Protozoa Classification

• Protozoa are single-celled eukaryotic microorganisms
• They are diverse in morphology, mode of nutrition, and reproduction
• The term "protozoa" is traditional and not a formal taxonomic rank, as it includes polyphyletic groups

Classification of Protozoa

• Protozoa were traditionally classified based on their means of locomotion
• Older classifications divided them into four main groups: Flagellates (Mastigophora), Amoeboids (Sarcodina), Ciliates (Ciliophora), and Sporozoans (Apicomplexa)
• Modern classifications use molecular data (e.g., rRNA sequences) to reflect evolutionary relationships
• Current classifications recognize several eukaryotic supergroups, with protozoa distributed among them
• Key protozoan groups include Excavata, Amoebozoa, SAR (Stramenopiles, Alveolates, and Rhizaria), and Archaeplastida
• Excavata includes flagellates like Giardia and Trichomonas
• Amoebozoa includes amoeboid organisms like Amoeba and Entamoeba
• SAR includes ciliates (e.g., Paramecium), apicomplexans (e.g., Plasmodium), and radiolarians
• Apicomplexa are characterized by an apical complex used for host cell invasion, and all members are parasitic

Life Cycle Stages

• Protozoa exhibit various life cycle stages, often adapted to different environments or hosts
• A common life cycle includes a trophozoite stage and a cyst stage
• The trophozoite is the active, feeding, and reproducing stage
• The cyst is a dormant, resistant stage that can survive harsh conditions and facilitate transmission
• Some protozoa have simple life cycles, while others have complex life cycles involving multiple hosts
• Apicomplexans like Plasmodium (malaria parasite) have complex life cycles involving both mosquito and vertebrate hosts
• Some protozoa reproduce asexually through binary fission, budding, or schizogony (multiple fission)
• Sexual reproduction (conjugation or syngamy) occurs in some protozoa, increasing genetic diversity

Habitat and Distribution

• Protozoa are found in diverse aquatic and terrestrial habitats worldwide
• They inhabit freshwater, marine, and soil environments
• Many protozoa are free-living, feeding on bacteria, algae, or organic matter
• Some protozoa live in symbiotic relationships with other organisms
• Termites, for example, harbor symbiotic protozoa in their guts that aid in cellulose digestion
• Many protozoa are parasitic, living in or on host organisms
• Parasitic protozoa can infect a wide range of hosts, including humans, animals, and plants
• Protozoa can be found in extreme environments, such as hot springs and acidic waters

Role in Ecosystems

• Protozoa play important roles in various ecosystems
• They are a vital component of the microbial food web, serving as both predators and prey
• Protozoa consume bacteria and algae, helping to regulate microbial populations
• They are a food source for larger organisms, such as small invertebrates
• Protozoa contribute to nutrient cycling by decomposing organic matter
• Some protozoa are used in wastewater treatment to remove bacteria and organic pollutants
• The presence and diversity of protozoa can indicate water quality and environmental health
• Symbiotic protozoa, such as those in termite guts, are crucial for the host's survival

Pathogenic Protozoa

• Many protozoa are pathogenic, causing diseases in humans and animals
• These diseases range from mild to life-threatening
• Important pathogenic protozoa include Plasmodium, Entamoeba, Giardia, Trichomonas, Trypanosoma, and Leishmania
• Plasmodium causes malaria, a mosquito-borne disease characterized by fever, chills, and anemia
• Entamoeba histolytica causes amoebic dysentery, an intestinal infection leading to diarrhea and abdominal pain
• Giardia lamblia causes giardiasis, an intestinal infection causing diarrhea, cramps, and nausea
• Trichomonas vaginalis causes trichomoniasis, a sexually transmitted infection causing vaginitis in women and urethritis in men
• Trypanosoma brucei causes African trypanosomiasis (sleeping sickness), transmitted by tsetse flies and affecting the central nervous system
• Trypanosoma cruzi causes American trypanosomiasis (Chagas disease), transmitted by triatomine bugs and affecting the heart and digestive system
• Leishmania species cause leishmaniasis, transmitted by sandflies and causing skin lesions or visceral infection
• Cryptosporidium parvum causes cryptosporidiosis, an intestinal infection causing diarrhea, especially in immunocompromised individuals
• Toxoplasma gondii causes toxoplasmosis, a tissue cyst-forming infection that can be dangerous to pregnant women and immunocompromised individuals
• Protozoan diseases are often transmitted through contaminated food, water, or insect vectors
• Prevention and control measures include sanitation, hygiene, vector control, and vaccination (for some diseases)
• Treatment options include antimicrobial drugs, but drug resistance is an increasing concern