Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Cytoplasm, prokaryotic

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tain the life of a bacterium, with the exception of those com-

ponents that reside in the membrane(s), and in the periplasm

of Gram-negative bacteria.

The cytoplasm is bounded by the cytoplasmic mem-

brane. Gram-negative bacteria contain another outer mem-

brane. In between the two membranes lies the periplasm.

When viewed in the light microscope, the cytoplasm of

bacteria is transparent. Only with the higher magnification

available using the transmission electron microscopedoes the

granular nature of the cytoplasm become apparent. The exact

structure of the cytoplasm may well be different than this

view, since the cytoplasm is comprised mainly of water. The

dehydration necessary for conventional electron microscopy

likely affect the structure of the cytoplasm.

The cytoplasm of prokaryotes and eukaryotesis similar

in texture. Rather than being a free-flowing liquid the cyto-

plasm is more of a gel. The consistency has been likened to

that of dessert gel, except that the bacterial gel is capable of

flow. The ability of flow is vital, since the molecules that

reside in the cytoplasm must be capable of movement within

the bacterium as well as into and out of the cytoplasm.

The genetic material of the bacteria is dispersed

throughout the cytoplasm. Sometimes, the deoxyribonucleic

acidgenome can aggregate during preparation for microscopy.

Then, the genome is apparent as a more diffuse area within the

granular cytoplasm. This artificial structure has been called

the nucleoid. Smaller, circular arrangements of genetic mate-

rial called plasmidscan also be present. The dispersion of the

bacterial genome throughout the cytoplasm is one of the fun-

damental distinguishing features between prokaryotic and

eukaryotic cells.

Also present throughout the cytoplasm is the ribonu-

cleic acid, various enzymes, amino acids, carbohydrates,

lipids, ions, and other compounds that function in the bac-

terium. The constituents of the membrane(s) are manufac-

tured in the cytoplasm and then are transported to their final

destination.

Some bacteria contain specialized regions known as

cytoplasmic inclusions that perform specialized functions.

These inclusions can be stored products that are used for the

nutrition of the bacteria. Examples of such inclusions are

glycogen, poly-B-hydroxybutyrate, and sulfur granules. As

well, certain bacteria contain gas-filled vesicles that act to

buoy the bacterium up to a certain depth in the water, or mem-

branous structures that contain chlorophyll. The latter function

to harvest light for energy in photosynthetic bacteria.

The cytoplasm of prokaryotic cells also houses the ribo-

somesrequired for the manufacture of protein. There can be

many ribosomes in the cytoplasm. For example, a rapidly

growing bacterium can contain upwards of 15,000 ribosomes.

The processes of transcription, translation, protein

import and export, and at least some degradation of com-

pounds occurs in the cytoplasm. In Gram-negative bacteria,

some of these functions also occur in the periplasmic fluid.

The mechanisms that underlie the proper sequential orchestra-

tion of these functions are still yet to be fully determined.

See alsoBacterial ultrastructure

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