Structure and Functions of Bacterial Cell Envelope
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).
The bacterial nucleus is neither enclosed in a nuclear membrane nor associated with any nucleolus. It is haploid and replicates by simple fission. The nucleus of the bacteria consists of a single circle of double-stranded deoxyribonucleic acid (DNA), arranged in a supercoiled circular structure. It measures about 1000 mm when straightened.
The chromosome is located in an irregularly shaped region called nucleoid, but often referred to as bacterial chromosome because of the analogy with the eukaryotic structure. The nucleoid is visible through the light microscope after staining with the Feulgen stain, which specifically reacts with DNA.
In actively growing bacteria, the bacterial DNA can account for up to 20% of the volume of the bacterium and has projec-tions that extend into the cytoplasmic matrix. Careful electron microscopic studies often have shown the nucleoid to be in contact with either the mesosome or the plasma membrane.
Many bacteria also possess smaller circles of extrachromo-somal DNA called plasmids. The plasmids are double-stranded DNA molecules, usually circular, that can exist and replicate independently. Plasmids are not required for host growth and reproduction, although they may carry genes that confer the bacterium with properties such as antibiotics resistance or the capacity to produce toxins or enzymes.