Size: The bacterial cells vary greatly in their size. An average cylindrical bacterial cell measures 3μ long and 1μ broad whereas an average spherical cell is 1μ in diameter. The average volume of a bacterial cell is 1 cubic micron. According to Buchanan (1946), one ml. of bacterial culture contains one trillion bacteria of average size.
Shape: The bacterial cells may be rod-like, spiral, or spherical in their shape.
1. Bacillus: The straight rod-like bacteria are called bacilli (singular bacillus) which possess rod-like, kidney or elongated cells. They vary greatly in their length and diameter ranging from 0.6μ -1.2μ long and 0.5μ -0.7μ wide to 3.8μ long and 1-1.2μ wide.
2. Spirillum: The spiral or curved forms are called spirilla in which the cells are spirally coiled or coma-shaped or variously curved. They vary in size from 1.5μ – 4μ long and 0.2μ-04μ wide in Vibrio to upto 50μ long in Spirillum.
3. Coccus: The rounded or spherical forms are called cocci in which the cells are more or less spherical. They are the smallest forms of bacteria. After division, the cells may either separate from each other or may remain joined together to form groups of two cells in Diplococcus. It is a tetrad of four cells in the Micrococcus tetra genus and a chain of cells in Streptococcus. The cocci range in diameter from 0.5μ-1.25μ.