All bacterial cells invariably have a cell membrane, cytoplasm, ribosome, and chromating bodies. The majority have a cell wall, which gives shape to the bacterial cell. Specific structures like capsule, slime, flagella, Pili, fimbriae and granules are not found in all bacteria.


Bacteria range in size from about 0.1 to 600 µm over a single dimension. Bacteria vary in size as much as in shape. The smallest (e.g., some members of the genus Mycoplasma) are about 100 to 200 nm in diameter, approximately the size of the largest viruses (poxviruses) Escherichia coli, a bacillus of about average size, is 1.1 to 1.5 µm wide by 2.0 to 6.0 µm long. Some spirochetes occasionally reach 500 µm in length whereas staphylococci are 0.75 – 1.25µ in diameter.

Recently a huge bacterium has been discovered in the intestine of the brown surgeonfish, Acanthurus nigrofuscus. Epulopiscium fishelsoni grows as large as 600 µm by 80 µm, a little smaller than a printed hyphen. It is now clear that a few bacteria are much larger than the average eukaryotic cell.

Shape of bacteria

On the basis of general shape, bacteria are classified into three categories. These shapes are known as cocci, bacilli and spiral. Although most of the bacterial species have fairly constant characteristic cell shape, yet some sells are pleomorphic and they can exist in a variety of shapes.

Exceptions to the above shapes are trachoma forming, sheathed, stalked, square, star-shaped, spindle-shaped, lobed and filamentous bacteria.

Fig. 6.1 Shapes of Bacteria

The cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division. If division is in one plane it will produce either a diplococcus or streptococcus arrangement is diplococcus, whereas when cocci form long chain of cells then arrangement is called as streptococci. When the division of cell is in two planes it will produce a tetrad arrangement. A tetrad is a suare of 4 cocci. Thirdly when the division is in three planes, it will produce a sarcina arrangement. Sarcina is a cube of 8 cocci. When division occurs in random planes, it will produce a staphylococcus arrangement in which cocci are arranged in irregular, often grape-like clusters. Diplocooccus pneumonia and staphylococcus aureus are some examples of cocci.


Fig. 6.2 cocci

Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or diplobacillus. Bacillus is a single cell of bacteria. Streptobacillus is a chain of bacilli. When rod shaped bacteria occur in pairs then arrangement of cells is know as diplobacilli examples of rod shaped bacterial are Escherichia coli, bacillus subtilis. Pseudomonas.


Fig. 6.3 Bacilli

The spiral shaped bacteria are spirally coiled. Spirals come in one of three forms, vibrio, a spirillum, or a spirochete. Vibrio is curved or comma-shaped rod. Spirillum is a thick, rigid spiral. Spirochete is a thin, flexible spiral. Examples of spiral shaped bacteria are Vibrio, hyphomicrobium.


Fig. 6.4 spirilla

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