Eukaryotic Microbes: Structure, Diversity, and Pathogenicity

Eukaryotic Microbes

Introduction to Eukaryotic Microbes

Eukaryotic microbes are a diverse group of organisms that include fungi, algae, protozoa, and helminths. These organisms are distinguished from prokaryotes by their complex cellular organization, including membrane-bound organelles and a defined nucleus. Some eukaryotic microbes are unicellular, while others are multicellular, and several are important human pathogens.

• Fungi: Unicellular (yeasts) or multicellular (molds)

• Algae: Photosynthetic, mostly aquatic

• Protozoa: Unicellular, microscopic, often motile

• Helminths: Multicellular animals (parasitic worms), not always considered microbes but included due to their medical importance

Prokaryotic vs. Eukaryotic Cells

Key Differences

Understanding the differences between prokaryotic and eukaryotic cells is fundamental in microbiology.

Feature	Prokaryote	Eukaryote
Chromosomes	One circular chromosome, not membrane-bound	Paired chromosomes, membrane-bound
Histones	Absent	Present
Organelles	No membrane-bound organelles	Have membrane-bound organelles
Cell Wall	Peptidoglycan (in bacteria)	Polysaccharide (if present, e.g., in fungi)
Division	Binary fission	Mitosis

Fungi

General Characteristics

Fungi are eukaryotic organisms that can be unicellular (yeasts) or multicellular (molds). Most fungi are saprophytic, decomposing dead organic matter, but some are pathogenic to humans and animals.

• Unicellular fungi: Yeasts (e.g., Saccharomyces cerevisiae)

• Multicellular fungi: Molds (e.g., Penicillium, Aspergillus)

• Beneficial roles: Decomposition, symbiosis with plants (mycorrhizae), food (mushrooms), and medicine production

Fungal Structures

• Thallus: The body of a fungus

• Hyphae: Filamentous structures; can be septate (with cross-walls) or coenocytic (without cross-walls)

• Mycelium: A mass of hyphae

• Pseudohyphae: Chains of yeast cells resulting from incomplete budding

Fungal Life Cycle

• Reproduction can be asexual (via spores, budding) or sexual (fusion of nuclei from two cells)

• Fungal spores are for reproduction, not survival (contrast with bacterial endospores)

Yeasts

• Unicellular fungi that reproduce by budding (asymmetric division)

• Dimorphic fungi: Exist in two forms depending on temperature

  • Yeast-like at 37°C

  • Mold-like at 25°C

• Candida albicans: Example of a dimorphic yeast causing candidiasis (oral thrush, vaginitis)

Problems Associated with Pathogenic Fungi

• Mycoses: Fungal infections of body tissues (e.g., aspergillosis, tinea/ringworm, candidiasis, cryptococcal meningitis)

• Mycotoxicoses: Poisoning by fungal toxins (e.g., aflatoxins, fumonisins)

• Hypersensitive allergic reactions: Allergies to fungal spores (e.g., Farmer's lung from Aspergillus)

Common Mycoses

• Fungi produce keratinase to degrade keratin in skin, nails, and hair

• Types of Mycoses:

  • Cutaneous mycoses (Dermatophytes): Infect skin, hair, nails (e.g., tinea infections such as athlete's foot, ringworm)

  • Subcutaneous mycoses: Infect deeper layers of skin

  • Systemic mycoses: Spread throughout the body

Type	Example Organisms	Diseases
Cutaneous	Trichophyton mentagrophyte, Microsporum canis, Epidermophyton floccosum	Tinea capitis (scalp), tinea corporis (body), tinea pedis (athlete's foot), tinea cruris (groin)
Subcutaneous	Various soil fungi	Infections beneath the skin
Systemic	Histoplasma, Coccidioides	Disseminated infections

Protozoa

General Characteristics

Protozoa are unicellular, eukaryotic organisms with complex life cycles. Many are free-living, but some are important human pathogens.

• Approximately 65,000 species

• Complex life cycles may require multiple hosts

• Definitive host: Host in which the parasite matures or reproduces

• Intermediate host: Host that harbors a developmental stage (e.g., larval stage)

Reproduction

• Asexual reproduction by fission, budding, or schizogony (multiple fission)

• Some protozoa form cysts for survival in harsh conditions

Examples of Pathogenic Protozoa

• Giardia lamblia: Causes giardiasis ("beaver fever"), forms cysts, infects intestines

• Trichomonas vaginalis: Causes trichomoniasis (STI), does not form cysts, transmitted via skin-to-skin contact

Trichomonas vaginalis

• Anaerobic, flagellated protozoan parasite

• Most common pathogenic protozoan in industrialized countries

• Causes trichomoniasis (STI); about 160 million infections annually worldwide

• Women usually symptomatic; men often asymptomatic

Toxoplasma gondii

• Definitive host: Felids (cats)

• Intermediate hosts: Warm-blooded animals (including humans)

• Oocysts from cat feces release sporozoites, which differentiate into tachyzoites (invade tissue) and bradyzoites (slow division)

Plasmodium (Malaria)

• Causes malaria, affecting ~300 million people/year

• Kills more people than HIV in Sub-Saharan Africa

• Definitive host: Anopheles mosquito

• Intermediate host: Humans

Pathogenic Helminths

General Characteristics

Helminths are multicellular parasitic worms with complex life cycles. They are characterized by reduced digestive, nervous, and locomotor systems, and complex reproductive strategies.

• Dioecious: Separate male and female animals

• Monoecious: Both male and female reproductive organs in one animal (hermaphroditic)

• Life cycle: Egg → larva(e) → adult

Group	Examples	Diseases
Flatworms	Tapeworms (Taenia saginata, Taenia solium), flukes	Taeniasis, schistosomiasis
Nematodes	Trichinella spiralis, pinworms	Trichinosis, enterobiasis

Tapeworms

• Adult tapeworms live in the intestines of humans (definitive host)

• Eggs from feces ingested by animals (e.g., pigs, cattle)

• Larvae lodge in animal muscle; humans infected by eating undercooked meat

• Attach to intestinal wall with hooks

• Taenia saginata (beef tapeworm) can grow up to 6 m

• Taenia solium (pork tapeworm)

Roundworms (Nematodes)

• Trichinosis (trichinellosis): Caused by eating raw or undercooked pork or wild game infected with Trichinella spiralis

• Pinworms: Common intestinal nematode infection

Additional info: Helminths are included in microbiology due to their life cycles involving microscopic stages and their importance as human pathogens.