Bacteria have reserved food material stored in cytoplasm. The food is concentrated organic deposits and also known as cytoplasmic inclusions. The reserved food is polymeric, high molecular weight and osmotically inert materials. Being osmotically inert do not decrease or increase osmolarity of cytoplasm and prevent the loss of cytoplasmic contents by cytolysis. The five types of cytoplasmic inclusions are known to present in bacteria, they are: Metachromatic granules, Poly-β-hydroxybutyrate and Polyglucan granules, Sulfur globules and Gas vacuoles. The first 3 types are referred as granules as they have granule like appearance. Cytoplasmic inclusions are found dispersed in the cytoplasm or sometimes enclosed by membrane. The basic requirement of reserved food material as the name itself suggests that they are energy reserves and readily available substrates for metabolic reactions to carry out during stress conditions.
Metachromatic granules: They are also known as Volutin or Polymetaphosphate granules. They are composed of polyphosphate, RNA and proteins. Their main function is to supply phosphate for nucleic acid synthesis, cell division, energy metabolism and as a source of phosphorous for nutrition. By staining with methylene blue, they represent hues of colors reddish purple, purple, bluish red or maroon under light microscope and hence the name 'metachromatic'. By electron microscopy, they appear as dark spheres. They are usually detected in old laboratory cultures stored at room temperature or refrigerator. Naturally, they are found in species of Corynebacterium, Spirillum, Rhizobium and Bacillus.
Poly-β-hydroxybutyrate (PHB) granules: They are also known as sudanophilic (stained by lipid stain Sudan) or lipid granules. Chemically, they are polyactides or polyesters. For light microscopic observation, they are stained by Sudan Black B or Nile blue to appear as dark blue dots in cell cytoplasm against reddish blue background. Electron microscopic observations show them as light round spots. Generally, PHB granules are formed during lipid synthesis, Acetate or butyrate metabolism, nitrogen deficiency conditions or denitrification. In lipid synthesis, acetyl CoA is condensed to aceto-acetyl CoA and it is further reduced to β-hydroxybutyryl CoA. Polymerization of this compound results in formation of PHB. Poly-β-hydroxybutyrate granules are important source of food during starvation conditions, particularly in soil and rhizosphere environment where nutrient stress is always prevalent. PHB granules are found in almost all species of Rhizobium, Bacillus, Alcaligenes and other soil bacteria.
Polyglucan granules: They are also known as iodophilic or polysaccharide granules. They are stained by iodine solution and appear brown or bluish under light microscope. They can be seen as dark round spots by electron microscopy. Polyglucan consists of repeated glucan units with α, 1-4 linkage and α, 1-6 branch points. They are deposited by bacteria themselves inside their cells when simple sugars like glucose, fructose or sucrose are present for polysaccharide (glucan) synthesis. They have been found in clostridia and coliform group of bacteria; they are very important source of substrate in carbohydrate metabolism during starvation conditions in these bacteria.
Sulfur globules: Sulfur globules are cytoplasmic globules of elemental sulfur. They are usually found in bacteria growing in environments rich in hydrogen sulfide (H2S) gas such as hydrothermal vents, thermal geysers, boiling water or sulfur springs. These habitats are always dominated by sulfate reducing (photosynthetic purple and green sulfur) bacteria like Chromatium and Chlorobium. They use H2S as electron donor to reduce carbon dioxide during photosynthesis process. The sulfur globules are also found in sulfur oxidising bacteria, like extremophile Thiobacillus thiooxidans which inhabit sulfur rich environments. They principally oxidize elemental sulfur to sulfates which is then assimilated by plants for synthesis of sulfur containing amino acids. Both sulfur reducing and oxidising bacteria are integral part of natural elemental sulfur cycle on the Earth.
Gas vacuoles: Aquatic bacteria like cyanobacteria possess gas vacuoles. They are present in the cytoplasm and hence considered as cytoplasmic inclusions. By light microscopy, they appear as bright refractile bodies and by electron microscopy as hollow cylindrical shapes with conical ends and striated protein boundary. Protein boundary is impermeable to water but allows exchange of various gases dissolved in water or at the air-water interface. Vacuoles may get collapsed under gas pressure or can be refilled by gases. Their refractility depends on pressure of internal gas. The main function of gas vacuole is to provide buoyancy to organism in aquatic habitat.
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