Protozoa

Protozoans
 Protozoans are unicellular eukaryotes.
 Protozoa lack a cell wall
 motile stage
 most ingest their food.

Protozoans
 Eukaryotic cell was developed through endosymbiosis.
 In distant past aerobic bacteria appear to have been
engulfed by anaerobic bacteria, but not digested.
Ultimately, the two developed a symbiotic relationship
with the engulfed aerobic bacteria becoming
mitochondria and eukaryotic cells developed.
 In a similar fashion, ancestors of chloroplasts formed
symbiotic union with other prokaryotes.

 So if mitochondria are within our cells and
they were not technically part of our cells,
so how are they copied from generation to
generation??
 How do we inherit mitochondria?

Protozoans
 Protozoans include both autotrophs and
heterotrophs. They include free-living and
parasitic forms.
 Reproduction can be asexual by fission or
budding or sexual by conjugation or
syngamy (fusion of gametes).

Types of Protozoa:
Flagellates (e.g., Giardia lamblia)
Amoeboids (e.g., Entamoeba histolytica)
Ciliates (e.g., Balantidium coli)
Sporozoans (e.g., Plasmodium knowlesi)
The Sporozoa are parasitic protozoans that lack
locomotor organs. They have no cilia, no flagella, no
pseudopods.

Movement in Protozoa
 Protozoa move mainly using cilia or
flagella and by using pseudopodia

Cilia and flagella
 No real morphological distinction between
the two structures, but cilia are usually
shorter and more abundant and flagella
fewer and longer.
 Each flagellum or cilium is composed of 9
pairs of longitudinal microtubules arranged
in a circle around a central pair.

Cilia and flagella
 The collection of tubules is referred to as
the axoneme and it is covered with a
membrane continuous with the rest of the
organism’s cell membrane.
 Axoneme anchors where it inserts into the
main body of the cell with a basal body.

Figure 11.09a
Protein spoke
Dynein motor
Basal body

Movement in Protozoa:
Pseudopodia
 Pseudopodia are chief means of
locomotion of amoebas but are also
formed by other protozoa and amoeboid
cells of many invertebrates.
 In amoeboid movement the organism
extends a pseudopodium in the direction it
wishes to travel and then flows into it.

Pseudopodia
 Amoeboid movement involves endoplasm and
ectoplasm. Endoplasm is more fluid than
ectoplasm which is gel-like.
 When a pseudopodium forms, an extension of
ectoplasm (the hyaline cap) appears and
endoplasm flows into it and fountains to the
periphery where it becomes ectoplasm. Thus, a
tube of ectoplasm forms that the endoplasm
flows through. The pseudopodium anchors to
the substrate and the organism moves forward.

Feeding in amebas
 Feeding in amoebas involves using
pseudopodia to surround and engulf a
particle in the process of phagocytosis.
 The particle is surrounded and a food
vacuole forms into which digestive
enzymes are poured and the digested
remains are absorbed across the cell
membrane.

Pseudopodia
- temporary cell extension used for movement and
gathering food
Types:
1. Lobopodia: broad extensions used for
locomotion and engulfing food
2. Filopodia: provide constant two-way streaming
that delivers food in a conveyor belt fashion
3. Reticulopodia: similar to filopodia, except they
extend out and form net-like series of extensions
4. Axopodia: thin, filamentous and supported by a
central axis of microtubules

Reproduction in protozoa
 The commonest form of reproduction is
binary fission in which two essentially
identical individuals result.
 In some ciliates budding occurs in which
a smaller progeny cell is budded off which
later grows to adult size.

Asexual
1. Fission
- characterized by division of the body into two or more parts, each of
which develops into a complete individual.
2. Budding
- self supportive cell sprouts from & detaches from the parent
organism.
3. Schizogony
- a multiple fission type; the nucleus undergoes multiple divisions
before the cell divides.
Sexual
1. Conjugation
- the simplest form of reproduction. Two single-celled organisms link
together, exchange genetic information, & then separate.
- Reproduction -

Binary fission
in various taxa

The life stages of protozoa are:-
– latent survival form.
- The active motile stage of
protozoan parasite or active pathogenic
vegetative state.

Encystment
- Some protozoa are
coated with a protective
capsule called cyst.

- Cyst is a reproductive structure in which new cells are
produced asexually.
- The breaking of cyst is called excystation.

Subphylum Mastigophora:
locomotion by one or more flagella

Subphylum Sarcodina: locomotion and
food gathered by pseudopodia (false foot)
- includes the Amoeba

Diseases caused by protozoa
 Many diseases are caused by protozaon
parasites
 These include:
 Malaria (caused by a sporozaon)
 Giardia, sleeping sickness (caused by
flagellates)
 Amoebic dysentry (caused by amoebae)

Disease caused by protozoa to
human beings
Disease Casual agents Organs affected vector
1. Amoebiasis Entamoeba
histolytica
Intestine, Liver Water
2.African Sleeping
Sickness
Trypanosoma brucei Blood, Brain Tsetse fly (
Glossina)
3.Leishmaniasis (
Kala - azar)
Leishmania
donovani
White Blood Cells,
Skin, Intestine
Sand fly (
Phlemotomus)
4.Malaria Plasmodium spp. Liver, Red Blood
Cells
Mosquito (
Anopheles)
5. Babesiosis Babesia microti Red Blood Cells Tick (Txode)

Assignment 2
 Significance of Protozoa

Malaria
 Malaria is one of the most important diseases in the
world.
 About 500 million cases and an estimated 700,000 to
2.7 million deaths occur worldwide each year (CDC).
 Malaria was well known to the Ancient Greeks and
Romans. The Romans thought the disease was caused
by bad air (in Latin mal-aria) from swamps, which they
drained to prevent the disease.

Malaria symptoms
 The severity of an infection may range from
asymptomatic (no apparent sign of illness) to the
classic symptoms of malaria (fever, chills,
sweating, headaches, muscle pains), to severe
complications (cerebral malaria, anemia, kidney
failure) that can result in death.
 Factors such as the species of Plasmodium and
the victims genetic background and acquired
immunity affect the severity of symptoms.

Malaria
 Despite humans long history with malaria
its cause, a sporozoan parasite called
Plasmodium, was not discovered until
1889 when Charles Louis Alphonse
Laveran a French army physician
identified it, a discovery for which he won
the Nobel Prize in 1907.

Malaria
 In 1897 an equally important discovery,
the mode of transmission of malaria, was
made by Ronald Ross.
 His identification of the Anopheles
mosquito as the transmitting agent earned
him the 1902 Nobel Prize and a
knighthood in 1911.

Plasmodium
 There are four species of Plasmodium: P.
falciparum, P. vivax, P.ovale and P.
malariae.
 P. falciparum causes severe often fatal
malaria and is responsible for most
deaths, with most victims being children.

Plasmodium
 Both Plasmodium vivax and P. ovale can go
dormant, hiding out in the liver. The parasites
can reactivate and cause malaria months or
years after the initial infection.
 P. malariae causes a long-lasting infection. If
the infection is untreated it can persist
asymptomatically for the lifetime of the host.

Life cycle of malaria
 Plasmodium has two hosts: mosquitoes
and humans.
 Sexual reproduction takes place in the
mosquito and the parasite is transmitted to
humans when the mosquito takes a blood
meal.

Life cycle of malaria: humans
 The mosquito injects Plasmodium into a human in the
form of sporozoites.
 The sporozoites first invade liver cells and asexually
reproduce to produce huge numbers of merozoites
which spread to red blood cells where more merozoites
are produced through more asexual reproduction.
 Some parasites transform into sexually reproducing
gametocytes and these if ingested by a mosquito
continue the cycle.

Life cycle of malaria: mosquitoes
 Gametocytes ingested by a mosquito combine in
the mosquito’s stomach to produce zygotes.
 These zygotes develop into motile elongated
ookinites.
 The ookinites invade the mosquito’s midgut wall
where they ultimately produce sporozoites,
which make their way to the salivary glands
where they can be injected into a new human
host.

How Plasmodium enhances
transmission rates
 The Plasmodium parasite engages in a
number of manipulative behaviors to
enhance its chances of being transmitted
between hosts.
 Such manipulations are a common feature
of parasite behavior, in general, as we will
see throughout the semester.

transmission rates
 Mosquitoes risk death when feeding and
attempt to minimize risk and maximize
reward when doing so.
 To obtain blood a mosquito must insert its
proboscis through the skin and then locate
a blood vessel. The longer this takes, the
greater the risk.

transmission rates
 As soon as the mosquito hits a blood
vessel the host’s body responds by
clotting the wound.
 Platelets clump around the proboscis and
release chemicals which cause the
platelets to clot together.

transmission rates
 To slow clotting and speed feeding, mosquitoes
inject anticoagulants including one called
apyrase that unglues the platelets. They also
inject other chemicals that expand the blood
vessels.
 Plasmodium in the host helps the mosquito feed
by releasing chemicals that also slow clotting.
The parasite’s help increases the chances of the
mosquito feeding successfully and sucking up
the parasite.

transmission rates
 Once in the mosquito, Plasmodium needs about
10 days to produce sporozoites that are ready to
be injected into a human.
 During this time, to reduce the chances of the
mosquito dying, Plasmodium apparently
discourages its host from eating. Although how
the parasite does this is not clear, mosquitoes
containing ookinites abandon feeding attempts
sooner than parasite-free mosquitoes.

transmission rates
 Once sporozoites are in the salivary
glands, however, Plasmodium wants the
mosquito to bite and bite often.
 In the salivary gland the parasite cuts off
the mosquito’s anticoagulant apyrase
supply. This makes it harder for the
mosquito to feed so it is hungrier and bites
more hosts.

transmission rates
 As a result, an infected mosquito is twice
as likely to bite two people in a single night
as an uninfected mosquito is.
 As a result, the parasite is spread more
widely.

Behavior of Plasmodium in humans
 Plasmodium enters the human blood stream
through a mosquito bite.
 The parasite must avoid the host’s immune
system. To do so while in the body it moves
from one hiding place to another.
 The parasite moves first to the liver. Can get
there in about 30 minutes, which is usually fast
enough to avoid triggering the immune system.

 At the liver Plasmodium enters a liver cell.
 The cell responds by grabbing
Plasmodium proteins and displaying the
antigens on its cell surface in a special cup
the major histocompatibility complex or
MHC.

 The immune system recognizes the
Plasmodium antigens and mounts an
immune response.
 However, in a week, before the immune
system has mounted its full response the
parasite has produced about 40,000
copies of itself and these burst out of the
liver to seek red blood cells.

 The parasites leave the liver, reenter the
bloodstream, and find a red blood cell to
enter.
 Each parasite spends two days in a red
blood cell consuming the hemoglobin and
reproducing.

Red blood cells
 Red blood cells (strictly red blood
corpuscles) are a challenging environment
to live in.
 They lack a nucleus and have little
metabolic activity. As a result, they have
few proteins for generating energy and
also lack most of a normal cell’s channels
for transporting fuel in and wastes out.

Red blood cells
 Red blood cells are specialized to
transport oxygen, which they carry by
binding and wrapping in hemoglobin
molecules.
 A red blood cell is pumped around the
body by the heart and travels about 300
miles over its lifetime.

Red blood cells
 Red blood cells are squeezed through
slender capillaries and compressed to one
fifth of their normal diameter before
rebounding.
 To survive this squeezing, red blood cells
have a network of proteins under their
membrane that can fold like a concertina
and allow the cell to stretch and squeeze
as needed.

Red blood cells
 Old red blood cells eventually lose their
elasticity and become stiff.
 Those that show signs of such aging are
filtered out as they pass through the
spleen and destroyed.

 Plasmodium cannot swim but uses hooks
to move along the blood vessels.
 At the parasite’s tip are sensors that
respond only to young red blood cells and
clasp on to proteins on the cell’s surface.

 The parasite uses a set of organelles
concentrated at its apical end to gain
entry. A suite of proteins are produced
that cause the red blood cell’s membrane
to open and let the parasite squeeze in.
 It takes only about 15 seconds for the
parasite to get in.

Figure 11.30
Plasmodium Sporozoite

 Inside in the red blood cell the
Plasmodium consumes the hemoglobin. It
takes in a small amount of hemoglobin,
slices it apart with enzymes and harvests
the energy released.
 The toxic core of the hemoglobin molecule
is processed into an inert molecule called
hemozoin.

 In order to reproduce, Plasmodium needs more
than hemoglobin.
 It modifies the red blood corpuscle so it can
obtain amino acids and make proteins.
 The parasite builds a series of tubes that
connect it to the surface of the cell and uses
these to bring in materials from the blood steam
and to pump out wastes.

 The parasite also produces proteins that help to
maintain the red blood cell’s springiness for as
long as possible so it is not eliminated by the
spleen.
 After a few hours, however, the red blood cell
has been too modified by the parasite to fool the
spleen. The parasite now produces sticky latch
proteins that glue the cell to blood vessel walls.

 Infected cells clump up in capillaries.
 After another day the contents of the cell have
been used up. The cell ruptures and 16 new
parasites burst out to infect other red blood cells.
 Some of these parasites transform into sexually
reproducing gametocytes and, as mentioned
previously, these if ingested by a mosquito will
continue the cycle.

 While in the red blood cells Plasmodium is
invisible to the immune system because
the red blood cells have no MHC and
cannot alert the immune system.
 The latch proteins however do stimulate
the immune system.

 The latch protein is made by a single
gene, but Plasmodium has over 100 such
genes each of which produces a unique
latch.
 In each generation some of the new
parasites switch on a new latch gene and
so the immune system is always playing
catch up.

Effects of malaria on human
evolution
 The intense selection pressure imposed
by malaria has resulted in a large number
of mutations that provide protection
against the parasite being selected for in
humans.
 The best known is sickle cell anemia.

Anti-malaria mutations: Sickle cell
anemia
 Sickle cell anemia is a condition common
in West Africans (and African Americans of
West African ancestry).
 In sickle cell anemia red blood cells are
sickle shaped as a result of a mutation
which causes hemoglobin chains to stick
together.

anemia
 People with the sickle cell allele are protected
against Plasmodium because their hemoglobin
under low oxygen conditions contracts into
needle-shaped clumps.
 This contraction not only causes the sickling of
the cell, but harms the parasite. Parasites are
impaled on the clumps and the cell loses its
ability to pump potassium, which the parasite
needs.

allele
 People with two copies of the sickle cell
allele usually die young, but heterozygotes
are protected against malaria.
 As a result the geographic distribution of
the allele and malaria in Africa match quite
closely.

Anti-malaria mutations: (G6PD)
deficiency
 Glucose-6-phosphate dehydrogenase
(G6PD) deficiency. There are hundreds
of alleles known and with more than 400
million people affected G6PD deficiency is
the commonest enzyme deficiency known.

Anti-malaria mutations:
Thalassemia
 Geographic distribution suggests it
protects against malaria and
epidemiological evidence also supports
this.
 People with G6PD-202A, a reduced
activity variant common in Africa, have a
much lower risk of suffering severe
malaria.

Thalassemia
 Thalassemia: People with thalassemia
make the ingredients of hemoglobin in the
wrong amounts.
 Too many or too few α or ß hemoglobin
chains are produced and when they are
assembled into hemoglobin molecules
spare chains are left over.

Other anti-malaria mutations:
Thalassemia
 Extra chains clump together and cause major
problems in the cell. These clumps grab oxygen,
but don’t enclose it and the oxygen often
escapes and because it is strongly charged, the
oxygen damages other molecules in the cell.
 Severe thalassemia is fatal, but mild forms
protect against malaria because the loose
oxygen severely damages the parasite and
renders it unable to invade new cells.

Ovalocytosis
 Ovalocytosis: Occurs in South east Asia and
has same genetic rules and consequences as
sickle cell anemia.
 People with ovalocytosis have blood cell walls
that are so rigid they can’t slip through
capillaries. The rigid cell walls make it hard for
the parasite to enter the cell and the cell’s rigidity
appears to prevent the parasite pumping in
phosphates and sulphates it needs to survive.

 One major advantage of these various anti-
malarial mutations appears to be that they
provide a natural vaccination program for
children.
 By slowing the development of the parasite
these mutations give a child’s naïve immune
system time to overcome Plasmodium’s
attempts to elude the immune system and mount
an immune response. Mild cases of malaria
thus immunize children to malaria and allow
them to survive to adulthood.

Mosquito nets save lives
 www.nothingbutnets.net or
www.nothingbutnets.org
 $10 gets a net to a family. 100% of your
donation goes to purchase and distribute
nets.

Human African Trypanosomiasis
(Sleeping sickness)
 Sleeping sickness is a protozoan disease, which
like malaria is spread by an insect vector, the
tsetse fly.
 The disease is endemic to sub-Saharan Africa
and an estimated 300,000 people are infected
annually with about 40,000 deaths.
 The disease organism is Trypanosoma brucei.

Trypanosoma forms in blood smear from patient with African trypanosomiasis
http://en.wikipedia.org/wiki/File:Trypanosoma_sp._PHIL_613_lores.jpg

Sleeping Sickness
 Symptoms:
 Begins with fever, headaches, and joint pains.
 Lymph nodes may swell enormously and parasite numbers
may be incredibly high. Greatly enlarged lymph nodes in
the back of the neck are tell-tale signs of the disease.
 If untreated the parasite may cross the blood-brain barrier,
which causes the characteristic symptoms the disease is
named for. The patient becomes confused and the sleep
cycle is disturbed with the patient alternating between
manic periods and complete lethargy. Progressive mental
deterioration is followed by coma and death.

Sleeping Sickness
 Trypanosome levels in infected patients show a
cycle of sharp peaks and valleys in parasite
numbers of approximately a week in length.
 The cycle occurs because the immune system
recognizes the glycoprotein in the trypanosomes
coat and mounts an immune response to it,
which eliminates parasites with that glycoprotein.

Sleeping Sickness
 Trypanosomes, however, possess about 1,000
different coat-building genes and periodically a
new one is turned on by a trypanosome that
produces a different coat, which the immune
system doesn’t recognize.
 Trypanosomes with this new coat reproduce
undetected until the immune system can mount
a response to the new coat.

Sleeping Sickness
 If the first generation of trypanosomes to infect a
host turned on their coat genes at random the
immune system could learn to recognize the
various possibilities quickly, remember them,
and eliminate the parasite.
 Instead the coat-building genes are turned on in
pre-set sequence. This means that the immune
system every week or so is faced with a new
coat that it has not seen before.

Sleeping Sickness
 As a result of the sequential coat-switching, the
immune system becomes chronically over-
stimulated and begins to attack the host’s body.
 The overstimulation of the immune system and
the movement of parasites into the central
nervous, where they escape the immune system
altogether, eventually kills the patient.

Protozoa

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