Shapes of Bacteria occur in three main shapes, spherical, rod-like and spiral.

(1) Spherical bacteria are called cocci (singular coccus). The cells may occur in pairs (diplococci), in groups of four (tetracocci), in bunches (staphylococci), in a bead-like  chain(streptococci)or in a cubical arrangement of eight or, more (sarcinae).

(2) Rod-like bacteria are called bacilli (singular bacillus). They generally occur singly, but may occasionally be found in pairs (diplo­bacilli) or chains (streptobacilli).

(3) .Spiral-shaped bacilli are called spirilla (singular, spirillum). Short incomplete spirals are called vibrios or comma bacteria.

Many bacteria are surrounded by a capsule.

Beneath the capsule is a cell wail which gives shape to the cell.

Many bacteria have fila­mentous appendages called fimbriae or pili.

Some bacteria have one, two or many flagella, which may arise from on or both ends of the cell (polar),or ma y arise from all over the cell (peritrichous).

Beneath­ the cell wall is a fine plasma membrane.

Folded invaginations of the plasma membrane, called mesosomes, are present in many bacteria.

The nucleoid is in the form of a highly folded DNA ring without a nuclear envelope.

The cytoplasm contains 70S ribsomes, an internal membrane system, storage granules and, in photosynthetic bacteria, chromatophores.

The details of the various structures will now be taken up.

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