Chapter 15: Gram-Positive Cocci

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 differentiated 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 Chapter 70.

STAPHYLOCOCCUS

1. Staphylococcus aureus

Diseases

S. 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, including the site of insertion of cardiac pacemakers. 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 a common cause of bacterial conjunctivitis.

Methicillin-resistant S. aureus (MRSA) is the most common cause of skin abscesses in the United States. ...