C h a p t e r 1 4
Mechanisms of Infectious Disease
303
Pathogenic
Treponema
species require no intermediates
and are spread from person to person by direct contact.
The most important member of the genus is
Treponema
pallidum
, the causative agent of syphilis.
Mycoplasmas.
The mycoplasmas are unicellular pro-
karyotes capable of independent replication. These
organisms are less than one-third the size of bacteria
and contain a small DNA genome approximately one-
half the size of the bacterial chromosome. The cell is
composed of cytoplasm surrounded by a membrane,
but, unlike bacteria, the mycoplasmas do not produce
a rigid peptidoglycan cell wall. As a consequence, the
microscopic appearance of the cell is highly variable,
ranging from coccoid forms to filaments, and the myco-
plasmas are resistant to cell wall–inhibiting antibiotics
such as penicillins and cephalosporins.
The mycoplasmas affecting humans are divided
into three genera:
Mycoplasma, Ureaplasma
, and
Acholeplasma.
The first two require cholesterol from
the environment to produce the cell membrane; the
acholeplasmas do not. In the human host, mycoplas-
mas are commensals. However, a number of species are
capable of producing serious diseases, including pneu-
monia (
Mycoplasma pneumoniae
), genital infections
(
Mycoplasma hominis
and
Ureaplasma urealyticum
),
and maternally transmitted respiratory infections to
infants with low birth weight (
U. urealyticum
).
Rickettsiaceae, Anaplasmataceae,
Chlamydiaceae, and Coxiella
This interesting group of organisms combines the char-
acteristics of viral and bacterial agents to produce dis-
ease in humans. All are obligate intracellular pathogens,
like the viruses, but produce a rigid peptidoglycan cell
wall, reproduce asexually by cellular division, and con-
tain RNA and DNA, similar to the bacteria.
The
Rickettsiaceae
depend on the host cell for essential
vitamins and nutrients, but the
Chlamydiaceae
appear
to scavenge intermediates of energy metabolism such as
adenosine triphosphate (ATP). The
Rickettsiaceae
infect
but do not produce disease in the cells of certain arthro-
pods such as fleas, ticks, and lice. The organisms are
accidentally transmitted to humans through the bite of
the arthropod (i.e., the vector) and produce a number of
potentially lethal diseases, including Rocky Mountain
spotted fever and epidemic typhus.
The
Chlamydiaceae
are slightly smaller than the
Rickettsiaceae
but are structurally similar and are trans-
mitted directly between susceptible vertebrates without
an intermediate arthropod host. Transmission and rep-
lication of
Chlamydiaceae
occur through a defined life
cycle. The infectious form, called an
elementary body
,
attaches to and enters the host cell, where it transforms
into a larger
reticulate body
. This undergoes active
replication into multiple elementary bodies, which are
then shed into the extracellular environment to initi-
ate another infectious cycle. Chlamydial diseases of
humans include sexually transmitted genital infections
(
Chlamydophila trachomatis;
see Chapter 41); ocular
infections and pneumonia of newborns (
C. trachoma-
tis
); upper and lower respiratory tract infections in chil-
dren, adolescents, and young adults (
Chlamydophila
pneumoniae
); and respiratory disease acquired from
infected birds (
Chlamydophila psittaci
).
Organisms within the family
Anaplasmataceae
(including the reorganized genera
Ehrlichia, Anaplasma,
Neorickettsia
, and
Wolbachia
) are also obligate intra-
cellular organisms that resemble the
Rickettsiaceae
in
structure and produce a variety of veterinary and human
diseases, some of which have a tick vector. These organ-
isms target host mononuclear and polymorphonuclear
white blood cells for infection and, similar to the
Chlamydiaceae
, multiply in the cytoplasm of infected
leukocytes within vacuoles called
morulae.
Unlike the
Chlamydiaceae
, however, the
Anaplasmataceae
do
not have a defined life cycle and are independent of
the host cell for energy production. The most com-
mon infections caused by
Anaplasmataceae
are human
monocytic and granulocytic ehrlichiosis. Human
monocytic ehrlichiosis is a disease caused by
Ehrlichia
chaffeensis
that can easily be confused with Rocky
Mountain spotted fever. Clinical disease severity
ranges from mild to life-threatening. Manifestations
include generalized malaise, anorexia and nausea,
fever, and headache. Decreases in white blood cells
(leukopenia) and platelets (thrombocytopenia) often
occur. Severe sequelae include severe respiratory fail-
ure, encephalopathy, and acute renal failure. The dis-
ease is usually more severe in the elderly and persons
with compromised immune function (e.g., those with
HIV/AIDS). Human granulocytic ehrlichiosis is also
transmitted by ticks, but is caused by
Anaplasma
phagocytophilum
. The symptoms are similar to those
seen with human monocytic ehrlichiosis.
FIGURE 14-7.
A photomicrograph of a Treponema pallidum
bacterium that causes syphilis.This microscopic bacterium
(spirochete) is a wormlike spiral-shaped microorganism that
wiggles vigorously when viewed under the microscope. (From
the Centers for Disease Control and Prevention Public Health
Images Library. No. 2323.)
