Classification of Bacteria

Bacteria and blue-green algae are prokaryotes—that is, they are organisms that lack membranes surrounding their genetic material. Today most scientists place prokaryotes in the kingdom Monera. Formerly, bacteria and blue-green algae were considered to be primitive plants and were classified in the plant kingdom.

Bacteria are generally classified by two methods. The simplest and oldest method is by shape. The three principal categories are:


(singular: Bacillus), rod-shaped bacteria; the most numerous of all types. They include coccobacilli and streptobacilli.


(singular: Coccus), spherical bacteria. This group is divided into bacteria that occur in pairs, such as the diplococci; in clusters, such as the staphylococci; and in chains, such as the streptococci.


(singular: Spirillum), spiral-shaped bacteria; the least numerous type.

Some scientists add a fourth category, the vibrios (S-shaped or comma-shaped bacteria) to this list.

The second method of classification is based on other characteristics, such as size, color, and chemical composition. An abbreviated and simplified version of this system follows.

Order Actinomycetales,

branching, filamentous bacteria that have by fungus-like growth. They are important in soil fertilization and as a source of antibiotics.

Order Beggiatoales,

filamentous bacteria that move along a surface in a creeping or gliding motion. They use photosynthesis to manufacture food. Many oxidize sulphur compounds.

Order Chlamydobacteriales,

bacteria encased in a protective covering called a sheath. They grow in algae-like clusters and use iron compounds for growth.

Order Eubacteriales,

rigid-cell bacteria that are rod-shaped or coccus-shaped. Most are immobile.

Order Hyphomicrobiales,

rigid-cell bacteria that reproduce by budding.

Order Myxobacteriales,

flexible, rod-shaped bacteria. They excrete slime and some produce appendages that are visible to the unaided eye.

Order Rickettsiales,

small, rod-shaped or coccus-shaped bacteria that are internal parasites in humans and other animals. Most are pathogenic.

Order Spirochaetales,

the spirochetes—long, thin, spiral-shaped bacteria. Most are pathogenic.

Most scientists classify bacteria in the phylum Schizophyta of the kingdom Monera.