2.1.1: Cellular Morphology and Structure

Morphology refers to the appearance or shape of a bacterium. All the cells of a given species will generally have the same shape and size. Three basic shapes are observed in bacteria: spheres, rods, and spirals.

Bacteria that are spherical in shape are referred to as cocci (singular, coccus). Those that are rod-shaped are referred to as bacilli (singular, bacillus). There are two categories of bacteria that appear spiral-shaped.

Those that have rigid cell walls are referred to as spirilla (singular, spirillum), while those that have flexible cell walls are known as spirochetes. Spirilla can also be differentiated from spirochetes, due to the presence of external flagella. Spirochetes, on the other hand have structures known as axial filaments endoflagellum), that allow their cells to rotate in a “corkscrew” manner. Coccobacillus bacteria are cells that have a shape intermediate to that of a coccus and a bacillus; they appear as short plump rods. Cells that appear as curved rods are referred to as vibrio. Some species will exhibit a variety of shapes and sizes, such bacteria are referred to as pleomorphic.

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