General Characteristics of Eukaryotic Microorganisms

Overview of Eukaryotic Microorganisms

Eukaryotic microorganisms include a diverse array of organisms that possess membrane-bound organelles and a true nucleus. These organisms are vital both as human pathogens and as contributors to ecological processes.

• Protozoa: Unicellular, motile, lack cell walls.

• Fungi: Non-photosynthetic, cell walls composed of chitin.

• Algae: Photosynthetic, cell walls often contain cellulose.

• Water molds and slime molds: Fungus-like protists.

• Helminths: Multicellular worms, studied in medical microbiology due to their pathogenicity.

These groups include both human pathogens and organisms essential for life processes.

Classification of Eukaryotes Over Time

The classification of eukaryotes has evolved, reflecting advances in molecular biology and phylogenetics.

Additional info: Modern classification uses genetic sequencing to define supergroups, improving accuracy in evolutionary relationships.

Cell Division in Eukaryotes

Types of Cytoplasmic Division

Eukaryotic cells divide by various mechanisms, including mitosis, budding, and schizogony.

• Mitosis: Produces two genetically identical daughter cells.

• Budding: Common in yeasts; a new cell forms as a protrusion from the parent cell.

• Schizogony: Multiple mitoses followed by cytokinesis, producing many daughter cells (merozoites), typical in protozoa such as Plasmodium.

Equation:

Other Eukaryotes of Microbiological Interest: Vectors

Arthropod Vectors

Arthropods are animals that transmit pathogens to humans. They are classified as mechanical (passive transfer) or biological (pathogen develops within vector).

• Arachnida: Four pairs of legs; includes ticks (most important arachnid vectors) and mites (some transmit rickettsial diseases).

• Insecta: Three pairs of legs, three body regions; includes fleas, lice, flies, mosquitoes (most important disease vectors), and kissing bugs.

Representative Arthropod Vectors

Protozoans

General Features

Protozoa are a diverse group of unicellular eukaryotes, defined by:

• Eukaryotic

• Unicellular

• Lack a cell wall

• Motile by cilia, flagella, or pseudopods

Some exceptions to these features exist.

Parasitology and Distribution

Protozoan and helminthic parasites are found worldwide, especially in moist environments such as ponds, lakes, and decaying organic matter. Most are chemoheterotrophic, obtaining food by phagocytosis. Parasitic infections are common in rural, undeveloped, or overcrowded areas and are emerging threats in developed nations.

Life Cycle of Protozoa

• Trophozoite: Motile, feeding, and reproducing stage within the host.

• Cyst: Infective, resting stage that survives in the environment. Undergoes excystment when ingested, developing into trophozoites. Trophozoites undergo encystment before leaving the host in feces.

Key Features of Protozoan Parasites of Humans

Distribution and Notable Protozoan Diseases

• Naegleria fowleri: Causes Primary Amebic Meningoencephalitis (PAM) when contaminated water is inhaled. High fatality rate; prevention is difficult due to environmental hardiness.

• Giardia intestinalis: Causes giardiasis, a diarrheal disease. Transmission is typically fecal-oral, often via contaminated water.

• Plasmodium spp.: Causative agent of malaria. Five species infect humans, with P. falciparum causing most deaths. Transmitted by Anopheles mosquitoes.

Plasmodium Life Cycle

• Sporozoites injected by mosquito enter bloodstream.

• Develop in liver, undergo schizogony to produce merozoites.

• Merozoites infect red blood cells, multiply, and cause symptoms.

• Some merozoites develop into gametocytes, which are taken up by mosquitoes.

Equation:

Flagellates: Trypanosoma cruzi and Trypanosoma brucei

• Trypanosoma cruzi: Causes Chagas' disease, transmitted by kissing bugs (Triatoma). Disease progresses through acute (chagomas), generalized, chronic asymptomatic, and symptomatic (congestive heart failure) stages.

• Trypanosoma brucei: Causes African sleeping sickness, transmitted by tsetse flies (Glossina). Two variants: T. brucei gambiense (West Africa) and T. brucei rhodesiense (East/Southern Africa). Life cycle involves maturation in the fly's salivary gland and direct injection into host; parasites remain extracellular.

Stages of African Sleeping Sickness

1. Lesion at bite site with dead tissue.

2. Parasitemia triggers fever, lymph node swelling, headaches.

3. Meningoencephalitis; death may occur within months if untreated.

Fungi

General Features

• Chemoheterotrophic: Obtain nutrients from organic sources.

• Do not perform photosynthesis.

• Cell walls typically composed of chitin.

Significance of Fungi

• Decompose dead organisms and recycle nutrients.

• Used in food production and manufacture of foods/beverages.

• Produce antibiotics and other drugs (e.g., penicillin).

• Can spoil food products.

Morphology of Fungi

• Molds: Composed of long filaments called hyphae.

• Yeasts: Small, globular, single-celled.

• Dimorphic fungi: Can switch between yeast-like and mold-like forms depending on environmental conditions.

Factors Predisposing to Opportunistic Mycoses

Disease in Depth: Candidiasis

Candidiasis is caused by Candida species, commonly Candida albicans. It can affect the mouth (oral thrush), skin, nails, and genital tract. Risk factors include immunosuppression, antibiotic use, and medical devices.

Helminths

General Features

• Macroscopic, multicellular, eukaryotic worms.

• Complex life cycles often involving intermediate hosts.

• Three main groups: Cestodes (tapeworms), Trematodes (flukes), Nematodes (roundworms).

Example: Ascaris lumbricoides

Ascaris lumbricoides is the most common nematode infection in humans, especially in the rural southeastern United States. Transmission occurs via ingestion of eggs in contaminated water or vegetables. Adult worms inhabit the small intestine; most infections are asymptomatic, but heavy worm burden can cause abdominal pain, nausea, vomiting, and intestinal obstruction.

Additional info: Helminth infections are diagnosed by identifying eggs or larvae in stool samples.