Page 142 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 16 Gram-Negative Cocci
meningococci, especially those in group A, are most likely
cocci are seen in a smear of spinal fluid (see Figure 16–4).
to cause epidemics of meningitis. Group B meningococci
The organism grows best on chocolate agar incubated at
cause many cases of meningitis in developed countries
37°C in a 5% CO atmosphere. A presumptive diagnosis of
because it is not present in the capsular polysaccharide vac-
2
Neisseria can be made if oxidase-positive colonies of gram-
cine (see “Prevention,” later). Overall, N. meningitidis ranks
second to S. pneumoniae as a cause of meningitis but is the
ferentiation between N. meningitidis and N. gonorrhoeae is
most common cause in persons between the ages of 2 and
18 years.
made on the basis of sugar fermentation: meningococci
Meningococci have four important virulence factors: negative diplococci are found (see Figure 16–5). The dif-
ferment maltose, whereas gonococci do not (both organ-
isms ferment glucose). Immunofluorescence can also be
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(1) A polysaccharide capsule that enables the organism
used to identify these species. Tests for serum antibodies
to resist phagocytosis by polymorphonuclear leukocytes
are not useful for clinical diagnosis. However, a procedure
(PMNs). The capsule is the immunogen in several com-
monly used vaccines against meningococci.
meningitis is the latex agglutination test, which detects
(2) Endotoxin, which causes fever, shock, and other
capsular polysaccharide in the spinal fluid.
pathophysiologic changes (in purified form, endotoxin can
reproduce many of the clinical manifestations of
Treatment
meningococcemia).
(3) An immunoglobulin A (IgA) protease that helps
Penicillin G is the treatment of choice for meningococcal
the bacteria attach to the membranes of the upper respira-
axone can also be used. Strains resistant to penicillin have
tory tract by cleaving secretory IgA.
(4) Factor H binding protein (FHBP) on meningo-
rarely emerged, but sulfonamide resistance is common. In
cocci binds Factor H, an inhibitor of complement factor infections. A third-generation cephalosporin such as ceftri-
2007–2008, strains of N. meningitidis resistant to cipro-
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floxacin emerged.
C3b. The presence of Factor H on the surface of meningo-
cocci reduces the opsonizing activity of C3b and reduces
the amount of membrane attack complex produced (see
Prevention
complement action in Chapter 63). FHBP is the immuno-
gen in the vaccine against group B meningococci.
prevent meningococcal disease. Either rifampin or cipro-
floxacin can be used for prophylaxis in people who have
Resistance to disease correlates with the presence of
antibody to the capsular polysaccharide. Most carriers
had close contact with the index case. These drugs are
develop protective antibody titers within 2 weeks of coloni-
preferred because they are efficiently secreted into the
saliva, in contrast to penicillin G.
zation. Because immunity is group-specific, it is possible to
The vaccines against groups A, C, Y, and W-135 menin-
have protective antibodies to one group of organisms yet be
susceptible to infection by organisms of the other groups.
Complement is an important feature of the host defenses,
gen. The vaccine against group B meningococci contains
because people with complement deficiencies, particularly gococci contain the polysaccharide capsule as the immuno-
Factor H binding protein as the immunogen.
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There are three forms of the polysaccharide vaccine for
in the late-acting complement components (C6–C9),
have an increased incidence of meningococcal bacteremia.
use in the United States, all of which contain the capsular
polysaccharide of groups A, C, Y, and W-135 as the immu-
Clinical Findings
actra contains the four polysaccharides conjugated to
The two most important manifestations of disease are
diphtheria toxoid as the carrier protein, whereas Menveo
meningococcemia (see Figure 16–1) and meningitis. The
contains the four polysaccharides conjugated to a nontoxic
most severe form of meningococcemia is the life-threatening
mutant of diphtheria toxin as the carrier protein. Meno-
Waterhouse–Friderichsen syndrome, which is character-
mune, the unconjugated vaccine, contains only the four
ized by high fever, shock, widespread purpura, dissemi-
nated intravascular coagulation, thrombocytopenia, and
conjugate vaccines induce higher titers of antibodies in chil-
adrenal insufficiency. Bacteremia can result in the seeding
dren than does the unconjugated vaccine. The vaccines
of many organs, especially the meninges. The symptoms of polysaccharides (not conjugated to a carrier protein). The
induce similar antibody titers in adults. Note that none of the
meningococcal meningitis are those of typical bacterial
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vaccines contain the group B polysaccharide because it is not
meningitis, namely, fever, headache, stiff neck, and an
immunogenic in humans. A fourth vaccine created for use in
increased level of PMNs in spinal fluid.
gate vaccine that contains only the group A polysaccharide.
Laboratory Diagnosis
In general, the conjugate vaccines are preferred over the
The principal laboratory procedures are smear and culture
unconjugated version. The unconjugated vaccine is effec-
tive in preventing epidemics of meningitis and in reducing
of blood and spinal fluid samples. A presumptive diagnosis
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