Cytoplasmic inclusions and vacuoles

Cytoplasmic inclusions and vacuoles

Concentrated deposits of certain substances are detectable in the cytoplasm of some bacteria. That they stain an intense reddish-purple color with dilute methylene blue and can be observed by light microscopy. By electron microscopy, they appear as round, dark areas. Volutin serves as a reserve source of phosphate. Another polymer often found in aerobic bacteria, especially under high-carbon, low-nitrogen culture conditions, is a chloroform-soluble, lipid-like material, poly--hydroxybutyrate, (PHB), which can serve as a reserve carbon and energy source. PHB granules can be stained with lipid-soluble dyes such as Nile blue. By electron microscopy, they appear as clear round areas. Polysaccharide granules, i.e., glycogen, can be stained brown with iodine. By electron microscopy, they appear as dark granules. Another type of inclusion is represented by the intracellular globules of elemental sulfur that may accumulate in certain bacteria growing in environments rich in hydrogen sulfide.

Some bacteria that live in aquatic habitats from gas vacuoles that provide buoyancy. By light microscopy these are bright, refractile bodies; by electron microscopy, they are seen to have a regular shape: hollow, rigid cylinders with more or less conical ends and having a striated protein boundary. This boundary is impermeable to water, but the various dissolved gasses in the culture medium can penetrate it to fill the cavity. The identifying feature of gas vacuoles is that they can be made to collapse under pressure and thereby lose their reflectivity.

Nuclear material

In contrast to eukaryotic cells, bacterial cells contain neither a distinct membrane-enclosed nucleus nor a mitotic apparatus. However, they do contain an area near the center of the cell that is regarded as a nuclear structure, and the DNA of the cell is confined to this area. Because it is not a discrete nucleus this nebulous structure has been designated by such terms as the nucleoid; the chromatic body: the nuclear equivalent: and even the bacterial chromosome, since it consists of a single, circular DNA molecule in which all the genes are linked. The nucleoid can be made visible under the light microscope by Feulgen staining, which is specific for DNA. By electron microscopy, it appears as a light area with delicate fibrillar structure. The behavior of the nucleoid in growing, dividing bacteria has been observed by use of phase-contrast microscopy with a medium having a high reference index.

Spores and cysts

Certain species of bacteria produce spores, either within the cell (endospores) or external to the cell (exospores). The spore is a metabolically dormant form which, under appropriate conditions, can undergo germination and outgrowth to form a vegetative cell.

Endospores

These structures are unique to bacteria. They are thick-walled, highly refractile bodies that are produced (one per cell) by Bacillus, Clostridium, Sporosarcina, Thermoactinomyces, and a few other genera. The shapes of endospores and also their location within the vegetative cell vary depending on the species. The structural changes that occur during the development of endospores have been extensively studied in Bacillus and Clostridium species. Endospore is usually produced by cells growing in rich media but which are approaching the end of active growth. Various factors such as aging or heat treatment are needed to activate the dormant spores (i.e., permit them to be able to undergo germination and outgrowth when they are placed in a suitable medium).

Endospores are extremely resistant to desiccation, staining, disinfecting chemicals, radiation, and heat. For example, the endospores of Clostridium botulinum type A have been reported to resist boiling for several hours. The degree of heat resistance of endospores varies with the bacterial species, but most can resist treatment at 80for at least 10 minutes. What causes this heat resistance has been a subject of intense study, but the explanation is still not clear. During sporulation, a dehydration process occurs in which most of the water in the developing spore is expelled; the resulting dehydrated state may be an important factor for heat resistance.

All endospores contain large amounts of dipicolinic acid (DPA), a unique compound that is undetectable in the vegetative cells yet can account for 10 to 15 percent of the spore’s dry weight. It occurs in combination with large amounts of calcium and is probably located in the core, i.e., in the central part of the spore. The calcium-DPA complex may possibly play a role in the heat resistance of endospores. Synthesis of DPA and the uptake of calcium occur during advanced stages of sporulation. During germination, endospores lose their resistance to heat and staining. Subsequent outgrowth occurs, characterized by synthesis of new cell material and development of the organism into a growing cell.

Exospores

Cells of the methane-oxidizing genus Methylosinus form exospores, ie.., spores external to the vegetative cell, by budding at one end of the cell. These are desiccation and heat-resistant, but unlike endospores, they do not contain DPA.

Conidiospores and sporangiospores

The large group of bacteria known as the actinomycetes forms branching hyphae; spores develop, singly or in chains, from the tips of these hyphae by cross wall formation (septation). If the spores are contained in an enclosing sac (sporangium), they are termed sporangiospores; if not, they are called conidiospores (or condition). The spores do not have the high heat resistance of endospores, but they can survive long periods of drying.

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.