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SUMMARIES OF MEDICALLY IMPORTANT BACTERIA

GRAM-POSITIVE COCCI (CHAPTER 15)

Staphylococcus aureus

  • Diseases—Abscesses of many organs, skin and soft tissue infections, endocarditis, osteomyelitis, septic arthritis, and impetigo. Also hospital-acquired pneumonia, surgical wound infections, and sepsis. Also exotoxin-mediated diseases such as gastroenteritis (food poisoning), toxic shock syndrome, and scalded skin syndrome. It is one of the most common causes of human infections.

  • Methicillin-resistant Staphylococcus aureus (MRSA) is the most common cause of skin abscesses in the United States. MRSA is also an important cause of pneumonia, necrotizing fasciitis, and sepsis in immunocompetent patients. S. aureus, especially MRSA, is the most common cause of infections in users of intravenous drugs.

  • Characteristics—Gram-positive cocci in clusters. Coagulase-positive. Catalase-positive. Most isolates produce β-lactamase. Some isolates have an altered penicillin-binding protein making it resistant to methicillin and nafcillin. These are called MRSA strains.

  • Habitat and Transmission—Main habitat is human nose; also found on human skin. Transmission is via hands.

  • Pathogenesis—Abscess containing pus is the most common lesion. These pyogenic lesions are often in the skin but can occur in any organ if S. aureus enters the bloodstream.

  • Three exotoxins can also be made if the specific strain contains the genes that encode the toxin. Toxic shock syndrome toxin is a superantigen and causes toxic shock syndrome by stimulating many helper T cells to release large amounts of lymphokines, especially interleukin (IL)-2. Enterotoxin, which causes food poisoning, is also a superantigen. Food poisoning has a short incubation period because it is preformed in food. Scalded skin syndrome toxin is a protease that cleaves desmoglein in tight junctions in the skin.

  • Protein A is an important virulence factor because it binds to the heavy chain of IgG. This reduces phagocytosis because the gamma heavy chain cannot bind to its receptor on the surface of neutrophils and macrophages. Predisposing factors to infection include breaks in the skin, foreign bodies such as sutures, neutrophil levels below 500/μL, intravenous drug use (predisposes to right-sided endocarditis), and tampon use (predisposes to toxic shock syndrome).

  • Laboratory Diagnosis—Gram-stained smear and culture. Yellow or gold colonies on blood agar; colonies often β-hemolytic. S. aureus is coagulase-positive; Staphylococcus epidermidis is coagulase-negative. Serologic tests not useful.

  • Treatment—Penicillin G for sensitive isolates; β-lactamase–resistant penicillins such as nafcillin for resistant isolates; vancomycin for isolates resistant to nafcillin. About 85% are resistant to penicillin G. Plasmid-encoded β-lactamase mediates most resistance. Resistance to methicillin and nafcillin is caused by changes in penicillin-binding proteins. Vancomycin-resistant strains have emerged.

  • Prevention—Cefazolin is used to prevent surgical wound infections. No vaccine is available. Handwashing reduces transmission.

Staphylococcus epidermidis

  • Diseases—Endocarditis on prosthetic heart valves, prosthetic hip infection, intravascular catheter infection, cerebrospinal fluid shunt infection, neonatal sepsis.

  • Characteristics—Gram-positive cocci in clusters. Coagulase-negative. Catalase-positive.

  • Habitat and Transmission—Normal flora of the human skin and mucous membranes. It is probably the patient’s ...

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