Shapes and Arrangement of Bacteria
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-likeÂ 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 (diploÂbacilli) 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 filaÂmentous 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).
BeneathÂ 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.