccharomyces cerevisiae (eukaryote, yeast).

Viruses and Their Relationship to Cellular Life

Comparison to Prokaryotes and Eukaryotes

Viruses are acellular entities that share some similarities with both prokaryotes and eukaryotes but are fundamentally different.

• Structure: Viruses lack cellular structure; composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid).

• Similarity: Some viruses infect prokaryotes (bacteriophages), others infect eukaryotes (animal and plant viruses).

• Disease: Both prokaryotes, eukaryotes, and viruses can cause disease in humans.

Categories of Microorganisms

Classification of Prokaryotes and Eukaryotes

• Prokaryotes: Includes bacteria and archaea.

• Eukaryotes: Includes fungi, protozoans, helminths (parasitic worms), and algae.

Example: Fungi (eukaryotes) include molds and yeasts; bacteria (prokaryotes) include Streptococcus and Bacillus.

Fungi, Protozoans, and Helminths

Structure and Appearance

• Fungi: Eukaryotic, cell wall contains chitin, may be unicellular (yeasts) or multicellular (molds).

• Protozoans: Unicellular eukaryotes, lack cell wall, variable shapes, motile via flagella, cilia, or pseudopodia.

• Helminths: Multicellular eukaryotic parasites; include nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes).

Classification of Helminths

• Nematodes: Roundworms (e.g., pinworm, hookworm).

• Cestodes: Tapeworms.

• Trematodes: Flukes.

Protozoan Diseases

• Malaria: Caused by Plasmodium species; transmitted by mosquitoes.

• Giardiasis: Caused by Giardia lamblia.

• Amoeboid brain infection: Caused by Naegleria fowleri.

• Toxoplasmosis: Caused by Toxoplasma gondii.

• Trichomonas vaginitis: Caused by Trichomonas vaginalis.

Fungal Diseases (Mycoses)

• Candidiasis: Caused by Candida albicans.

• Ringworm: Dermatophyte infection.

• Athlete's foot: Caused by Trichophyton species.

• Histoplasmosis: Caused by Histoplasma capsulatum.

Helminthic Diseases

• Pinworm: Enterobius vermicularis (nematode).

• Hookworm: Ancylostoma or Necator species.

Cellular Structures and Functions

Fungal vs Bacterial Cellular Structure

• Fungi: Eukaryotic, cell wall contains chitin, mitochondria present.

• Bacteria: Prokaryotic, cell wall contains peptidoglycan, no mitochondria.

Fungus feeds on: Organic matter; often decomposers.

Protozoans and moisture: Thrive in moist environments; many are aquatic.

Helminths and hermaphroditic: Some helminths possess both male and female reproductive organs.

Trophozoite: The active, feeding stage of protozoans.

Microscopy and Size

Protozoan Size and Light Microscopy

• Protozoans are generally larger than bacteria; visible under light microscopes.

Reproduction and Spores

Fungal Spores vs Bacterial Spores

• Fungal spores: Used for reproduction; can be sexual or asexual.

• Bacterial spores (endospores): Survival structures; highly resistant to environmental stress.

Reproductive fungal spores: Examples include conidia, sporangiospores.

Bacterial Cell Wall and Gram Staining

Peptidoglycan and Gram-Positive Bacteria

• Peptidoglycan: Major component of bacterial cell wall; provides structural strength.

• Gram-positive rods: Thick peptidoglycan layer; retain crystal violet stain.

• Gram-positive rods and endospores: Some, like Bacillus and Clostridium, form endospores.

Microbial Diseases and Examples

Bacterial Diseases

• Strep throat: Caused by Streptococcus pyogenes.

• Botulism: Caused by Clostridium botulinum.

• Tetanus: Caused by Clostridium tetani.

• Anthrax: Caused by Bacillus anthracis.

• Meningitis: Can be caused by bacteria, fungi, or protozoa.

Fungal diseases: See "Fungal Diseases (Mycoses)" above.

Comparison Table: Prokaryotes vs Eukaryotes vs Viruses

Feature	Prokaryotes	Eukaryotes	Viruses
Nucleus	No	Yes	No
Membrane-bound organelles	No	Yes	No
Cell wall	Peptidoglycan (bacteria)	Chitin (fungi), cellulose (plants), absent in animals	None
Ribosomes	70S	80S	None
Reproduction	Binary fission	Mitosis/meiosis	Requires host cell
Diseases in humans	Yes	Yes	Yes
Examples	Escherichia coli, Streptococcus	Candida, Plasmodium, Enterobius	Influenza virus, HIV

Additional info:

• Malaria eradication efforts have reduced cases globally, but the disease persists in tropical regions.

• Motility in eukaryotes is achieved via flagella, cilia, or pseudopodia.

• Fungi cause diseases called mycoses; some are superficial (skin), others systemic.

• Ribosomes are essential for protein synthesis; some pathogens target host ribosomes.

• Bacterial endospores are highly resistant to heat, desiccation, and chemicals.