Eukaryotic Microbes: Protists

• Eukaryotic microbes have diverse life cycles, morphologies, and nutritional needs.
• Protists, parasitic worms, and fungi are eukaryotic microbes that cause disease

Clinical Focus Part 1

• Sarah, a 7-year-old, was diagnosed with ringworm after a red, itchy spot on her arm was examined with a Wood's lamp, which caused the spot to fluoresce.
• Ringworm presents as a raised ring, gray or brown on darker skin, and red on lighter skin.

Characteristics of Protists

• Protist is an informal term for diverse microscopic eukaryotic organisms without shared evolutionary origin.
• Historically, protists were grouped into "animal-like" protozoans, "plant-like" algae, and "fungus-like" protists.
• Plankton are microorganisms that drift in water, including motile, nonphotosynthetic zooplankton and photosynthetic phytoplankton.
• Protozoans inhabit aquatic and terrestrial habitats; some are free-living, parasitic, or beneficial symbionts.
• Trophozoites are the feeding and growth form of protozoa and feed on small particulate food sources.
• Some protozoa can develop into an encapsulated cyst stage under harsh conditions and revert to trophozoites when conditions are favorable.
• Encystment is the process by which a trophozoite becomes a cyst.
• Excystment is when cysts are triggered by environmental cues to become active again.
• Eimeria is a protozoan genus capable of encystment and includes human and animal pathogens and oocysts are shed in feces.
• Protozoans reproduce asexually (binary fission, budding, or schizogony) or sexually.
• Schizogony involves multiple nuclear divisions before the cell divides into smaller cells called merozoites, stored in schizonts.
• Sexual reproduction in protozoans includes syngamy (fusion of haploid gametes) or conjugation (exchange of DNA).
• Protist conjugation is a form of eukaryotic sexual reproduction between cells of different mating types found in ciliates
• The plasma membrane (plasmalemma) is a feature of all protozoans.
• Pellicle is a rigid protein band inside the membrane of some protozoans.
• Ectoplasm is the outer gel layer of cytoplasm, whereas endoplasm is the inner fluid region of cytoplasm.
• A cytostome is a specialized structure for phagocytosis and a cytoproct is used for exocytosis of wastes.
• Oral grooves lined with cilia sweep food particles into cytostomes.
• Protozoans are heterotrophic, ingesting whole food particles (holozoic) or soluble food molecules (saprozoic).
• Flagella or cilia made of microtubules are used for locomotion; cytoplasmic extensions (pseudopodia) are also used for movement.
• Contractile vacuoles are organelles used for osmotic regulation.
• Mitochondria may be absent, altered to kinetoplastids, or hydrogenosomes

Taxonomy of Protists

• Protists are a polyphyletic group scattered across different taxonomic groups within Eukarya.
• Eukarya is divided into six supergroups: Amoebozoa, Excavata, Chromalveolata, Opisthokonta, Rhizaria, and Archaeplastida.
• Amoebozoa, Excavata, and Chromalveolata contain many protozoans of clinical significance.

Amoebozoa

• Amoebozoa includes protozoans that use amoeboid movement via pseudopodia formed by actin microfilaments.
• Entamoeba includes commensal or parasitic species, such as E. histolytica, which causes amoebic dysentery.
• Acanthamoeba can cause keratitis and blindness.
• Naegleria fowleri is a "brain eating amoeba" classified in the phylum Percolozoa.
• Eumycetozoa (slime molds) include cellular and plasmodial slime molds.
  • Cellular slime molds exist as individual amoeboid cells that aggregate into a mobile slug, forming a fruiting body with haploid spores.
  • Plasmodial slime molds exist as large, multinucleate amoeboid cells that form reproductive stalks to produce spores that divide into gametes.
• Dictyostelium discoideum is a cellular slime mold used to study cell differentiation, with single-celled and multicelled life stages.

Chromalveolata

• Chromalveolata is a supergroup including apicomplexans, ciliates, diatoms, and dinoflagellates, united by similar origins of plastids.
• Apicomplexans are intra- or extracellular parasites with an apical complex for entering host cells.
  • They have complex life cycles with an infective sporozoite that undergoes schizogony to produce merozoites
  • The genus Plasmodium is an example.
• Cryptosporidium parvum causes intestinal symptoms and epidemic diarrhea when cysts contaminate drinking water.
• Theileria (Babesia) microti, transmitted by Ixodes scapularis, causes recurring fever and is a common transfusion-transmitted pathogen.
• Toxoplasma gondii causes toxoplasmosis, transmitted from cat feces, unwashed produce, or undercooked meat, and can cause birth defects.
• Ciliates (Ciliaphora) are a diverse group with cilia on their cell surface for locomotion or feeding.
  • Balantidium coli is a parasitic ciliate that affects humans, causing intestinal illness.
  • Paramecium is a motile ciliate with a visible cytostome and cytoproct.
  • Stentor is a sessile ciliate that uses cilia for feeding.
• Ciliates have a diploid, somatic micronucleus (for sexual reproduction) and a polyploid macronucleus derived from the micronucleus (for metabolic genes).
• Ciliates reproduce through conjugation, exchanging micronuclei to form a genetically different diploid micronucleus.
• Öomycetes (water molds) have similarities to fungi but have cellulose cell walls and are generally diploid.
  • Phytophthora is a plant pathogen that caused the Irish potato famine.

Excavata

• Excavata includes primitive eukaryotes and parasites with limited metabolic abilities.
• These organisms have complex cell shapes, often including a depression on the cell surface called an excavate
• Fornicata lack mitochondria but have flagella, including Giardia lamblia, which causes diarrheal illness.
• Parabasalia are animal endosymbionts with basal bodies, modified mitochondria (kinetoplastids), undulating membranes, and many flagella.
  • Trichomonads include Trichomonas vaginalis, which causes the sexually transmitted disease trichomoniasis.
• Euglenozoa include photosynthetic and nonphotosynthetic species with two flagella, a pellicle, a stigma (eyespot), and chloroplasts.
  • Euglena is typically not pathogenic.
• Trypanosomes (Trypanosoma) are parasitic pathogens.
  • T. brucei causes African trypanosomiasis (African sleeping sickness), transmitted by the tsetse fly (Glossina spp.).
  • T. cruzi causes American trypanosomiasis (Chagas disease), transmitted by Triatoma spp. ("kissing bugs").
• Leishmania includes trypanosomes that cause disfiguring skin disease and sometimes systemic illness

Eye on Ethics: Neglected Parasites

• The CDC has identified five neglected parasitic infections (NPIs): toxoplasmosis, Chagas disease, toxocariasis, cysticercosis, and trichomoniasis.
• These NPIs are prioritized due to the number of people infected, the severity of the illness, and whether the illness can be treated or prevented.