Page 155 - Review of Medical Microbiology and Immunology ( PDFDrive )

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mebooksfree.com mebooksfree.com mebooksfree.com LISTERIA MONOCYTOGENES mebooksfree.com mebooksfree.com
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PART II Clinical Bacteriology
Cutaneous diphtheria causes ulcerating skin lesions
Immunization consists of three doses given at 2, 4, and
covered by a gray membrane. These lesions are often indo-
6 months of age, with boosters at 1 and 6 years of age.
lent and often do not invade surrounding tissue. Systemic
Because immunity wanes, a booster every 10 years is
symptoms rarely occur. In the United States, cutaneous
recommended. Immunization does not prevent naso-
diphtheria occurs primarily in the indigent.
pharyngeal carriage of the organism.
Laboratory Diagnosis
Laboratory diagnosis involves both isolating the organism Diseases
and demonstrating toxin production. It should be empha-
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sized that the decision to treat with antitoxin is a clinical
Listeria monocytogenes causes meningitis and sepsis in
one and cannot wait for the laboratory results. A throat
newborns, pregnant women, and immunosuppressed
swab should be cultured on Loeffler’s medium, a tellurite
plate, and a blood agar plate. The tellurite plate contains a
is a major cause of concern for the food industry.
tellurium salt that is reduced to elemental tellurium within
the organism. The typical gray-black color of tellurium in
Important Properties
the colony is a telltale diagnostic criterion. If C. diphtheriae
Listeria monocytogenes is a small gram-positive rod
is recovered from the cultures, either animal inoculation or
arranged in V- or L-shaped formations similar to coryne-
an antibody-based gel diffusion precipitin test is performed
to document toxin production. A PCR assay for the pres-
movement that distinguishes it from the corynebacteria,
ence of the toxin gene in the organism isolated from the
which are nonmotile. Colonies on a blood agar plate pro-
patient can also be used. bacteria. The organism exhibits an unusual tumbling
duce a narrow zone of β-hemolysis that resembles the
Smears of the throat swab should be stained with both
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hemolysis of some streptococci.
Gram stain and methylene blue. Although the diagnosis of
Listeria grows well at cold temperatures, so storage of
diphtheria cannot be made by examination of the smear,
the finding of many tapered, pleomorphic gram-positive
of gastroenteritis. This paradoxical growth in the cold is
rods can be suggestive. The methylene blue stain is excel-
called “cold enhancement.”
lent for revealing the typical metachromatic granules.
Pathogenesis
Treatment
Listeria infections occur primarily in two clinical settings: (1)
The treatment of choice is antitoxin, which should be
in the fetus or in a newborn as a result of transmission across
given immediately on the basis of clinical impression
because there is a delay in laboratory diagnostic proce-
and immunosuppressed adults, especially renal transplant
dures. The toxin binds rapidly and irreversibly to cells and,
patients. (Note that pregnant women have reduced cell-
once bound, cannot be neutralized by antitoxin. The func- the placenta or during delivery; and (2) in pregnant women
mediated immunity during the third trimester.)
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tion of antitoxin is therefore to neutralize unbound toxin in
The organism is distributed worldwide in animals,
the blood. Because the antiserum is made in horses, the
plants, and soil. From these reservoirs, it is transmitted to
patient must be tested for hypersensitivity, and medications
for the treatment of anaphylaxis must be available. Serum
ucts, undercooked meat, and raw vegetables. Contact with
sickness (see Chapter 65) may occur after administration of
domestic farm animals and their feces is also an important
antiserum made in horses.
source. In the United States, listeriosis is primarily a food-
Treatment with penicillin G or erythromycin is also rec-
borne disease associated with eating unpasteurized cheese
ommended, but neither is a substitute for antitoxin. Antibi-
and delicatessen meats. Following ingestion, the bacteria
otics inhibit growth of the organism, reduce toxin production,
appear in the colon and then can colonize the female geni-
and decrease the incidence of chronic carriers.
membranes rupture or infect the neonate during passage
Prevention
through the birth canal.
Diphtheria is very rare in the United States because chil- tal tract. From this location, they can infect the fetus if
The pathogenesis of Listeria depends on the organism’s
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dren are immunized with diphtheria toxoid (usually
ability to invade and survive within cells. Invasion of cells
given as a combination of diphtheria toxoid, tetanus
is mediated by internalin made by Listeria and E-cadherin
toxoid, and acellular pertussis vaccine, often abbreviated
on the surface of human cells. The ability of Listeria to pass
as DTaP). Diphtheria toxoid is prepared by treating the
testinal tract depends on the interaction of internalin and
exotoxin with formaldehyde. This treatment inactivates
E-cadherin on those tissues.
the toxic effect but leaves the antigenicity intact.
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