Protozoa Flashcards
Outline what protozoan parasites are and state the four classified groups defined by their features and give an example of each.
- Amoeba(N. Fowleri); movement via pseudopods.
- Flagellates(Giardia); movement via flagella.
- Ciliates(Balantidiasis); movement via cilia.
- Sporozoa(P. Falciparum - Malaria); has a complex life cycle with an actin myosin motor for movement within a host.
Briefly draw/outline typical protozoan lifecycle.
There are 2 keys stage of any protozoan life cycle:
- Trophozoites; these are the active cells that metabolize and multiple via binary vision like bacteria(some might have complex life cycle that include re-production sexually).
- Cysts; if cells are found in an adverse condition/environment they will form cysts which are typically dormant cells with a thick cell wall for protection.
Trophozoites typically inhabit the small intestine but if they get passed to large intestines they form cysts that if excreted can last in the environment for months. If re-ingested by another organism the cells will activate again via excystation.
There are 2 different protozoan life cycles, most have the direct life cycle.
Direct; find a host, multiple (eggs/cysts), pass on to infect other hosts. (E.g. giardia and N. Fowleri)
Indirect; here 2 different species are required to complete the life cycle. (e.g. Plasmodium usually sexually reproduce in a non-human definitive host, and asexual reproduction occurs in the human intermediate host)
Refer to diagrams in notion
General notes on P. Falciparum.
Group of obligate parasites, is defined asa parasitic organism that cannot complete its life-cycle without exploiting a suitable host.
- Plasmodium spp. (Malaria)
- Transmitted by Anopholes mosquitoes. Causes the highest number of deaths.
What disease does P. Fal cause? Describe the LIFECYCLE of P. Fal. … including merozoites.
Disease causes: Malaria
The malaria parasite life cycle involves two hosts.
- During a blood meal, a malaria-infected femaleAnophelesmosquito inoculates sporozoites into the human host.
- Sporozoites infect liver cells
- and mature into schizonts,
- which rupture and release merozoites. (Of note, inP. vivaxandP. ovalea dormant stage [hypnozoites] can persist in the liver (if untreated) and cause relapses by invading the bloodstream weeks, or even years later.)
- After this initial replication in the liver (exo-erythrocytic schizogony), the parasites undergo asexual multiplication in the erythrocytes (erythrocytic schizogony).
- Merozoites infect red blood cells.
- The ring stage trophozoites mature into schizonts, which rupture releasing merozoites. Some parasites differentiate into sexual erythrocytic stages (gametocytes).
- Blood stage parasites are responsible for the clinical manifestations of the disease. The gametocytes, male (microgametocytes) and female (macrogametocytes), are ingested by anAnophelesmosquito during a blood meal.
- The parasites’ multiplication in the mosquito is known as the sporogonic cycle. While in the mosquito’s stomach, the microgametes penetrate the macrogametes generating zygotes.
- The zygotes in turn become motile and elongated (ookinetes)
- which invade the midgut wall of the mosquito where they develop into oocysts.
- The oocysts grow, rupture, and release sporozoites,
- (cycle renews)which make their way to the mosquito’s salivary glands. Inoculation of the sporozoitesinto a new human host perpetuates the malaria life cycle.
(DRAW FROM NOTION)
Erythrocyte Invasion by Merozoites(Miler et al 2019)
Merozoites on their own aren’t motile, however, they do practice a certain mobility when bound to an RBC. This process usually takes 5-10 minutes upon first contact with the RBC to be fully in its cytoplasm. This process is also RECEPTOR MEDIATED(Look later).
Merozoites are a type of polarized cell in which the tip needs to be in direct contact with the membrane, however, initial attachment like this is unlikely. So merozoites engage in a type of movement similar to the myosin-actin bridge in human muscles to orient themselves in the appropriate manner. They will form tight junctions and endocytose in a vacuole in the RBC cytoplasm. From there they’ll import glucose and breakdown myoglobin to be used as a protein source. IMPORTANT NOTE: the breakdown of glucose here is anaerobic meaning lactic acid is produced resulting in the acidification of blood (acidosis which can cause death).
Motility:
- An interaction between the host and parasites occurs, the parasite attaches to the cell via specific adhesion proteins:
- For sporozoites, it’s the TRAP,
- Merozoite TRAP or Plasmodium thrombospondin-related apical merozoite protein.
- *Not fully sure what proteins are actually involved here as per the primary literature however there is potential for novel therapies to target these proteins so that the parasite can never eve attach to these cells.
- Myosin anchored in the Inner Membrane Complex attached to microtubules
- Force drive parasite along cell surface.
Explain P. Fal asexual blood stages
There is no sexual process that occurs within the human host, this only occurs in the mosquito. There is a complex mechanism by which the RBC is remodelled by P. fal infections, and this is actually one of its key points of pathogenicity.
- Initial merozoites invasion of RBCs.
- hours after we get Maurer’s clefts forming on the inner surface of the RBC membrane.
- more proteins forming within the cell, including knobs with PfEMP1 adhesion proteins (look later).
- 36-40 hrs visibly see new merozoites form before eventual lysis of RBC.
Explain P. Fal host remodelling and PfEMP1. (NOTION DRAW)
The pathogenicity of P. Falciparum involves RBC Membrane re-modeling(this is the primary reason people die from this pathogen).
Parasite secretes a number of proteins into RBC cytosol:
- PfTRiC; Parasite Group II Chaperonin
- PfEMP; P. falciparum erythrocyte membrane protein(see later)
- MC; Maurer Cleft
Host Cell Remodeling (Cytoadherence)
- To avoid clearance by the spleen:
- PfEMP is an issue because its expression leads to an affinity for uninfected cells → Infected RBCs bind with endothelial cells(sequestration).
- Platelet-mediated clumping of infected erythrocytes.
- Rosetting of infected and non-infected RBCs. A major issue arising is thrombosis(clots) in various tissues (An example is cerebral malaria where you get lost in the brain - this protein has variants which may be expressive for endothelial receptors within the brain(see later).
PfEMP1
- P. falciparum erythrocyte membrane protein (PfEMP1) expression on the surface of infected red blood cell. It’s associated with severe malaria (SM) and is considered a key virulence factor in malaria, as it binds to various host receptors on the endothelium to sequester infected erythrocytes from circulation and destruction in the spleen. PfEMP1 is a target of antibody-mediated immunity, and in response, PfEMP1 molecules have diversified extensively(Shabani, 2017).
- 60+ var genes encoding different variants of PfEMP1 proteins.
- One variant per infected RBC is expressed.
- Cerebral malaria associated with high expression of variants that bind endothelial protein C receptor (EPCR) and intercellular adhesion molecule 1 (ICAM-1 aka CD54), found on brain microvascular endothelial cells.
Outline the plasmodium metabolism.
Another major cause of death is acidosis(the acidification of the blood). The Plasmodiu metabolism is relatively simple, most of the energy they provide for thesleves is through scavedning glucose from cytoplasm of RBC, they form ‘food vacules’ by endocytosing some of the cytoplasm of the RBC take into vacoule where the glcuse brakdons (this breakdown is primary through glycolysis). Note that oxygen availability in thsi circumstance isn’t ideal which creates build up of lactiv acid. glycolysis can keep going as long as there’s a steady supply of NAD+ to form NADH(from thsi point if there is no oxidation pathway to follow there a build up of NADH and no NAD+ thus resulting in lactic acid build up). eventually when the RBCs lyse, all of that acid is released into the bloodstream.
DISCUSS the diagnosis, treatment, and prevention of Malaria.
Diagnosis:
-Blood Smear and/or Giemsa Stain is the gold standard.
- Rapid diagnostics tests detect Plasmodium antigens:
- P. falciparum histidine-rich protein 2(PfHRP2).
- Parasites lactate dehydrogenase(Pf-pLDH). (Makes sense - think about acidosis)
*Due to AMR these rapid diagnostic strips may be becoming less effective due to ongoing mutations of these organisms.
Treatment (Tse et al., 2019)
- Quinine
- inhibits hemozoin formation(parasite breakdown Hb as a protein source but cant use heme in anyway - iron is very reactive in the form and is toxic however they avoid this typically by turning it into hamzoin but with this drug this formation is inhibited and thus will be toxic to the cell/parasite) - note that malaria is developing resistance to this for an unknown reason.
- Free heme and hemin poison parasite through oxidative stress.
- inhibits hemozoin formation(parasite breakdown Hb as a protein source but cant use heme in anyway - iron is very reactive in the form and is toxic however they avoid this typically by turning it into hamzoin but with this drug this formation is inhibited and thus will be toxic to the cell/parasite) - note that malaria is developing resistance to this for an unknown reason.
- Artemisinin
- specifically for P. fal - mode of action unsure.
- Curenlt the mode of action is uncertain, however its been hypotheztied that the endoperoxide ring of artmisinin → the double oxyen group within the ring gets cleaved and produces free radicals.
- Kills merozoites and gametocytes, reduced transmission, increasing resistance
-
WHO recommends that therapy is delivered as a concoction of drugs namely:
- Artemisinin + longer lasting drug
- Artemether/mefloquine or Artemether/lumefantrine(more resistant strains)
-
WHO recommends that therapy is delivered as a concoction of drugs namely:
- specifically for P. fal - mode of action unsure.
*Resistance: From 2008-Present there’s an increasing number of clinical cases showing resistance. Two mechanisms are thought to be involved: (1) Efflux of drug away from target? (2) Change in parasite target?
Vaccine
RTS,S/AS01 - Mosquirix
- Recombinant protein-based vaccine which produced antibodies against P. falciparum sporozoites(but no other species). The highest efficacy was found only 86.7% in adults and 50% or less in minors.
Generally define N. Fowleri and what disease it causes.
Naegleria fowleri, colloquially known as the “brain-eating amoeba”, is technically classified as a shape-shifting ameboflagellate excavate, rather than a true amoeba.
- Causes meningoencephalitis - Rare (34 cases in USA 2010-2019 (CDC 2019))
- Transmission ONLY via inhalation through nasal passage.
- Trophozoites migrate to the brain via olfactory nerve tissue, travelling to the brain through the cribriform plate thus consuming neurons.
- Proteases, phospholipases, pore-forming proteins
Explain/draw N. Fowleri. Lifecycle.
Outline the symptoms, diagnosis, and treatment of N. Fowleri.
- Lives in fresh warm water.
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Symptoms
- Headaches, delirium, seizures
- Increases in intracranial pressure and cerebral spinal fluid
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Diagnosis
- Presence of trophozoites in the CSF (using trichrome or Giemsa stain)
- Neutrophils in high number
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Treatments
- No clinical trails
- Amphotericin B – forms pores in the cell membrane
- Other anti-fungal agents
Even with treatment, 97% of patients die.
Generally define B. Coli and what disease it causes.
Balantidium coli is a parasitic species of ciliate alveolates that causes the disease balantidiasis.
- Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine(invade intestinal mucosa).
Outline the symptoms, diagnosis, and treatment of B. Coli
- Diagnosis; examination of stool for cysts ().
- Treatment; Metronidazole(crosses the cell membranes of anaerobic and aerobic pathogens but is only antimicrobial in anaerobes/microaerophilic organisms. Reductive activation – leads to the formation of free radicals -> Free radicals cause DNA breakage).
Generally define G. Intestinalis and what disease it causes.
Giardia intestinalis and Giardia lamblia, is a flagellated parasitic microorganism that colonizes the small intestine, causing a diarrheal condition known as giardiasis.
- Most prevalent intestinal protozoan parasite in humans.
- 3,000-4,000 cases per year UK
- ~1 billion cases in developing countries
- Severe impact on child physical development
- Cysts are activated from stomach acid and bile, and trophozoites emerge and attach to the small intestine epithelium.
Outline the VF of G. Intestinalis.
Attachment; the ventral adhesive disc and surface lectins enable attachment to and colonization of the intestinal endothelium.
Alteration of host innate defenses; released arginine demitasse and other Giardia spp. products downregulate epithelial production of nitric oxide
Antigenic variation; VSP on the trophozoite surface switches to avoid IgA-directed clearance
