Bacterial cytology cell inclusions

Bacterial cytology cell inclusions

1. Bacterial Cytology : Cell Inlusions Vishrut S. Ghare ( M.Sc Microbiology, SET) Asst. Professor, S.B.B alias A. Jedhe College, Pune

2. Cell inclusions : Gas vesicles, Carboxysomes , PHB granules, Metachromatic granules, Glycogen bodies, Starch granules, Magnetosomes , Sulfur granules, Chlorosomes

3. Cell inclusions : Within the cytoplasm of prokaryotic cells are several kinds of reserve deposits, known as inclusions. Cells may accumulate certain nutrients when they are plentiful and use them when the environment is deficient. Some inclusions are common to a wide variety of bacteria, whereas others are limited to a small number of species and therefore serve as a basis for identification. Some inclusions, such as magnetosomes , are membrane-enclosed organelles, while other inclusions, such as carboxysomes , are enclosed in protein complexes. Inclusions function as energy reserves and as reservoirs of structural building blocks. Inclusions can often be seen directly with the light microscope and are usually enclosed by single layer membranes that partition them off in the cell.

4. Gas vesicles: Some prokaryotes are planktonic , meaning that they live a floating existence within the water column of lakes and the oceans. These organisms can float because they contain gas vesicles. These structures confer buoyancy on cells, allowing them to position themselves in a water column in response to environmental cues. The most dramatic examples of gas- vesiculate bacteria are cyanobacteria that form massive accumulations called blooms in lakes or other bodies of water Gas vesicles are spindle-shaped structures made of protein; they are hollow yet rigid and of variable length and diameter. Gas vesicles in different organisms vary in length from about 300 to more than 1000 nm and in width from 45 to 120 nm, but the vesicles of a given organism are more or less of constant size.

5. Gas vesicles may number from a few to hundreds per cell and are impermeable to water and solutes but permeable to gases. The presence of gas vesicles in cells can be determined either by light microscopy, where clusters of vesicles, called gas vacuoles, appear as irregular bright inclusions, or by transmission electron microscopy. Each vacuole consists of rows of several individual gas vesicles, which are hollow cylinders covered by protein ( GvpA and GvpC ). Gas vacuoles maintain buoyancy so that the cells can remain at the depth in the water appropriate for them to receive sufficient amounts of oxygen, light, and nutrients. They descend by simply collapsing vesicles and float upward when new ones are constructed. Examples: Aquatic prokaryotes, cyanobacteria , anoxygenic photosynthetic bacteria and halobcteria has gas vacuoles

6. B. Carboxysomes : Several autotrophic prokaryotes that use the Calvin cycle produce polyhedral cell inclusions called carboxysomes . The inclusions, about 100 nm in diameter, are surrounded by a thin, protein membrane and consist of a crystalline array of RubisCO , with about 250 RubisCO molecules present per carboxysome . Carboxysomes appear to be a mechanism for concentrating CO 2 in the cell and making it readily available to RubisCO (ribulose-1, 5-bisphosphate carboxylase / oxygenase ). Associated with the shell is the enzyme carbonic anhydrase that converts carbonic acid and bicarbonate into CO 2 . Carboxysomes are present in many cyanobacteria and other CO 2 -fixing bacteria. Thus the carboxysome serves as a site for CO 2 fixation.

7. Among the bacteria containing carboxysomes are nitrifying bacteria, cyanobacteria , and acidithiobacilli .

8. C. PHB granules One of the most common inclusion bodies in prokaryotic organisms is poly-β- hydroxybutyric acid (PHB), a lipid that is formed from β- hydroxbutyric acid units. The monomers of PHB bond by ester linkage to form the PHB polymer, and then the polymer aggregates into granules; it can be observed by either light or electron microscope. The monomer in the polymer is not only hydroxybutyrate (C4) but can vary in length from as short as C3 to as long as C18. Thus, the more generic term poly-β- hydroxyalkanoate (PHA) is often used to describe this class of carbon- and energy-storage polymers. PHAs are synthesized by cells when there is an excess of carbon and are broken down for biosynthetic or energy purposes, Many prokaryotes, including species of both Bacteria and Archaea , produce PHAs.

9. Several types of PHA granules have been identified, but the most common contain poly-β- hydroxybutyrate (PHB). The structure of PHB inclusions has been well studied, and PHB granules are surrounded by a single-layered shell composed of proteins. Much of the interest in PHB and other PHA granules is due to their industrial use in making biodegradeable plastics. Azotobacter , B. cereus, B. megaterium , Corynebacterium , Pseudomonas, Rhodospirillum , Methylobacterium contains PHB. PHB was first isolated and characterized by Maurice Lemoigne in 1952. Sudan black stain is used to detect presence of PHB granules in cytoplasm. Biosynthesis of PHB starts with condensation of 2 molecules of acetyl – CoA to produce acetoacetyl CoA which is reduced to hydroxybutyryl-CoA , this monomer is later used to make PHB

10. PHB monomer Pathway of PHB synthesis

11. D. Metachromatic granules/ volutin / Polyphosphate Metachromatic granules are large inclusions that take their name from the fact that they sometimes stain red with certain blue dyes such as methylene blue. Collectively they are known as volutin . Volutin represents a reserve of inorganic phosphate (polyphosphate) that can be used in the synthesis of ATP, nucleic acid and phospholipid biosyntheses. It is generally formed by cells that grow in phosphate-rich environments. Phosphate is often a limiting nutrient in natural environments. Thus if a cell happens upon an excess of phosphate, it is advantageous to be able to store it as polyphosphate for future use. Metachromatic granules are found in algae, fungi, and protozoa, as well as in bacteria. These granules are characteristic of Corynebacterium diphtheriae .

12. E. Glycogen bodies and F. Starch Granules Glycogen and starch, which are a polymer of glucose, acts as C storage. Like PHA, glycogen and starch are a storehouse of both carbon and energy. Glycogen is produced when carbon is in excess in the environment and is consumed when carbon is limited. Glycogen resembles starch, the major storage reserve of plants, but differs slightly from starch in the manner in which the glucose units are linked together. when iodine is applied to the cells, in the presence of iodine, glycogen granules appear reddish brown and starch granules appear blue. Starch granules are found in E.coli . Glycogen bodies are found in Proteus, Klebsiella , Shigella , Bacillus , Salmonella and cyanobacteria

13. G. Magnetosomes Some bacteria can orient themselves specifically within a magnetic field because they contain magnetosomes . These structures are intracellular particles of the iron mineral magnetite—Fe 3 O 4. Magnetosomes are formed by several Gram-negative bacteria such as Magnetospirillum magnetotacticum and act like magnets. Bacteria may use magnetosomes to move downward until they reach a suitable attachment site. Magnetosomes impart a magnetic dipole on a cell, allowing it to respond to a magnetic field. Bacteria that produce magnetosomes exhibit magnetotaxis , the process of orienting and migrating along Earth’s magnetic field lines. Magnetosomes are surrounded by a thin membrane containing phospholipids, proteins, and glycoproteins

14. Northern Hemisphere bacteria use their magnetosome chain to determine northward and downward directions, and swim down to nutrient-rich sediments or locate the optimum depth in freshwater and marine habitats. Magnetosomes are intracellular chains of magnetite (Fe 3 O 4 ) or greigite (Fe 3 S 4 ) particles. They are around 35 to 125 nm in diameter and enclosed within invaginations of the plasma membrane. In vitro , magnetosomes can decompose hydrogen peroxide, which forms in cells in the presence of oxygen.

15. H. Sulfur granules Many Gram-negative prokaryotes can oxidize reduced sulfur compounds , such as hydrogen sulfide (H 2 S). The oxidation of sulfide is linked to either reactions of energy metabolism ( chemolithotrophy ) or CO 2 fixation (autotrophy). In either case , elemental sulfur (S ) may accumulate in the cell in microscopically visible globules. However, as the reduced sulfur source becomes limiting , the sulfur in the granules is oxidized to sulfate ( SO4 2- ), and the granules slowly disappear as this reaction proceeds. Interestingly, although the sulfur globules appear to be in the cytoplasm they actually reside in the periplasm . The periplasm expands outward to accommodate the globules as H 2 S is oxidized to S and then contracts inward as S is oxidized to SO 4 2- . Examples: Chromatium vinosum , Thibacillus sp.

16. I. Chlorosomes In some bacteria, bacteriochlorophylls are located in membranous vesicles called chlorosomes . It is found in anoxygenic green bacteria, green sulphur bacteria, green filamentous anoxygenic phototrophs . Chlorosomes were discovered in 1964 and described as rectangular bodies attached to the inner side of cytoplasmic membrane in thin sections of cells of chlorobium sp . A chlorosome is a photosynthetic light harvesting complex. It is composed of proteins, lipids, carotenoids and quinones . In green sulphur bacteria their length varies from 100 to 200 nm, width of 50-100 nm and height of 15-30 nm.