Fundamentals of Microbiology

1.2.1 Prokaryotic Cell Structure

1.2.1 Prokaryotic Cell Structire

Bacteria vary in size and shape. The most common shapes are coccus, bacillus and spirillum.

Coccus: Roughly spherical, Cocci cells exist as individual cells or in characteristic arrangements.

1. In pairs which is called diplococcus. eg. Neisseria.sp.

2. As long chains – eg. Streptococcus sp.

3. In irregular grape like clumps eg. Staphylococcus sp.

4. In tetrads eg. Micrococcus sp.

Bacillus: rod shape eg. Bacillus spp.

Bacilli differ in their length – to width ratio. Coccobacilli – resemble cocci as they are short and wide. The shape of the rod’s end often varies – may be flat, rounded, and cigar-shaped or bifurcated. Rods may occur in pairs or chains. Rods may also are curved to form commas or incomplete spirals.

Some bacteria form long multinucleate filaments or hyphae – eg. Actinomycetes.

Sprillum: Some are shaped like long rods twisted into spirals or helix. They are called spirilla (rigid) and spirochetes (if flexible). There are also other shapes like oval or pear shaped bacteria, or even flat, square to rectangular box shaped.

There are some bacteria which are variable in shape and lack a characteristic form. They are called pleomorphic.

Size:

Some members of Mycoplasma are only 100 - 200 nm in diameter.

Escherichia coli is 1.1 to 1.5 µ m wide by 2.0 to 6.0 µm long. There are a few large bacteria whose size reaches 500 µm in length – eg. Spirochetes.

Epulopiscium fishelsoni is an exceptionally long bacterium measuring 80 µm x 600 µm.