Page 44 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 7 Pathogenesis
members of the normal flora and, as such, are not transmit-
ted directly prior to the onset of infection.
frequently from the respiratory and gastrointestinal tracts;
hence transmission to the new host usually occurs via air-
A generalized sequence of the stages of infection is as
follows:
borne respiratory droplets or fecal contamination of food
and water. Organisms can also be transmitted by sexual
(1) Transmission from an external source into the portal
of entry.
nated needles, or biting insects. The transfer of blood,
(2) Evasion of primary host defenses such as skin or
either by transfusion or by sharing needles during intrave-
stomach acid.
nous drug use, can transmit various bacterial and viral
(3) Adherence to mucous membranes, usually by bacte- contact, urine, skin contact, blood transfusions, contami-
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pathogens. The screening of donated blood for Treponema
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rial pili.
pallidum, human immunodeficiency virus (HIV), human
(4) Colonization by growth of the bacteria at the site of
T-cell lymphotropic virus, hepatitis B virus, hepatitis C
adherence.
(5) Disease symptoms caused by toxin production or
infection by these organisms.
invasion accompanied by inflammation.
The major bacterial diseases transmitted by ticks in the
(6) Host responses, both nonspecific and specific
United States are Lyme disease, Rocky Mountain spotted
(immunity), during steps 3, 4, and 5.
fever, ehrlichiosis, relapsing fever, and tularemia. Of these
(7) Progression or resolution of the disease.
five diseases, Lyme disease is by far the most common. Ticks
of the genus Ixodes transmit three infectious diseases: Lyme
DETERMINANTS OF BACTERIAL
PATHOGENESIS
Bacteria, viruses, and other microbes can also be trans-
mitted from mother to offspring, a process called vertical
1. Transmission disease, ehrlichiosis, and babesiosis, a protozoan disease.
transmission. The three modes by which organisms are
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transmitted vertically are across the placenta, within the
An understanding of the mode of transmission of bacteria
and other infectious agents is extremely important from a
birth canal during birth, and via breast milk. Table 7–2
public health perspective, because interrupting the chain of
transmitted vertically. (Horizontal transmission, by con-
transmission is an excellent way to prevent infectious
trast, is person-to-person transmission that is not from
diseases. The mode of transmission of many infectious
mother to offspring.)
diseases is “human-to-human,” but infectious diseases are
also transmitted from nonhuman sources such as soil,
There are four important portals of entry: respiratory
water, and animals. Fomites are inanimate objects, such as
tract, gastrointestinal tract, genital tract, and skin (Table 7–3).
Important microorganisms and diseases transmitted by
towels, that serve as a source of microorganisms that can
cause infectious diseases. Table 7–1 describes some impor-
The important bacterial diseases transmitted by foods
tant examples of these modes of transmission.
Although some infections are caused by members of the
are listed in Table 7–5, and those transmitted by insects are
normal flora, most are acquired by transmission from water are described in Table 7–4.
listed in Table 7–6. The specific mode of transmission of
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TABLE 7–1 Important Modes of Transmission
Clinical Example
Mode of Transmission
I. Human to human

A. Direct contact
Gonorrhea
Intimate contact (e.g., sexual or passage through birth canal)
Dysentery
Fecal–oral (e.g., excreted in human feces, then ingested in food or water)
B. No direct contact
C. Transplacental
Congenital syphilis
Bacteria cross the placenta and infect the fetus
D. Bloodborne
screening of blood for transfusions has greatly reduced this risk
II. Nonhuman to human

Tetanus
A. Soil source Syphilis Transfused blood or intravenous drug use can transmit bacteria and viruses;
Spores in soil enter wound in skin
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Legionnaire’s disease
B. Water source
Bacteria in water aerosol are inhaled into lungs
C. Animal source

1. Directly
Cat-scratch fever
Bacteria enter in cat scratch
Lyme disease
2. Via insect vector
3. Via animal excreta
Hemolytic–uremic syndrome
Bacteria in cattle feces are ingested in undercooked hamburger
caused by E. coli O157
Bacteria on an object (e.g., a towel) are transferred onto the skin
D. Fomite source
Staphylococcal skin infection
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