Bacterial Morphology Bacteria can be broadly divided into four types: cocci (round), bacilli (rod)and spirilla (spiral).
Cocci Cocci are spherical, oval, kidney, and chestnut-shaped, and measure approximately 0.5 to 7.0 μm in diameter. They are classified as monococci, streptococci, diplococci, tetra-generic, sarcina, and staphylococci.
Bacilli Bacilli are shaped like cylinders or rods. They may be observed individually, in pairs as diplobacilli, or in chains as seen in streptobacilli. These rod-shaped bacteria include rounded, spiral, and rectangular-shaped forms.
Spirillum Spirilla include spiral and comma-shaped bacteria, where the spiral does not form a complete circle
Bacterial Cytological Structure Somatic antigen (O antigen) The somatic antigen is a hard elastic layer that covers the outside of the cell to maintain the bacterium’s shape and protect its internal structure.
Capsule (K antigen) A polypeptide mucosal layer made from polysaccharide produced by the bacteria is found in some bacteria. When this mucoidal layer is thick and shows a clear structure, it is referred to as the capsule.
Flagellum (H antigen) Certain bacilli such as enteric bacteria possess one or more tails, known as flagellum (pl. flagella), by which they are motile. Depending on their number and position, bacteria are grouped into monotrichous, amphitrichous, and peritrichous bacilli. Vibrio cholera and Vibrio parahaemolyticus are monotrichous while Escherichia coli and Salmonella are peritrichous.
Cell membrane This is a thin membrane inside the cell wall, which covers the outside of the cell, composed mainly of lipids and lipoproteins.
Cytoplasm Cytoplasm contains mitochondria-like granules that consist of RNA and TCA cycles, and oxidation-reduction enzymes.
