What Are Different Types of Bacteria?

Scientists commonly classify bacteria based on three factors: the bacteria's need for oxygen, its reactions to the Gram test, and its shape. Shape is perhaps the easiest way to classify bacteria, which may look like spheres, rods, or spirals. There are also exceptions to these shapes.

Spheres

• Spherical bacteria are called coccoid bacteria, and scientists give them more specific names depending on how they divide. For example, if coccoid bacteria stay attached in pairs, they are called diplococci. If they stay attached in groups of four, they are called tetrads. And if they stay attached in groups of eight (usually in cubic form), they are called sarcinae. Coccoid bacteria can also form other shapes. Streptococci forms chains, and staphylococci have no regular shape at all. Certain types of staphylococci are common on human skin.

Rods

• Rods come in many different types. Some rods, for instance, are curved, and others are shaped like cigars, ovals, or staffs. Like coccoid bacteria, they can be further classified based on how they divide. Diplobacilli stay attached end-to-end, forming pairs; streptobacillia form chains; and coccobacillia have no distinct shape.

  One important type of rod-shape bacteria is called a nitrogen-fixing bacteria. Through chemical processes, these bacteria change nitrogen gas into the ammonium found in the earth's soil.

Spirals

• Spiral-shaped bacteria come in three different forms: vibrio, spirillum, or spirochete. A vibrio looks like a comma; take for example, the Bifidobacterium spp., which occurs naturally in the large intestine. A spirillum, on the other hand, is a thick, stiff spiral. The spirochete is thin and supple.

Other Shapes

• With the billions of different bacteria on and in the earth, other shapes also exist. To name just a few, bacteria may be lobed, star-shaped, square-shaped, spindle-shaped, or filamentous. Filamentous bacteria, for example, are used in sewer and waste water management systems.

Pleomorphic Bacteria

• Bacteria that retain their shape are called monomorphic; thos that do not are called polymorphic. Polymorphic bacteria, like the Mycoplasma pneumoniae, can assume a number of different shapes, even within the same culture. As its name suggests, Mycoplasma pneumoniae causes walking pneumonia, and it is so small that it resembles a large virus in size. Other types of polymorphic bacteria include Rickettsia rickettsii, which causes Rocky Mountain spotted fever, and Bacillus anthracis, which causes anthrax.