Page 128 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 15 Gram-Positive Cocci

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mebooksfree.com mebooksfree.com mebooksfree.com FIGURE 15–10 Endocarditis. Note vegetations (black arrows) mebooksfree.com
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FIGURE 15–8
Pharyngitis. Note erythema of soft palate, uvula,
and posterior pharynx and swelling of the uvula. The most common
bacterial cause of pharyngitis is Streptococcus pyogenes. Note: The
on mitral valve. Viridans streptococci are the most common cause of
curved white lines on the uvula and the palate are artifacts of pho-
subacute bacterial endocarditis. (Reproduced with permission from Longo DL
tography. (Source: Centers for Disease Control and Prevention. CDC #6323.)
2012, pg 1052. Copyright © 2012 by The McGraw-Hill Companies, Inc.)

(2) a-Hemolytic streptococci form a clear zone around et al (eds): Harrison’s Principles of Internal Medicine. 18th ed. New York: McGraw-Hill,
is determined by an amino sugar. For example, Group A
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mebooksfree.com mebooksfree.com mebooksfree.com S. pyogenes. It protrudes from the outer surface of the cell mebooksfree.com
their colonies because complete lysis of the red cells occurs.
β-hemolytic streptococci (S. pyogenes) is distinguished
β-Hemolysis is due to the production of enzymes (hemoly-
from Group B β-hemolytic streptococci (S. agalactiae)
sins) called streptolysin O and streptolysin S (see “Patho-
because it has a different C carbohydrate.
genesis” later).
(2) M protein is the most important virulence factor of
(3) Some streptococci are nonhemolytic (γ-hemolysis).
and blocks phagocytosis (i.e., it is antiphagocytic). It inac-
There are two important antigens of β-hemolytic
tivates C3b, a component of complement that opsonizes
streptococci:
the bacteria prior to phagocytosis (see Chapter 63). Strains
(1) C carbohydrate determines the group of β-hemolytic
of S. pyogenes that do not produce M protein are
streptococci. It is located in the cell wall, and its specificity
nonpathogenic.
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mebooksfree.com mebooksfree.com mebooksfree.com FIGURE 15–11 Streptococcus pyogenes—Gram stain. Arrow mebooksfree.com
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FIGURE 15–9
Cellulitis. Note erythema and swelling of the dor-
sum of the foot. Streptococcus pyogenes is the most common cause of
cellulitis. (Reproduced with permission from Usatine, RP et al: The Color Atlas of Family
points to a long chain of gram-positive cocci. (Used with permission from
Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Medicine, New York: McGraw-Hill, 2009. Courtesy of Richard P. Usatine, MD.)

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