Bacteria a very small, unicellular micro-organisms. Unicellular means they are made of one cell. Bacteria are typically between 1-2 µm. 1 -2 micrometre is 0.001-0.002 mm.

Most bacteria are beneficial (useful to humans), but some are harmful (pathogenic) and cause disease and illness.

Bacteria have one of four common shapes: - bacilli (rod- shaped), cocci (spherical), vibrio (comma-shaped), and spirilla (spiral shaped).

Some bacteria clump or group together.

Shapes of Bacteria

Cocci bacteria

Cocci bacteria (singular is coccus bacterium) are spherical shaped bacteria.

Diplococcus

These are two cocci bacteria joined together. E.g. Neisseria meningitidis bacteria cause bacterial meningitis.

Streptococcus

These are many cocci bacteria joined together in chains. E.g. Streptococcus pyogenes bacteria cause Strep throat infections.

Staphylococcus

These are many cocci bacteria joined together in clumps. E.g. Staphylococcus aureus bacteria cause skin infections, but can cause pneumonia, bone and joint infections.

Bacilli bacteria

These are rod shaped bacteria that can have additional structures, such as flagella or pili.

E.g. Salmonella species & Escherichia coli (E. coli).

One bacillus bacterium, lots of bacilli.

Vibrio bacteria

Vibrio

Vibrio bacteria form bent rods or are comma shaped bacteria. E.g. Vibrio cholerae bacteria cause the disease cholera.

Spirilla bacteria

Spirilla

Spirilla bacteria (singular is spirillum bacterium) are spiral-shaped bacteria. E.g. Treponema pallidum which causes syphilis.

Classification of Bacteria

Bacteria can also be classified according to whether they stain pink or not with a stain called the Gram stain. Bacteria that stain pink or red are called Gram negative. Those that do not stain pink (i.e. stain stays purple) are called Gram positive.

Gram positive vs Gram negative

Gram negative bacteria (left image) have a thicker cell wall than Gram positive bacteria (right image).

Structure of a Typical Bacterial Cell.

Bacterial cells have some structures/organelles in common with animal and plant cells, such as: cytoplasm, cell membrane (plasma membrane), DNA and a cell wall. However, it also can have other specialised structures.

The diagram below shows some of these structures and their function (job).

Video 1 - Bacterial Cell Structure & Function

Bacterial Life Processes

Click on the buttons below to find out how carry out their life processes.

Nutrition

Growth

Reproduction

Respiration

Excretion

Movement & Sensitivity

Bacterial Diseases

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