Special units of measurement are used to state the size of microorganisms. Size
is usually measured in microns and the symbol for micron is , the Greek letter mu. A
micron is equivalent to 1/1000th of a millimeter or 1/25,400th of an inch. To give you a
better idea of the size of a micron, the diameter of a metal pinhead is about 2000
microns. The range in size of bacteria is wide. Some cells are so small that they can
barely be seen by the best compound light microscopes whereas others are large
enough to be almost visible with the unaided eye. Among the ball- or round-shaped
bacteria, the diameter varies from 0.5 to 2.5 microns. The variations are much greater
in the rod-like species--from 0.2 to 2.0 microns in width and from 1 to 15 microns in
length. Most of the bacteria of importance in foods range from 2 to 10 microns in length
and 0.5 to 2.5 microns in diameter. The size of viruses is much smaller, the largest
being about 0.2 micron and the smallest about 0.012 micron.
BACTERIA MORPHOLOGY AND PHYSIOLOGY
a. Definition. Bacteria are microscopic, unicellular microorganisms, containing
no well-defined nucleus. Most bacteria are devoid of chlorophyll and reproduce
asexually by division (binary fission).
(1) The cell (figure 1-1) is the structural unit of all living organisms. The
typical cell is composed of cytoplasm surrounded by a cell wall, and there is a nucleus
near the center of the cell. Because of their small size and controversial nature, there is
no generally accepted opinion that the bacterial cell has a nucleus. The outer part of
the bacterial cell is made up of three structures we call the cell wall, the cytoplasmic
membrane, and the capsule. The cell wall limits the volume occupied by the cytoplasm
and gives shape and rigidity to the cell. Inside the cell wall is the cytoplasmic
membrane that surrounds the cytoplasm.
Figure 1-1. Bacterial cell structure.