Page 60 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 7 Pathogenesis
proposed must be satisfied to confirm the causal role of an
organism. These criteria are as follows:
etiologic diagnosis. This approach can be illustrated with
two examples: (1) in a patient with a sore throat, the pres-
(1) The organism must be isolated from every patient
ence of a few β-hemolytic streptococci is insufficient for
with the disease.
a microbiologic diagnosis, whereas the presence of many
(2) The organism must be isolated free from all other
would be sufficient, and (2) in a patient with fever,
organisms and grown in pure culture in vitro.
(3) The pure organism must cause the disease in a
part of the normal flora, whereas the same organisms in
healthy, susceptible animal.
the blood are likely to be the cause of bacterial
(4) The organism must be recovered from the inocu- α-hemolytic streptococci in the throat are considered
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endocarditis.
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lated animal.
In some infections, no organism is isolated from the
patient, and the diagnosis is made by detecting a rise in
The second type of situation pertains to the practical,
everyday problem of a specific diagnosis of a patient’s ill-
(amount) of antibody in the second or late serum sample
ness. In this instance, the signs and symptoms of the ill-
should be at least four times the titer (amount) of antibody
ness usually suggest a constellation of possible causative
agents. The recovery of an agent in sufficient numbers
in the first or early serum sample.
PEARLS
those infections that occur at a much higher rate than usual,
• The term pathogen refers to those microbes capable of caus-
ing disease, especially if they cause disease in immunocompe-
and pandemics are those infections that spread rapidly over
large areas of the globe.
tent people. The term opportunistic pathogen refers to Determinants of Bacterial Pathogenesis
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vector. mebooksfree.com
microbes that are capable of causing disease only in immuno-
compromised people.
• Virulence is a measure of a microbe’s ability to cause disease
(i.e., a highly virulent microbe requires fewer organisms to
• The modes of transmission of microbes include both human-
cause disease than a less virulent one). The ID 50 is the number
to-human and nonhuman-to-human processes. Nonhuman
of organisms required to cause disease in 50% of the popula-
sources include animals, soil, water, and food.
tion. A low ID 50 indicates a highly virulent organism.
• Human-to-human transmission can occur either by direct con-
• The virulence of a microbe is determined by virulence factors,
tact or indirectly via a vector such as an insect, notably ticks or
such as capsules, exotoxins, or endotoxins.
mosquitoes. Animal-to-human transmission can also occur
• Whether a person gets an infectious disease or not is deter-
either by direct contact with the animal or indirectly via a
mined by the balance between the number and virulence of
the microbes and the competency of that person’s host • The main “portals of entry” into the body are the respiratory
defenses.
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tract, gastrointestinal tract, skin, and genital tract.
• Many infections are asymptomatic or inapparent because
• Human diseases for which animals are the reservoir are called
our host defenses have eliminated the microorganism before it
zoonoses.
could multiply to sufficient numbers to cause the symptoms of
disease.
• The term infection has two meanings: (1) the presence of
microbes in the body and (2) the symptoms of disease. The
• Pili are the main mechanism by which bacteria adhere to
human cells. They are fibers that extend from the surface of
presence of microbes in the body does not always result in
bacteria that mediate attachment to specific receptors on
symptoms of disease (see the previous bullet).
cells.
• Bacteria cause the symptoms of disease by two main mecha-
nisms: production of toxins (both exotoxins and endotoxins)
and induction of inflammation.
strains of bacteria that mediates strong adherence to certain
structures such as heart valves, prosthetic implants, and
• Most bacterial infections are communicable (i.e., capable of • Glycocalyx is a polysaccharide “slime layer” secreted by some
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catheters.
spreading from person to person), but some are not (e.g., botu-
lism and Legionella pneumonia).
Invasion, Inflammation, & Intracellular Survival
• Three epidemiologic terms are often used to describe infec-
tions: endemic infections are those that occur at a persistent,
usually low level in a certain geographic area, epidemics are
For example, hyaluronidase produced by S. pyogenes degrades
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