Protozoa, Helminths & Related Microbiology Vocabulary

Whale Sharks (Opening Context)

• Mentioned as an introductory example: whale sharks are herbivores, eating only plankton, algae, etc.

• Serves to contrast with parasitic organisms that feed on living tissues.

Protozoa — General Characteristics

• Single-celled, eukaryotic parasites.

• Possess all major eukaryotic organelles except chloroplasts (some species may rarely contain chloroplasts).

• Size: microscopic; transparent membrane allows direct observation of internal activity.

• Habitat breadth:

  • Freshwater, marine water, soil, plants, animals, feces.

  • Fecal-oral transmission = dominant route of human infection.

• Most species are harmless; a minority are pathogenic, causing millions of human infections annually.

Specialized Structures & Organelles

• Highly specialized organelles can mimic multicellular systems:

  • "Mouth" parts / cytostomes for suction and ingestion.

  • Visible digestive tract (food visibly enters, is digested, then expelled).

  • Reproductive tracts that include sex pili for DNA exchange.

• Locomotor & attachment structures:

  • Pseudopods (“false feet”) – also function in feeding.

  • Flagella – whip-like motility apparatus.

  • Cilia – rhythmic movement, pattern maintenance, adhesion.

  • Suction-cup-like pads used to cling to host tissues.

Metabolism & Feeding Strategy

• Heterotrophic: must obtain complex organic nutrients.

• Free-living forms are saprophytes (a.k.a. saprobes) — scavenge dead plants & animals.

• Parasitic species exploit:

  • Fluids (plasma, digestive juice, cerebrospinal fluid).

  • Host tissues → chronic bleeding, anemia, weight loss, abdominal pain.

Life-Cycle Forms

• Trophozoite

  • Active, motile, feeding stage.

  • Requires abundant food & moisture.

• Cyst

  • Dormant, non-feeding, stress-resistant stage.

  • Initiates disease spread; survives gastric acidity.

• Typical human cycle (fecal-oral model):

  1. Ingest cyst on raw/contaminated food (e.g., salad handled with unwashed hands).

  2. Cyst survives stomach; reaches small intestine.

  3. In large intestine → excysts → trophozoite.

  4. Trophozoite feeds, multiplies, erodes mucosa → dysentery.

  5. If untreated: wall perforation → peritonitis OR systemic dissemination (brain, liver, bloodstream).

Reproduction & Gene Exchange

• Asexual: binary fission/mitosis inside trophozoite.

• Sexual: occasionally via conjugation-like processes.

• Trophozoites re-encyst → cysts excreted → new hosts.

• Intermediate hosts (snails, fish, arthropods) often carry cysts/larvae; humans infected via ingestion, bites, or entry through wounds (e.g., wading with skin cuts).

Environmental & Practical Notes

• Many parasites are killed at 60^\circ - 70^\circ\text{C} → ensure thorough cooking.

• Outdoor/wet environments: organisms may enter wounds or natural orifices (e.g., Naegleria swimming up nasal passages).

• Water-safety devices (e.g., LifeStraw) filter up to \sim 1000 gallons; useful in endemic zones.

Classification of Protozoa (Motility Based)

• Amoeboids → move with pseudopods.

• Flagellates → move via flagella.

• Ciliates → move via cilia.

• Apicomplexans (non-motile in adult form) → glide or rely on vectors (e.g., Plasmodium, Cryptosporidium).

Clinically Important Protozoa

• Entamoeba histolytica (Amoebiasis)

  • Intestinal dysentery: watery/bloody/mucoid diarrhea, fever.

  • Extra-intestinal: amoebic liver abscess (“anchovy-paste” aspirate).

  • Treatment: Metronidazole 750\,\text{mg} PO q8h × 7 days (kills trophozoites) + Paromomycin (kills cysts).

• Naegleria fowleri

  • Free-living thermophilic amoeba in warm lakes.

  • Entry: swims up nose → brain → Primary Amoebic Meningoencephalitis.

  • Mortality ≈ 100\%; no effective cure.

• Acanthamoeba spp.

  • Similar brain/eye infections; often contact-lens associated.

• Giardia lamblia

  • Giardiasis: malodorous flatulence, fatty diarrhea; milder than E. histolytica.

• Trichomonas vaginalis

  • Sexually transmitted trichomoniasis; vaginal irritation, altered pH; not identical to bacterial vaginosis.

• Trypanosoma brucei (African Sleeping Sickness) & Trypanosoma cruzi (Chagas Disease)

  • Vector borne (tsetse fly / triatomine bug).

  • Features: hypersomnia (sleeping >20 h/day), cardiomyopathy (LV hypertrophy). Advanced cases may require heart transplant.

• Leishmania donovani

  • Cutaneous lesions resembling leprosy; may disseminate to visceral organs.

  • Vector: sandfly (lecture referenced snails).

• Plasmodium spp. (Malaria)

  • Species: P. falciparum, P. vivax, P. ovale (treatment distinctions not exam-critical).

  • Vector: Anopheles mosquito.

  • Sickle-cell trait confers protection; parasite cannot invade sickle-shaped RBCs.

• Toxoplasma gondii

  • Source: cat feces (domestic & feral); under-cooked meat.

  • Flu-like → brain cysts visible on CT/MRI.

  • Risk groups: pregnant women (congenital toxoplasmosis), AIDS.

• Cryptosporidium parvum / Cyclospora cayetanensis

  • Non-motile, intestinal protozoa; watery diarrhea; often self-limiting in immunocompetent hosts.

Helminths (Parasitic Worms)

• Macroscopic (visible), may reach 25 m.

• Body plan category by segmentation:

  • Flatworms (Platyhelminthes)

  • Cestodes (tapeworms): segmented; generally treatable.

  • Trematodes (flukes): unsegmented; greater morbidity (e.g., bladder & liver flukes → cancers).

  • Roundworms (Nematodes): cylindrical, unsegmented.

Life Stages

1. Embryo (fertilized egg).

2. Larva.

3. Adult.

• Many are hermaphroditic.

Representative Helminths & Clinical Points

• Schistosome / bladder fluke (e.g., Schistosoma haematobium)

  • Water-borne; larvae can swim up urethra → bladder wall → chronic inflammation, cancer.

• Trichinella spiralis

  • Encysts in striated muscle; acquired via under-cooked pork/wild game.

• Ascaris lumbricoides

  • Large intestinal roundworm; visible; anecdote: worm extracted from child’s nose.

• Taenia solium (pork tapeworm)

  • Causes intestinal obstruction and cysticercosis (brain & muscle cysts resembling “spaghetti”).

• Clonorchis sinensis (liver fluke, lectured as “norcus sunensis”)

  • Biliary tract obstruction; cholangiocarcinoma risk.

• Schistosoma japonicum / mansoni / haematobium (blood flukes)

  • Systemic eosinophilia; travel history key for diagnosis.

Diagnostic Clues & Laboratory Findings

• Stool O&P (ova & parasites) with special stains → visualize trophozoites/cysts.

• Occult blood (guaiac) for micro-bleeding.

• CBC findings: marked eosinophilia + elevated monocytes → helminthic suspicion.

• Imaging (CT, MRI) for cysts/abscesses (liver, brain).

Prevention & Control Summary

• Cook food above 60^\circ\text{C}; avoid raw salads in endemic areas.

• Use boiled/filtered water; LifeStraw filters \ge\,1000 gallons.

• Personal hygiene: hand washing, proper food handling.

• Avoid swimming / nasal exposure in warm stagnant waters.

• Pregnant women: avoid cat-litter exposure.

Parasitology vs Other Microbial Fields

• Parasitology = study of protozoa & helminths.

• Classification

  • Protozoa → by locomotion.

  • Helminths → by body segmentation.

Glycocalyx & Cell Biology Review

• Glycocalyx of microbes composed of polysaccharides.

• Fungal infection sites: respiratory tract, skin, nails, eyes, brain (systemic potential).

Microbial Genetics Essentials

• Chromosomal DNA: essential genes (~4{,}000 in E. coli).

• Plasmids: small, circular, non-chromosomal DNA; impart virulence & antibiotic resistance.

• Genotype: gene set; phenotype: expressed traits.

Lac Operon (E. coli Example)

• Inducible operon regulating lactose metabolism.

• Components:

  • Promoter.

  • Operator.

  • Repressor protein binds operator when lactose absent → transcription OFF.

  • Presence of lactose in medium removes repressor → genes transcribed → lactose-digesting enzymes synthesized.

Bacterial Conjugation

• “Bacterial sex”: direct DNA (plasmid) transfer donor → recipient via sex pilus.

• Mechanism of rapid spread of resistance genes.

Key Vocabulary Quick-Hits

• Saprophyte (Saprobes): organism feeding on dead organic matter.

• Pseudopod: temporary cytoplasmic projection for movement/feeding.

• Dysentery: painful, bloody diarrhea.

• Peritonitis: inflammation of peritoneum; potential after intestinal perforation.

• Hypertrophic cardiomyopathy: enlarged left ventricle (Trypanosoma cruzi sequel).

• Anchovy-paste abscess: classic description of amoebic liver abscess aspirate.

• Hermaphroditic: organism possessing both male & female reproductive organs.