Protoplasts, Spheroplasts, the cytoplasm and Cysts

Protoplasts

A Protoplasts is that portion of the bacterial cell consisting of the cytoplasmic membrane and the cell material bounded by it. Protoplasts can be prepared from gram-positive bacteria by treating the cells with an enzyme such as lysozyme, which selectively dissolves the cell wall, or by culturing the bacteria in the presence of an antibiotic such as penicillin, which prevents the formation of the cell wall. In either case, the osmotic pressure of the medium must be sufficiently high to protect the organisms from bursting. Bacteria normally occur in hypotonic environments (i.e., environments having a lower osmotic pressure than that within the bacterial cells) and they continually take up water by osmosis; this. They tend to expand, pressing the cytoplasmic membrane tightly against the rigid cell wall. In the absence of rigid cell wall, there is nothing to prevent the continued expansion and eventual bursting of a protoplast. This bursting can be prevented by preparing protoplasts in an isotonic medium, i.e., in a medium that has an osmotic pressure similar to that of the protoplast. Such osmotically protected protoplasts are soft and fragile and are spherical, regardless of the original shape of the cell.

Spheroplasts

Round, osmotically fragile forms of gram-negative bacteria can be prepared by procedures similar to those used for the protoplasts of gram-positive bacteria. However, the cell walls of gram-negative bacteria differ from those of gram-positive bacteria possessing an outer membrane. Although the peptidoglycan of the cell wall may be destroyed by lysozyme or its synthesis inhibited by antibiotics, the flexible outer membranes, the cytoplasmic membrane of the protoplasts plus the outer membrane of the cell wall, the cell is called a spheroplast rather than a protoplast.

Some bacteria, the mycoplasmas, never have cell walls and are bounded by only a cytoplasmic membrane; therefore, they have many of the properties of protoplasts, yet they manage to thrive nonetheless. Most mycoplasmas are parasites of animals, plants, or arthropods, and therefore live in osmotically favorable or isotonic environments. Some are able to attain a degree of rigidity by incorporating cholesterol into their cytoplasmic membranes. Most mycoplasmas have a more or less spherical shape, but one genus. Spiroplasma, consist of helical cells. How such cells are able to maintain this shape in the absence of cell wall is unknown.

Membranous intrusions and intracellular membrane system

Bacterial cells do not contain membrane-enclosed organelles corresponding to the mitochondria and chloroplasts of eukaryotic cells. However, bacteria may have specialized invaginations of the cytoplasmic membrane that can increase their surface are for certain functions.

Many bacteria, especially gram-positive bacteria, possess membrane invaginations in the form of systems of convoluted tubules and vesicles termed mesosomes. Those known as central mesosomes penetrates deeply into the cytoplasm, are located near the middle of the cell, and seem to be attached to the cell’s nuclear, material; they are thought to be involved in DNA replication and cell division. In contrast, peripheral mesosomes show only a shallow penetration into the cytoplasm, are not restricted to a central location, and are not associated with the nuclear material; they seem to be involved in export of exocellular enzymes such as penicillinase.

Extensive intracellular membrane systems occur in methane-oxidizing bacteria, in certain Chemoautrophic bacteria, and in nearly all phototrophic bacteria. They serve to increase surface area for various metabolic activities. For example, in phototrophic bacteria they are the site of the photosynthetic apparatus of the cell; the infoldings provide a large surface area to accommodate a high content of light-absorbing pigments. In the phototrophs known as cyanobacteria, special intracellular membrane (thylakoids) occur that seem to be separate from the cytoplasmic membrane.

The cytoplasm

The cell material bounded by the cytoplasm membrane may be divided into (1) the cytoplasmic area, granular in appearance and rich in the macromolecular RNA-protein bodies known as ribosomes, on which proteins are synthesized; (2)the chromatic area, rich in DNA; and (3)the fluid portion with dissolved substances. Unlike animal or plant cells, there is no endoplasmic reticulum to which ribosomes are bound; some ribosomes are free in the cytoplasm, and others, especially those involved in the synthesis of proteins to be transported out of the cell, are associated with the inner surface of the cytoplasmic membrane. When the ribosomes of prokaryotes undergo sedimentation in a centrifuge, they have a sedimentation coefficient of 70 Svedberg units (the 70s) and are composed of two subunits, a 50S, and a 30S subunit. This is in contrast to the ribosomes of eukaryotic organisms, which have a sedimentation coefficient of the 80s and are composed of a 60S and a 40S subunit.

Cysts

Cysts are dormant, thick-walled, desiccation-resistant forms that develop by differentiation of a vegetative cell and which can later germinate under suitable conditions. In some ways cysts resemble endospores; however, their structure and chemical composition are different and they do not have the high heat resistance of endospores. The classic example of cysts is the structurally complex type produced by the genus Azotobacter. Several other bacteria can differentiate into cystlike forms, but these seem to lack the degree of structural complexity characteristics of Azotobacter cysts.

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.