of important human diseases caused by each group of
pathogens are found at the end of this chapter.
BACTERIA (SEE TABLE 22–3)
Bacteria are very simple unicellular organisms. All are
microscopic in size, and a magnification of 1000 times
is usually necessary to see them clearly. A bacterial cell
consists of watery cytoplasm and a single chromosome
(made of DNA) surrounded by a cell membrane.
Enclosing all of these structures is a cell wall, which is
strong and often rigid, giving the bacterium its char-
acteristic shape.
Based on shape, bacteria are classified as one of
three groups: coccus, bacillus, or spirillum (Fig. 22–2).
A coccus(plural: cocci) is a sphere; under the micro-
scope, cocci appear round. Certain prefixes may be
used to describe the arrangement of spheres. Staphylo
means clusters, streptorefers to chains of cells, and
diplomeans pairs of cells.
A bacillus (plural: bacilli) is a rod-shaped bac-
terium; rods may vary in length depending on the
genus. A spirillum(plural: spirilla) is a long cell with
one or more curves or coils. Some spirilla, such as
those that cause syphilis and Lyme disease, are called
spirochetes. Many of the bacilli and spirilla are capa-
ble of movement because they have flagella. These
are long, thread-like structures that project from the
cell and beat rhythmically.
Bacteria reproduce by the process of binary fis-
sion, in which the chromosome duplicates itself, and
the original cell divides into two identical cells. The
presence or absence of oxygen may be important for
bacterial reproduction. Aerobicbacteria can repro-
duce only in the presence of oxygen, and anaerobic
bacteria can reproduce only in the absence of oxygen.
Facultatively anaerobicbacteria are not inhibited in
either situation; they are able to reproduce in either
the presence or absence of oxygen. This is obviously
an advantage for the bacteria, and many pathogens
and potential pathogens are facultative anaerobes.
The Gram Stain
Based on the chemicals in their cell walls, most bacte-
ria can be put into one of two groups, called gram
positiveor gram negative. A simple laboratory pro-
cedure called the Gram stain(see Fig. 22–2) shows us
the shape of the bacteria and their gram reactions.
Gram-positive bacteria appear purple or blue, and
gram-negative bacteria appear pink or red. Some bac-
teria do not stain with the Gram method (many spir-
illa and spirochetes do not), but for those that do, each
genus is either gram positive or gram negative. This
does not change, just as the characteristic shape of the
bacteria does not change. The genus Streptococcus, for
example, is always a gram-positive coccus; the genus
Escherichiais always a gram-negative bacillus. If a
Gram stain is done on a sputum specimen from a
patient with pneumonia, and a gram-positive coccus is
found, this eliminates all of the gram-negative cocci
and bacilli that may also cause pneumonia. The Gram
stain, therefore, is often an important first step in the
identification of the pathogen that is causing a partic-
ular infection. In Table 22–3 (at the end of this chap-
ter), the gram reaction (where applicable) and shape
are included for each pathogen.Special Characteristics
Although bacteria are simple cells, many have special
structural or functional characteristics that help them
to survive. Some bacilli and cocci have capsules (see
Fig. 22–2); a capsuleis a gelatinous sheath that enclo-
ses the entire cell. Capsules are beneficial to the bac-
teria because they inhibit phagocytosis by the host’s
white blood cells. This gives the bacteria time to
reproduce and possibly establish themselves in the
host. This is notbeneficial from our point of view
(remember that we are the hosts), but bacterial cap-
sules are also antigenic, which means that they stim-
ulate antibody production by our lymphocytes. This
starts the destruction of bacteria by our immune
responses. We take advantage of this by using bacter-
ial capsules in some of our vaccines, such as those used
to prevent pneumonia and meningitis.
Some bacilli are able to survive unfavorable envi-
ronments by forming spores. A sporeis a dormant
(inactive) stage that consists of the chromosome and a
small amount of cytoplasm surrounded by a thick wall.
Spores can survive conditions such as heat (even boil-
ing), freezing, or dehydration, which would kill the
vegetative (active) forms of the bacterial cells.
Fortunately for us, most pathogens are unable to form
spores, but some that do are the causative agents of gas
gangrene, botulism, tetanus, and anthrax. These bac-
teria are decomposers in the soil environment, and
their spore-forming ability enables them to survive the
extremes of temperature and lack of water that may
occur in the soil.
Many bacteria cause disease because they produce
toxins, which are chemicals that are harmful to host
tissues. Often these toxins are the equivalent of our508 An Introduction to Microbiology and Human Disease