Page 61 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART I Basic Bacteriology
50
hyaluronic acid in the subcutaneous tissue, allowing the organ-
ism to spread rapidly.
targets within the cell and therefore cause a variety of diseases
with characteristic symptoms (see Tables 7–9 and 7–10). Sev-
• IgA protease degrades secretory IgA, allowing bacteria to
eral exotoxins are enzymes that attach ADP-ribose to a cell
attach to mucous membranes.
component (ADP-ribosylation). Some exotoxins act by pro-
• The capsule surrounding bacteria is antiphagocytic (i.e., it
teolytic cleavage of a cell component, whereas others act as
retards the phagocyte from ingesting the organism). Mutant
superantigens, causing the overproduction of cytokines.
strains of many pathogens that do not produce capsules are
nonpathogenic. Endotoxins
mebooksfree.com mebooksfree.com mebooksfree.com • Lipid A is the toxic component of LPS. It induces the overpro- mebooksfree.com
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• Inflammation is an important host defense induced by the
• Endotoxins are lipopolysaccharides (LPS) located in the
presence of bacteria in the body. There are two types of inflam-
outer membrane only of gram-negative bacteria. They are not
mation, pyogenic and granulomatous, and bacteria typically
secreted by bacteria.
elicit one type or the other. Pyogenic inflammation, the host
defense against pyogenic (pus-producing) bacteria such as S.
duction of cytokines, such as tumor necrosis factor, interleu-
pyogenes, consists of neutrophils (and antibody and comple-
kin-1, and nitric oxide, from macrophages, which causes the
ment). Granulomatous inflammation, the host defense
symptoms of septic shock, such as fever and hypotension. In
against intracellular, granuloma-producing bacteria, such as M.
addition, LPS activates the complement cascade (alternate
tuberculosis, consists of macrophages and CD4-positive T cells.
pathway), resulting in increased vascular permeability, and the
The type of inflammatory lesion is an important diagnostic
coagulation cascade, resulting in increased vascular permea-
criterion.
bility and disseminated intravascular coagulation.
• Bacteria can evade our host defenses by a process called intra-
cellular survival (i.e., bacteria that can live within cells are • Endotoxins are poorly antigenic, do not induce antitoxins, and
do not form toxoids.
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protected from attack by macrophages and neutrophils). Note
that many of these bacteria (e.g., M. tuberculosis) are not obli-
Typical Stages of an Infectious Disease
gate intracellular parasites (which can grow only within cells),
but rather have the ability to enter and survive inside cells.
the time between the moment the person is exposed to the
Exotoxins
microbe (or toxin) and the appearance of symptoms. The pro-
• Exotoxins are polypeptides secreted by certain bacteria that
drome period is the time during which nonspecific symptoms
alter specific cell functions resulting in the symptoms of dis-
occur. The specific-illness period is the time during which the
ease. They are produced by both gram-positive and gram-
characteristic features of the disease occur. The recovery
negative bacteria, whereas endotoxin is found only in
period is the time during which symptoms resolve and health
gram-negative bacteria.
is restored.
• Exotoxins are antigenic and induce antibodies called antitox-
ins. Exotoxins can be modified to form toxoids, which are • After the recovery period, some people become chronic carri-
ers of the organism and in others latent infections develop.
antigenic but not toxic. Toxoids, such as tetanus toxoid, are
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• Some people have subclinical infections during which they
used to immunize against disease.
remain asymptomatic. The presence of antibodies reveals that
• Many exotoxins have an A–B subunit structure in which the A
a prior infection has occurred.
subunit is the active (toxic) one and the B subunit is the one
that binds to the cell membrane and mediates the entry of the
A subunit into the cell.
SELF-ASSESSMENT QUESTIONS
1. Handwashing is an important means of interrupting the chain
following bacteria is most likely to be transmitted vertically?
of transmission from one person to another. Infection by which
(A) Chlamydia trachomatis
one of the following bacteria is most likely to be interrupted by mother to fetus or newborn child. Infection by which one of the
(B) Clostridium tetani
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handwashing?
(C) Haemophilus influenzae
(A) Borrelia burgdorferi
(D) Shigella dysenteriae
(B) Legionella pneumophila
(E) Streptococcus pneumoniae
(C) Staphylococcus aureus
(D) Streptococcus agalactiae (group B streptococcus)
(E) Treponema pallidum
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