Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Protozoa

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Scientists have discovered fossilized specimen of proto-

zoa that measured 0.78 in (20 mm) in diameter. Whatever the

size, however, protozoans are well-known for their diversity

and the fact that they have evolved under so many different

conditions.

One of the basic requirements of all protozoans is the

presence of water, but within this limitation, they may live in

the sea, in rivers, lakes, stagnant ponds of freshwater, soil, and

in some decaying matters. Many are solitary organisms, but

some live in colonies; some are free-living, others are sessile;

and some species are even parasitesof plants and animals

(including humans). Many protozoans form complex, exqui-

site shapes and their beauty is often greatly overlooked on

account of their diminutive size.

The protozoan cell body is often bounded by a thin pli-

able membrane, although some sessile forms may have a

toughened outer layer formed of cellulose, or even distinct

shells formed from a mixture of materials. All the processes of

life take place within this cell wall. The inside of the mem-

brane is filled with a fluid-like material called cytoplasm, in

which a number of tiny organs float. The most important of

these is the nucleus, which is essential for growth and repro-

duction. Also present are one or more contractile vacuoles,

which resemble air bubbles, whose job it is to maintain the

correct water balance of the cytoplasm and also to assist with

food assimilation.

Protozoans living in salt water do not require contractile

vacuoles as the concentration of salts in the cytoplasm is simi-

lar to that of seawater and there is therefore no net loss or gain

of fluids. Food vacuoles develop whenever food is ingested

and shrink as digestion progresses. If too much water enters the

cell, these vacuoles swell, move towards the edge of the cell

wall and release the water through a tiny pore in the membrane.

Some protozoans contain the green pigment chlorophyll

more commonly associated with higher plants, and are able to

manufacture their own foodstuffs in a similar manner to

plants. Others feed by engulfing small particles of plant or ani-

mal matter. To assist with capturing prey, many protozoans

have developed an ability to move. Some, such as Euglena and

Trypanosoma are equipped with a single whip like flagella

which, when quickly moved back and forth, pushes the body

through the surrounding water body. Other protozoans (e.g.,

Paramecium) have developed large numbers of tiny cilia

around the membrane; the rhythmic beat of these hairlike

structures propel the cell along and also carry food, such as

bacteria, towards the gullet. Still others are capable of chang-

ing the shape of their cell wall. The Amoeba, for example, is

capable of detecting chemicals given off by potential food par-

ticles such as diatoms, algae, bacteria or other protozoa. As the

cell wall has no definite shape, the cytoplasm can extrude to

form pseudopodia (Greek pseudes, “false”; pous, “foot”) in

various sizes and at any point of the cell surface. As the

Amoeba approaches its prey, two pseudopodia extend out

from the main cell and encircle and engulf the food, which is

then slowly digested.

Various forms of reproduction have evolved in this

group, one of the simplest involves a splitting of the cell in a

process known as binary fission. In species like amoeba, this

process takes place over a period of about one hour: the

nucleus divides and the two sections drift apart to opposite

ends of the cell. The cytoplasm also begins to divide and the

cell changes shape to a dumb-bell appearance. Eventually the

cell splits giving rise to two identical “daughter” cells that then

resume moving and feeding. They, in turn, can divide further

in this process known as asexual reproduction, where only one

individual is involved.

Some species that normally reproduce asexually, may

occasionally reproduce through sexual means, which involves

the joining, or fusion, of the nuclei from two different cells. In

the case of paramecium, each individual has two nuclei: a

larger macronucleus that is responsible for growth, and a

much smaller micronucleus that controls reproduction. When

paramecium reproduce by sexual means, two individuals join

in the region of the oral groove—a shallow groove in the cell

membrane that opens to the outside. When this has taken

place, the macronuclei of each begins to disintegrate, while

the micronucleus divides in four. Three of these then degener-

ate and the remaining nucleus divides once again to produce

two micronuclei that are genetically identical. The two cells

then exchange one of these nuclei that, upon reaching the

other individual’s micronucleus, fuse to form what is known

as a zygote nucleus. Shortly afterwards, the two cells separate

but within each cell a number of other cellular and cytoplas-

mic divisions will continue to take place, eventually resulting

in the production of four daughter cells from each individual.

Protozoans have evolved to live under a great range of

environmental conditions. When these conditions are unfavor-

able, such as when food is scarce, most species are able to

enter an inactive phase, where cells become non-motile and

secrete a surrounding cyst that prevents desiccationand pro-

tects the cell from extreme temperatures. The cysts may also

serve as a useful means of dispersal, with cells being borne on

the wind or on the feet of animals. Once the cyst reaches a

more favorable situation, the outer wall breaks down and the

cell resumes normal activity.

Many species are of considerable interest to scientists,

not least because of the medical problems that many cause.

The tiny Plasmodiumprotozoan, the cause of malaria in

humans, is responsible for hundreds of millions of cases of ill-

ness each year, with many deaths occurring in poor countries.

This parasite is transferred from a malarial patient to a healthy

person by the bite of female mosquitoes of the genus

Anopheles. As the mosquito feeds on a victim’s blood the par-

asites pass from its salivary glands into the open wound. From

there, they make their way to the liver where they multiply and

later enter directly into red blood cells. Here they multiply

even further, eventually causing the blood cell to burst and

release from 6-36 infectious bodies into the blood plasma. A

mosquito feeding on such a patient’s blood may absorb some

of these organisms, allowing the parasite to complete its life

cycle and begin the process all over again. The shock of the

release of so many parasites into the human blood stream

results in a series of chills and fevers—typical symptoms of

malaria. Acute cases of malaria may continue for some days or

even weeks, and may subside if the body is able to develop

immunityto the disease. Relapses, however, are common and

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