Types of Morphological Bacteria

There are about 1,650 known species of bacteria--of these, about 900 constitute "true bacteria" or eubacteria, while the remainder constitutes a group called "higher bacteria." True bacteria are the simplest and the most common. They are either spherical, ovoid or rod-shaped (i.e., the rod being straight, curved or spiral). Most true bacteria are disease-causing microorganisms in humans. The higher bacteria are grouped into five orders (a) actinomycetales or fungus-like bacteria, (b) chlamydobacteriales or alga-like bacteria, (c) myxobacteriales or slime bacteria, (d) spirochaetales or protozoa-like bacteria, and (e) ricketsiales or minute and rod-shaped bacteria.

General Morphology of the True Bacteria

• True bacteria are the smallest and structurally simplest of all living organisms. True bacteria are strictly unicellular and solitary. In some forms, group of cells are embedded in a mucilage layer. Unicellular bacterial cells vary in shape and fundamentally are of the four groups: (a) spherical or coccus type, (b) bacillus type or rod-shaped, (c) spirillum type or spiral-shaped, and (d) vibrio or comma-like.

Spherical or Coccus type

• Coccus bacteria are spherical or ellipsoidal and such cells are called "cocci." Cocci may be of different forms. When the coccus (singular) occurs singly, it is celled micrococcus, but normally cocci occur in colonies; if coccus exists in pairs, the organism is called diplococcus (i.e., Diplococcus pneumoniae) which causes pneumonia. When coccus occurs in a chain, it is known as streptococcus. If the colony occurs in the form of grape-like clusters, it is called staphylococcus (e.g., Staphylococcus aureus which causes boils). Cocci forming colonies in cubical packets of eight or more are called sarcina (e.g., the air-borne saprophyte, Sarcina lutea). All these types of association are due to the differences in the plane of cell divisions and non-separation of daughter cells.

Rod-shaped or Bacillus Type

• Bacillus type bacteria are elongated and either cylindrical or straight rod-like and such cells are called bacilli. It includes important genera such as: Bacillus, Salmonella, Clostridium and Pseudomonas. Bacilli again may be of different types: single, diplobacilli (when in pairs of cells), and streptobacilli (when in groups of cells forming a chain).

Spiral-Shaped

• Spirillum type bacteria are spirally coiled or long corkscrew-shaped, spiral cells known as spirilla. Important genera under this are: Spirillum, Microspira, Rhodospirillum and Leptospira.

Vibrio Type

• In the comma-like or vibrio type bacteria, rod-like cells are simply curved so that they look like comma. Important species of this type include Vibrio comma.