The outer layer or cell envelope provides a structural and physiological 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).
Many bacteria, both Gram-positive and Gram-negative, possess a gel-like layer outside the envelope when growing in their natural environments. When a gel-like layer forms a well-defined condensed layer around the bacterial envelope, it is called a capsule and is demonstrable by a light microscope. When this gel-like layer is narrower, detectable only by indirect serological methods or by electron microscope but not by light microscope, it is called a microcapsule. An amorphous viscid colloidal material secreted by some bacteria extracellularly is termed as loose or free slime or glycocalyx.
The capsule is mostly made up of polysaccharides, often referred to collectively as exopolysaccharides. Exopolysaccharides are some-times neutral homopolysaccharides (e.g., the glucans and fruc-tans of many oral streptococci) or negatively charged (Table 2-4).
However, Bacillus anthracis has a capsule comprising of polyamino acids, such as D-glutamic acid. The D-glutamic acid is probably analogous to the negatively charged polysaccharide capsule.
The capsule is fully hydrated and can be demonstrated by light microscopy in either living or stained bacteria as follows:
Slime layer (S-layer) is a structured paracrystalline protein layer shown by electron microscopy. These are generally composed of a single kind of protein molecule, sometimes with carbohy-drates attached. They are resistant to proteolytic enzymes and protein-denaturing agents. The slime layer protein protects the cell from wall-degrading enzymes and bacteriophages. It plays an important role in the maintenance of cell shape, and it may be involved in cell adhesion to host epidermal surfaces.
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