Page 120 - Review of Medical Microbiology and Immunology ( PDFDrive )

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Gram-Positive Cocci

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CHAPTER C ONTENT S

Introduction
Summaries of Organisms
Staphylococcus
Practice Questions: USMLE & Course Examinations
Streptococcus
Streptococcus pneumoniae

INTRODUCTION
There are two medically important genera of gram-positive
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cocci: Staphylococcus and Streptococcus. Two of the most
important human pathogens, Staphylococcus aureus and
Streptococcus pyogenes, are described in this chapter. Staph-
ylococci and streptococci are nonmotile and do not form
spores.
Both staphylococci and streptococci are gram-positive
cocci, but they are distinguished by two main criteria:
(1) Microscopically, staphylococci appear in grapelike
clusters, whereas streptococci are in chains.
(2) Biochemically, staphylococci produce catalase (i.e.,
they degrade hydrogen peroxide), whereas streptococci
whitish pus surrounded by erythema. An abscess is the classic lesion
do not. FIGURE 15–1 Abscess on foot. Note central raised area of
caused by Staphylococcus aureus. (Reproduced with permission from Wolff K,
Johnson R (eds): Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 6th ed.
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mebooksfree.com mebooksfree.com mebooksfree.com States. It is also an important cause of pneumonia, necrotiz- mebooksfree.com
Additional information regarding the clinical aspects of
New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)
infections caused by the organisms in this chapter is pro-
vided in Part IX entitled Infectious Diseases beginning on
page 593.
STAPHYLOCOCCUS
ing fasciitis, and sepsis in immunocompetent patients.
Staphylococcus epidermidis causes prosthetic valve
Diseases
endocarditis and prosthetic joint infections. It is the most
Staphylococcus aureus causes abscesses (Figure 15–1), vari-
common cause of central nervous system shunt infections
ous pyogenic infections (e.g., endocarditis, septic arthritis,
and osteomyelitis), food poisoning, scalded skin syndrome
cus saprophyticus causes urinary tract infections, especially
(Figure 15–2), and toxic shock syndrome. It is one of the
cystitis. Kawasaki syndrome is a disease of unknown etiol-
most common causes of hospital-acquired pneumonia, and an important cause of sepsis in newborns. Staphylococ-
ogy that may be caused by certain strains of S. aureus.
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mebooksfree.com mebooksfree.com mebooksfree.com Staphylococci are spherical gram-positive cocci arranged in 109 mebooksfree.com
septicemia, and surgical-wound infections. It is an impor-
tant cause of skin and soft tissue infections, such as follicu-
Important Properties
litis (Figure 15–3), cellulitis, and impetigo (Figure 15–4). It
is the most common cause of bacterial conjunctivitis.
irregular grapelike clusters (Figure 15–5). All staphylococci
Methicillin-resistant Staphylococcus aureus (MRSA) is
produce catalase, whereas no streptococci do (catalase
the most common cause of skin abscesses in the United

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