Section II. BACTERIA
Bacteria are microscopic, unicellular organisms of the Kingdom Monera. They
are widely distributed in nature, occurring nearly everywhere. They are found on and in
our bodies, in the air we breathe, in the food we eat, and in the water we drink. They
are part of the natural flora of the skin, naso-pharyngeal mucosa, and alimentary tract.
They are particularly plentiful in the upper layers of the soil, playing an important part in
the decomposition of organic matter and in nitrogen fixation. There are several
thousand known species of bacteria; however, of this number, only about 100 species
are pathogenic to man. Some also produce disease in the lower animals. The vast
majority of bacteria, however, do not attack man or beast. They are not only harmless,
but they are quite beneficial; without their help, all plant and animal life would cease. In
this section, however, we are concerned with those bacteria that cause disease in man.
Bacteria produce disease in man by invading the body and reproducing (colonizing),
thereby causing damage to the host cells. Reproduction is by binary fission (paragraph
2-3e). Under favorable conditions, bacteria reproduce very rapidly. Some newly
formed bacteria can mature and divide to form two new bacteria every 20 to 30 minutes.
Most bacteria have independent metabolism, enabling them to live and multiply in a
favorable medium outside of a host cell.
MORPHOLOGY OF BACTERIA
Bacteria range in size from 0.15 (microns) in diameter for some of the smaller,
pathogenic cocci, to 20 in length for the larger, nonpathogenic bacilli. The three
principal shapes of bacteria are shown in figure 2-5.
a. Coccus (Plural, Cocci). Spherical bacteria occurring in any of the following
Singly (coccus). Figure 2-5 A.
In pairs (diplococcus). Figure 2-5 B.
In chains (streptococcus). Figure 2-5 C.
In clusters (staphyloccus) Figure 2-5 D.
In clusters of four (tetrad) or eight (cube). Figure2-5 E.
b. Bacillus (Plural, Bacilli). Rod-shaped bacteria occurring in any of the
Singly (bacillus). Figure 2-5 F.