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Structure and Functions of Bacterial Cell Envelope
The outer layer or cell envelope provides a structural and physi-ological barrier between the protoplasm (inside) of the cell and the external environment. The cell envelope protects bacteria against osmotic lysis and gives bacteria rigidity and shape. The cell envelope primarily consists of two components: a cell wall and cytoplasmic or plasma membrane. It encloses the proto-plasm, which consists of (i) cytoplasm, (ii) cytoplasmic inclu-sions (mesosomes, ribosomes, inclusion granules, vacuoles), and (iii) a single circular DNA (Fig. 2-7).
Bacterial cytoplasm is a colloidal suspension of a variety of organic and inorganic solutes in a viscous watery solution. The matrix is largely formed by nearly 70% water. Cytoplasm contains all the biosynthetic components required by a bacterium for growth and cell division, together with genetic material. Prokaryotic cytoplasm, unlike that of eukaryotes, lacks endoplasmic reticulum and mitochon-dria. It also does not show any protoplasmic streaming. Bacteria lack a true cytoskeleton. The cytoplasm consists of ribosomes, mesosomes, and intracytoplasmic inclusions bodies.
Ribosomes: The cytoplasmic matrix often is packed with ribo-somes. Ribosomes look like small, featureless particles at low magnification in electron micrographs. They are smaller than their eukaryotic counterpart with sedimentation of 70S, com-pared with 80S in eukaryotes. They consist of two subunits of 30S and 50S, giving a net 70S. Ribosomes are important because:
· They serve as the sites of protein synthesis; matrix ribo-somes synthesize proteins destined to remain within the cell, whereas plasma membrane ribosomes make proteins for transport to the outside.
· They are also the sites of actions of several antibiotics, such as amino glycosides, macrolides, and tetracyclines.
Mesosomes: These are vesicular convoluted or multilaminatedstructures formed as invagination of the plasma membrane into the cytoplasm. Mesosomes are of two types—septal and lateral. The septalmesosome attached to the bacterial DNA is believed to coordinate nuclear and cytoplasmic divisionsdur-ing binary fission. The function of lateral mesosomes still remains to be known. Mesosomes are analogous to the mito-chondria of eukaryotes and are the principal sites of respiratory enzymes in bacteria.
Intracytoplasmic inclusion bodies: Intracytoplasmicinclu-sion bodies are present in the protoplasm of bacteria. Their main function is believed to be of storage. This occurs when their main constituent element is present in excess in the cul-ture medium. Since inclusion bodies are used for storage, their quantity can vary depending on the nutritional status of the cell. They are the sources of carbon, inorganic substances, and energy. Some inclusion bodies also function to reduce osmotic pressure.
They may be of two types: (i) organic inclusion bodies, which usually contain either glycogen or polyhydroxybutyrate, and (ii) inorganic inclusion bodies, which may be of polyphosphate granules or sulfur granules. Examples of intracytoplasmic inclusion bodies include metachromatic granules or volutin granules, starch inclusions, and lipid inclusions. Volutin granules, typically present in C. diphtheriae, can be demonstrated by Albert’s stain. Similarly, starch granules present in the bacteria can be demonstrated on staining with iodine. Lipid inclusion found in Mycobacteria is demonstrated by Sudan black dye.
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