Q.Describe the classification of bacteria based on their morphology (shape).

Bacteria are single-celled organisms built on a small number of basic shapes: cocci (spherical), bacilli (rod-shaped) and spirilla (spiral). Most bacteria keep one of these shapes throughout their life, but some, called pleomorphic bacteria, can vary in shape rather than showing one fixed form - Rhizobium, the nitrogen-fixing bacterium found in the root nodules of legumes, is a commonly cited example of a pleomorphic bacterium.

Bacteria are also classified by the number and arrangement of the whip-like flagella on their surface, which provide motility. A cell with no flagella is atrichous; one with a single flagellum at one pole is monotrichous; one with flagella at both poles is amphitrichous; one with a tuft of flagella clustered at one pole is lophotrichous; and one with flagella spread over its entire surface is peritrichous. A 'peritrichous bacillus', for example, is simply a rod-shaped cell whose flagella cover its whole surface rather than being confined to one end or pole.

Structurally, a bacterial cell is bounded by a cell wall and a plasma membrane enclosing the cytoplasm. It has no true nucleus; instead its single, usually circular DNA molecule lies free in the cytoplasm and is called the genophore, since it performs the role of a chromosome without the membrane-bound organisation of a eukaryotic one. Many bacteria additionally carry plasmids - small, circular pieces of extra-chromosomal DNA that replicate independently and often carry non-essential but useful genes, such as those for antibiotic resistance; because plasmids can be transferred between cells and engineered to carry a chosen gene, they are of great practical significance in biotechnology. During conjugation, a donor cell extends a tube-like sex pilus that draws it against a recipient cell and provides the channel through which DNA passes between them.