SHAPE &SIZE OF BACTERIA

Bacteria are classified by shape into three basic groups: coci,

bacilli-and spirochetes. The cocci are round,

the bàcilli are rods, and the spirochetes are spiral-shaped. Some

bacteria are variable in shape and are said to be pleomorphic

(heterogeneous shape). The shape of a bacterium is determined

by its rigid cell wall. The microscopic appearance of a bacteríum

is one of the most important criteria used in its identification.

In addition to their characteristic shapes, the arrangement

of bacteria is important. For example, certain cocci occur in

pairs (diplococci), some in chains (streptococci), and others

in grapelike clusters (staphylococci). These arrangements are

determined by the orientation and degree of attachment of the

bacteria at the time of cell division. The arrangement of rods

and spirochetes is medically less important and is not described

in this introductory chapter

Bacteria range in size from about 0.2 to 5 um ).

The smallest bacteria (Mycoplasma) are about the same size as

the largest viruses (poxviruses) and are the smallest organisms

capable of existing outside a host. The longest bacteria rods are

the size of some yeasts and human red blood cells (7 um).