Protists Flashcards
What is encystment and what are its purposes?
Encystment is the formation of a cyst around parasitic protists
They protect the organism from adverse environmental conditions
provide shelter for reproductive processes
allow transmission of the parasite from one host to another
Define the term protist?
Protists are eukaryotic organisms that are either single-celled or have only simple multicellular structures.
Huge genetic diversity
Found in fresh water, salt water, soil and as symbionts in plants, animals and fungi
Feeding strategies vary. Some can photosynthesise, some are heterotrophs and feed of other organisms
Can reproduce via sexual and asexual means depending on the environmental conditions
What is excystment?
The emergence of a vegetive parasitic protist from a cyst. This protist is a trophozoite
The ingestion of a cyst is a common way for humans to become infected. Excystment occurs in the gut
What structure is common between the majority of protists?
They have one of more flagella at some point within their life cycle which are used for movement and feeding
They are structurally different to bacterial flagella
What is a flagellate?
A protist which has a flagella during the adult stage of their life cycle.
What are Protozoa?
Single called animal like cells
Unable to photosynthesise
Contain one or more nuclei
Why is antiprotist therapy often unsuccessful?
Protists are eukaryotes, so have the same type of molecules and metabolism as their hosts. Thus it is difficult to achieve selective toxicity.
Two treatments which are successful are folic acid antagonists as the protists are unable to use the hosts folic acid where as the host can obtain what they require from their diet
Melarsopol is a drug based on arsenic which inhibits protist enzyme activity
What is Giardiasis?
Diarrhoeal disease causes by the protist Giardia spp - usually Giardia lamblia in humans
Describe the Giardia protist and it’s life cycle?
Very ancient protist.
Has no mitochondria - has a mitosome instead which is evolved from mitochondria
Cysts are ingested in contaminated food or water.
Excystment is initiated in the stomach by the high pH but does not occur until the lower pH of the small intestine
The protist has four flagella which is uses to move to the gut epithelium where is attaches via sucker pads and specific receptors.
It prevents fat absorption and cause loose fatty stools
If the protist is moved through the digestive system the decrease in pH triggers encystment where a thick mucous layer forms around the cyst and binary fission occurs to form four nuclei. The cyst allows safe transmission to a new host when excreted.
Cysts can survive in the soil etc for months
When infection occurs metronidazole can be administered but the infection can clear by itself
What is Trichomoniasis?
Sexually transmitted disease caused by the protist Trichomonas vaginalis.
It has four flagella and no mitochondria, instead it has a hydrogenosome
No cyst stage - transmitted in trophozoite stage
Attach to the vaginal mucosa via adhesins
Cause irritation and foul discharge
Diagnosis via fluorescent antibodies in urine or vaginal swab and PCR procedure analysing the urine or vaginal swab
What is Trypanosomiasis?
Disease caused by the protist trypanosomes
Causes sleeping sickness or human African trypanosomiasis (HAT) in Africa and Chagas’ disease in Central and South America
Has a complex life cycle which involves more than one type of host and an insect vector
Describe the four different morphological forms of Trypanosomes?
Typically, the trypomastigote form occurs extracellularly in both the denitive host and the vector (e.g. in humans infected with Trypanosoma
spp.).
The amastigote is an intracellular form occurring in the denitive host
The promastigote and epimastigote are forms found in the insect vectors (Figure 6.2a).
The position of the kinetoplast relative to the nucleus and the origin of the agellum differ in each of the four morphological types.
How is trypanosomiasis transmitted?
It is transmitted in its trypomastigote form to the host via the bite of a tsetse fly
Once in the blood stream, what two forms do the trypomastigote take?
trypomastigotes develop into two forms:
a long slender form that multiplies by binary fission
a stumpy form that is non-dividing but is the only form that is infective for a biting tsetse fly.
What are the differences between the two strains of Trypanosoma species?
Trypanosoma brucei gambiense Chronic disease Kills in three years Attacks the CNS and causes lethargy Transmitted by the tsetse vector
Trypanosoma brucei rhodesinese
Acute
Kills within a year
Inflammation of lymph nodes and kills small blood vessels supplying the brain and heart
Transmitted by the tsetse fly for a reservoir of wild and domesticated livestock.
How are African trypanosomes able to produce prolonged infections in vertebrate hosts?
The prime means of producing prolonged infections relates to the trypanosomes ability to vary a major surface antigen (the variant surface
antigen, VSG) within a single infection, thus progressively evading the immune response of the host.
How can the African trypanosome achieve this compared with the situation in the Influenza virus?
The African trypanosomes have an extensive repertoire of genes (perhaps up to 1700) for these VSGs in their genome, of which, in any individual
trypanosome, just a single gene is expressed at any one time, whereas the influenza virus depends on mutation and also recombination between the
genome of different viruses.
