Microbiology and Immunology

Protoplasts and spheroplasts WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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person through female Anopheles mosquitoes. People who

suffer from malaria experience symptoms such as shivering,

sweating, high fevers, and delirium. African sleeping sick-

ness, also known as African trypanosomiasis, is caused by

another sporozoan, Trypanosoma. Trypanosoma is transmitted

through the African tsetse fly. This organism causes high fever

and swollen lymph nodes. Eventually the protist makes its

way into the victim’s brain, where it causes a feeling of uncon-

trollable fatigue. Giardiasis is another example of a disease

caused by a protist. This illness is caused by Giardia, a sporo-

zoan carried by muskrats and beavers. Giardiasis is character-

ized by fatigue, cramps, diarrhea, and weight loss. Amoebic

dysenteryoccurs when a certain amoeba, Entamoeba histolyt-

ica,infects the large intestine of humans. It is spread through

infected food and water. This organism causes bleeding, diar-

rhea, vomiting, and sometimes death.

Members of the kingdom Protista can also be very ben-

eficial to life on Earth. Many species of red algae are edible

and are popular foods in certain parts of the world. Red algae

are rich in vitamins and minerals. Carageenan, a polysaccha-

ride extracted from red algae, is used as a thickening agent in

ice cream and other foods. Giant kelp forests are rich ecosys-

tems, providing food and shelter for many organisms.

Trichonymphs are flagellates that live in the intestines of ter-

mites. These protozoans break down cellulose in wood into

carbohydrates the termites can digest.

The kingdom Protista is a diverse group of organisms.

Some protists are harmful, but many more are beneficial.

These organisms form the foundation for food chains, produce

the oxygen we breathe, and play an important role in nutrient

recycling. Many protists are economically useful as well. As

many more of these unique organisms are discovered, humans

will certainly enjoy the new uses and benefits protists provide.

See alsoEukaryotes

PProtoplasts and spheroplastsROTOPLASTS AND SPHEROPLASTS

Protoplasts and spheroplasts are altered forms of bacteriaor

yeast, in which the principal shape-maintaining structure of

the bacteria is weakened. Each bacterium forms a sphere,

which is the shape that allows the bacterium to withstand the

rigors, particularly osmotic, of the fluid in which it resides.

The term protoplast refers to the spherical shape

assumed by Gram-positive bacteria. Spheroplast refers to the

spherical shape assumed by Gram-negative bacteria. The dif-

ference is essentially the presence of a single membrane, in the

case of the protoplast, and the two membranes (inner and

outer) of the Gram-negative spheroplasts. It is also possible to

generate a gram-negative protoplast by the removal of the

outer membrane. Thus, in essence, protoplast refers to a bac-

terial sphere that is bounded by a single membrane and spher-

oplast refers to a sphere that is bounded by two membranes.

Bacteria are induced to form protoplasts or spheroplasts

typically by laboratory manipulation. However, formation of

the structures can occur naturally. Such bacteria are referred to

as L-forms. Examples of bacterial genera that can produce L-

forms include Bacillus, Clostridium, Haemophilus,

Pseudomonas, Staphylococcus, and Vibrio.

The peptidoglycanis the main stress-bearing layer of

the bacterial cell wall and the peptidoglycan also gives the

bacterium its shape. In the laboratory, weakening the peptido-

glycan network in the cell wall generates both protoplasts and

spheroplasts.

By exposing bacteria to an enzyme called lysozyme,

the interconnecting strands of the two particular sugars that

form the peptidoglycan can be cut. When this is done, the

peptidoglycan loses the ability to serve as a mechanical

means of support.

The situation in yeast is slightly different, as other com-

ponents of the yeast cell wall are degraded in order to form the

protoplast.

The process of creating protoplasts and spheroplasts

must be done in a solution in which the ionic composition and

concentration of the fluid outside of the bacteria is the same

as that inside the bacteria. Once the structural support of the

peptidoglycan is lost, the bacteria are unable to control their

response to differences in the ionic composition between the

bacterial interior and exterior. If the inner concentration is

greater than the outer ionic concentration, water will flow

into the bacterium in an attempt to achieve an ionic balance.

The increased volume can be so severe that the bacteria will

burst. Conversely, if the inner ionic concentration is less than

the exterior, water will exit the bacterium, in an attempt to

dilute the surroundings. The bacteria can shrivel to the point

of death.

Preservation of ionic balance is required to ensure that

bacteria will not be killed during their transformationinto

either the protoplast or the spheroplast form. Living proto-

plasts and spheroplasts are valuable research tools. The mem-

brane balls that are the protoplasts or spheroplasts can be

induced to fuse more easily with similar structures as well as

with eukaryotic cells. This facilitates the transfer of genetic

material between the two cells. As well, the sequential manu-

facture of spheroplasts and protoplasts in Gram-negative bac-

teria allows for the selective release of the contents of the

periplasm. This approach has been popular in the identifica-

tion of the components of the periplasm, and in the localiza-

tion of proteins to one or the other of the Gram-negative

membranes. For example, if a certain protein is present in a

spheroplast population—but is absent from a protoplast popu-

lation—then the protein is located within the outer membrane.

See also Bacterial ultrastructure; Biotechnology; Trans-

formation

PProtozoaROTOZOA

Protozoa are a very diverse group of single-celled organisms,

with more than 50,000 different types represented. The vast

majority are microscopic, many measuring less than 1/200

mm, but some, such as the freshwater Spirostomun, may reach

0.17 in (3 mm) in length, large enough to enable it to be seen

with the naked eye.

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