Bacteria are very small organisms. The smallest bacteria are about 0.1 micron in diameter, while the largest may be 60*6 microns. Bacteria are classified according to their shapes:
(1) Spherical bacteria: these are called cocci (singular coccus). The cells may occur in pairs (diplococci), in groups of four (tetracocci), in bunches (staphylococci), in a bead-like chin (streptococci) or in a cubical arrangement of eight or more (sarcinae).
(2) Rod-like bacteria: these are called bacilli (singular bacillus). They generally occur singly, but may occasionally be found in pairs (diplobacilli) or chains (streptobacilli).
(3) Spiral -shaped bacilli: These are called spirilla (singular, spirillum). Short incomplete spirals are called vibrios or comma bacteria.