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INTRODUCTION

There are two medically important genera of gram-positive cocci: Staphylococcus and Streptococcus. Two of the most important human pathogens, Staphylococcus aureus and Streptococcus pyogenes, are described in this chapter. Staphylococci 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 do not.

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 593.

STAPHYLOCOCCUS

Diseases

Staphylococcus aureus causes abscesses (Figure 15–1), various pyogenic infections (e.g., endocarditis, septic arthritis, and osteomyelitis), food poisoning, scalded skin syndrome (Figure 15–2), and toxic shock syndrome. It is one of the most common causes of hospital-acquired pneumonia, septicemia, and surgical-wound infections. It is an important cause of skin and soft tissue infections, such as folliculitis (Figure 15–3), cellulitis, and impetigo (Figure 15–4). It is the most common cause of bacterial conjunctivitis.

Methicillin-resistant Staphylococcus aureus (MRSA) is the most common cause of skin abscesses in the United States. It is also an important cause of pneumonia, necrotizing fasciitis, and sepsis in immunocompetent patients.

FIGURE 15–1

Abscess on foot. Note central raised area of whitish pus surrounded by erythema. An abscess is the classic lesion caused by Staphylococcus aureus. (Reproduced with permission from Wolff K, Johnson R (eds): Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)

FIGURE 15–2

Scalded skin syndrome. Note widespread areas of “rolled up” desquamated skin in infant. Caused by an exotoxin produced by Staphylococcus aureus. (Reproduced with permission from Wolff K, Johnson R (eds): Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology. 6th ed. New York: McGraw-Hill, 2009. Copyright © 2009 by The McGraw-Hill Companies, Inc.)

FIGURE 15–3

Folliculitis. Note the multiple, small pustules on the chin and neck. Staphylococcus aureus is the most common cause of folliculitis. (Reproduced with permission from Wolff K, Goldsmith LA, Katz SI et al (eds): Fitzpatrick’s Dermatology in General Medicine. 7th ed. New York: McGraw-Hill, 2008, pg 1699. Copyright © 2008 by The McGraw-Hill Companies, Inc.)

FIGURE 15–4

Impetigo. Lesions of impetigo are crops of vesicles with a “honey-colored” crust. Impetigo is caused by either Staphylococcus aureus or Streptococcus pyogenes. (Reproduced with permission from Wolff K, Johnson R ...

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