Microbiology and Immunology

Parasites WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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organism. In this way, the amount of water inside the parame-

cia can be controlled. The operation of the contractile vacuole

is another feature that is visible by the light microscopic obser-

vation of living paramecia.

On the exterior lies a membrane that is called the pelli-

cle. The pellicle is both stiff, to provide support and to main-

tain the shape of the organism, and is flexible, to allow some

flexing of the surface. Also on the surface are hundreds of tiny

hairs called cilia. The cilia wave back and forth, and act to

sweep food particles (bacteriaand smaller protozoa) towards

the primitive mouth of the organism (the gullet). The cilia are

also important in locomotion, acting analogous to the oars of

a rowboat. The beating of the cilia is easily visible under light

microscopic examination, especially if the movement of the

organism has been retarded by the addition of a viscous com-

pound such as glycerol to the sample.

See alsoEukaryotes

PParasitesARASITES

A parasite is an organism that depends upon another organism,

known as a host, for food and shelter. The parasite usually

gains all the benefits of this relationship, while the host may

suffer from various diseases and discomforts, or show no signs

of the infection. The life cycle of a typical parasite usually

includes several developmental stages and morphological

changes as the parasite lives and moves through the environ-

ment and one or more hosts. Parasites that remain on a host’s

body surface to feed are called ectoparasites, while those that

live inside a host’s body are called endoparasites. Parasitism is

a highly successful biological adaptation. There are more

known parasitic species than nonparasitic ones, and parasites

affect just about every form of life, including most all animals,

plants, and even bacteria.

Parasitology is the study of parasites and their rela-

tionships with host organisms. Throughout history, people

have coped with over 100 types of parasites affecting

humans. Parasites have not, however, been systematically

studied until the last few centuries. With his invention of the

microscopein the late 1600s, Anton von Leeuwenhoek was

perhaps the first to observe microscopic parasites. As

Westerners began to travel and work more often in tropical

parts of the world, medical researchers had to study and treat

a variety of new infections, many of which were caused by

parasites. By the early 1900s, parasitology had developed as

a specialized field of study.

Light micrograph of a paramecium.

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