302
U N I T 4
Infection and Immunity
disassemble a clogged sink drain to see a perfect exam-
ple of a bacterial biofilm.
The physical appearance of a colony of bacteria
grown on an agar plate can be quite distinctive for dif-
ferent species. Some produce pigments that give colonies
a unique color. Some bacteria produce highly resistant
spores when faced with an unfavorable environment.
The spores can exist in a quiescent state almost indefi-
nitely until suitable growth conditions are encountered,
at which time the spores germinate and the organism
resumes normal metabolism and replication.
Classification.
Bacteria are extremely adaptable life
forms. They are found not just in humans and other
hosts, but in almost every environmental extreme on
earth. However, each individual bacterial species has a
well-defined set of growth parameters, including nutri-
tion, temperature, light, humidity, and atmosphere.
Bacteria with extremely strict growth requirements are
called
fastidious.
For example,
Neisseria gonorrhoeae
,
the bacterium that causes gonorrhea, cannot live for
extended periods outside the human body.
Aerobes
are
bacteria that require oxygen for growth and metabolism;
anaerobes
cannot survive in an oxygen-containing envi-
ronment. An organism capable of adapting its metabo-
lism to aerobic or anaerobic conditions is
facultatively
anaerobic.
It is of interest to note that the vast majority of
bacterial species found in humans and in the surrounding
environment have not been cultivated on artificial media.
In the laboratory, bacteria are generally classified
according to the microscopic appearance and staining
properties of the cell. The Gram stain, originally devel-
oped in 1884 by Danish bacteriologist Christian Gram,
is still the most widely used staining procedure. Bacteria
are designated as
gram-positive
organisms if they are
stained purple by a primary basic dye (usually crystal
violet); those that are not stained by the crystal violet
but are counterstained red by a second dye (safranin)
are called
gram-negative
organisms. Staining character-
istics and microscopic morphology are used in combi-
nation to describe bacteria. For example,
Streptococcus
pyogenes
, the agent of pharyngitis and rheumatic fever,
is a gram-positive streptococcal organism that is spheri-
cal, grows in chains, and stains purple by Gram stain.
Legionella pneumophila
, the bacterium responsible for
Legionnaires’ disease, is a gram-negative rod.
Another means of classifying bacteria according to
microscopic staining properties is the
acid-fast stain
.
Because of the unique fatty acid content and composition
of their cell membrane, certain bacteria are resistant to
the decolorization of a primary stain (either carbol fuch-
sin or a combination of auramine and rhodamine) when
treated with a solution of acid alcohol. These organisms
are termed
acid-fast
and include a number of significant
human pathogens, most notably
Mycobacterium tubercu-
losis
(the cause of tuberculosis) and other mycobacteria.
For purposes of taxonomy (i.e., identification and
classification), each member of the bacterial kingdom
is categorized into a small group of genetically related
organisms called the
genus
, and further subdivided
into distinct individuals within the genus called
spe-
cies.
The genus and species assignment of the organ-
ism is reflected in its name (e.g.,
Staphylococcus
[genus]
aureus
[species]).
Spirochetes.
The spirochetes are an eccentric category
of bacteria that are mentioned separately because of
their unusual cellular morphology and distinctive mech-
anism of motility. Technically, the spirochetes are gram-
negative rods but are unique in that the cell’s shape is
helical and the length of the organism is many times its
width (Fig. 14-7). A series of filaments are wound about
the cell wall and extend the entire length of the cell.
These filaments propel the organism through an aque-
ous environment in a corkscrew motion.
Spirochetes are composed of four genera:
Leptospira,
Borrelia, Treponema
, and
Brachyspira
. Each genus has
saprophytic and pathogenic strains. The pathogenic
leptospires infect a wide variety of wild and domestic
animals. Infected animals shed the organisms into the
environment through the urinary tract. Transmission
to humans occurs by contact with infected animals or
urine-contaminated surroundings. Leptospires gain
access to the host directly through mucous membranes
or breaks in the skin and can produce a severe and
potentially fatal illness called
Weil syndrome.
In con-
trast, the borreliae are transmitted from infected ani-
mals to humans through the bite of an arthropod vector
such as lice or ticks. (A vector is any organism that car-
ries a pathogen from a source to a host.) Included in the
genus
Borrelia
are the agents of relapsing fever (
Borrelia
recurrentis
) and Lyme disease (
Borrelia burgdorferi
).
Included in the genus
Borrelia
are the agents of relapsing
fever (
Borrelia recurrentis
) and Lyme disease (
Borrelia
burgdorferi
is the most common in North America).
FIGURE 14-6.
Electron micrograph depicting large numbers
of Staphylococcus aureus bacteria, which were found on
the luminal surface of an indwelling catheter. Of importance
are the sticky-looking substances woven between the round
cocci bacteria, which were composed of polysaccharides
and are known as biofilm.This biofilm has been found to
protect the bacteria that secrete the substance from attacks by
antimicrobial agents such as antibiotics (magnified × 2363).
(From the Centers for Disease Control and Prevention Public
Health Images Library. No. 7488. Courtesy of Rodney M.
Donlan, Janice Carr.)
