Page 133 - Review of Medical Microbiology and Immunology ( PDFDrive )

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PART II Clinical Bacteriology
122
“smoky” urine (due to red cells in the urine). Most patients
recover completely. Reinfection with streptococci rarely
nonenterococci do not.
Although cultures remain the gold standard for the
leads to recurrence of glomerulonephritis.
diagnosis of streptococcal pharyngitis, a problem exists
The disease is initiated by antigen–antibody complexes
on the glomerular basement membrane. Complement is
because the results of culturing are not available for at least
activated and C5a attracts neutrophils that secrete enzymes
that damage the endothelium of the glomerular capillaries.
the office whether antibiotics should be prescribed. For this
reason, rapid tests that provide a diagnosis in approxi-
It can be prevented by early eradication of nephritogenic
streptococci from skin colonization sites but not by admin- 18 hours, and it is beneficial to know while the patient is in
mately 10 minutes were developed.
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istration of penicillin after the onset of symptoms.
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The rapid test detects bacterial antigens in a throat swab
specimen. In the test, specific antigens from the group A
streptococci are extracted from the throat swab with cer-
Acute Rheumatic Fever
Approximately 2 weeks after a group A streptococcal
gens bound to latex particles. Agglutination of the colored
infection—usually pharyngitis—rheumatic fever, charac-
latex particles occurs if group A streptococci are present in
terized by fever, migratory polyarthritis, and carditis, may
the throat swab. The specificity of these tests is high, but the
develop. The carditis damages myocardial and endocardial
sensitivity is low (i.e., false-negative results can occur). If
tissue, especially the mitral and aortic valves, resulting in
the test result is negative but the clinical suspicion of strep-
vegetations on the valves. Uncontrollable, spasmodic move-
ments of the limbs or face (chorea) may also occur. ASO
A rapid test is also available for the detection of group B
titers and the erythrocyte sedimentation rate are elevated.
streptococci in vaginal and rectal samples. It detects the
Note that group A streptococcal skin infections do not
DNA of the organism, and results can be obtained in
cause rheumatic fever. Most cases of pharyngitis caused by tococcal pharyngitis is high, a culture should be done.
approximately 1 hour.
group A streptococci occur in children age 5 to 15 years,
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Viridans group streptococci form α-hemolytic colonies
and hence rheumatic fever occurs in that age group.
on blood agar and must be distinguished from S. pneumoniae
Rheumatic fever is due to an immunologic cross-
reaction between antibodies formed against M proteins of
streptococci are resistant to lysis by bile and will grow in the
S. pyogenes and proteins on the surface of joint, heart and
presence of optochin, whereas pneumococci will not. The
brain tissue. It is an autoimmune disease, greatly exacer-
various viridans group streptococci are classified into spe-
bated by recurrence of streptococcal infections. If strepto-
cies by using a variety of biochemical tests.
coccal infections are treated within 8 days of onset, rheumatic
fever is usually prevented. After a heart-damaging attack of
Serologic
rheumatic fever, reinfection must be prevented by long-
term prophylaxis. In the United States, fewer than 0.5% of
infections. In patients suspected of having rheumatic
group A streptococcal infections lead to rheumatic fever,
fever, an elevated ASO titer is typically used as evidence
but in developing tropical countries, the rate is higher ASO titers are high soon after group A streptococcal
than 5%.
of previous infection because throat culture results are
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often negative at the time the patient presents with rheu-
matic fever. Titers of anti-DNase B are high in group A
Laboratory Diagnosis
Microbiologic
previous streptococcal infection in patients suspected of
Gram-stained smears are useless in streptococcal pharyngi-
having AGN.
tis because viridans streptococci are members of the nor-
mal flora and cannot be visually distinguished from the
Treatment
pathogenic S. pyogenes. However, stained smears from skin
Group A streptococcal infections can be treated with either
lesions or wounds that reveal streptococci are diagnostic.
Cultures of swabs from the pharynx or lesion on blood agar
AGN patients benefit from penicillin treatment after the
plates show small, translucent β-hemolytic colonies in 18 to
onset of the two diseases. In mild group A streptococcal
48 hours. If inhibited by bacitracin disk, they are likely to
infections, oral penicillin V can be used. In penicillin-
be group A streptococci (see Figure 15–13). penicillin G or amoxicillin, but neither rheumatic fever nor
allergic patients, erythromycin or one of its long-acting
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Group B streptococci are characterized by their ability
derivatives (e.g., azithromycin) can be used. However,
to hydrolyze hippurate and by the production of a protein
erythromycin-resistant strains of S. pyogenes have emerged
that causes enhanced hemolysis on sheep blood agar when
combined with β-hemolysin of S. aureus (CAMP test).
drugs in the treatment of streptococcal pharyngitis.
Group D streptococci hydrolyze esculin in the presence of
Clindamycin can also be used in penicillin-allergic patients.
bile (i.e., they produce a black pigment on bile-esculin
Streptococcal pyogenes is not resistant to penicillins.
agar). The group D organisms are further subdivided: the
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