Skin and Soft Tissue InfectionsS. aureus causes a variety of cutaneous infections characterized by pus-containing blisters, many of which can also be caused by group A streptococci and other streptococcal species. Predisposing factors include skin disease (e.g., eczema), skin damage (e.g., minor trauma), injections, and poor personal hygiene.

• Infections can be superficial (e.g., folliculitis, cellulitis, impetigo) or deep and painful (e.g., furuncles, carbuncles, hidradenitis suppurativa).
  • - Carbuncles (often located in the lower neck) are more severe and painful than furuncles (boils that extend from hair follicles) and are due to coalesced lesions extending to deeper SC tissue.
  • - Mastitis in lactating women can range from superficial cellulitis to abscess.

Musculoskeletal Infections See Chap. 84. Infections of the Skin, Soft Tissues, Joints, and Bones for additional details.

• S. aureus is among the most common causes of osteomyelitis arising from either hematogenous dissemination or contiguous spread from a soft tissue site (e.g., diabetic or vascular ulcers).
  • - Hematogenous osteomyelitis in adults is often vertebral and occurs in pts with endocarditis, pts undergoing hemodialysis, injection drug users, or diabetics. Intense back pain and fever can occur, but infections may also be clinically occult.
  • - Epidural abscess is a serious complication that can present as trouble voiding or walking or as radicular pain in addition to symptoms of osteomyelitis; neurologic compromise can develop in the absence of timely treatment, which often requires surgical intervention.
  • - Osteomyelitis from contiguous soft tissue infections is suggested by exposure of bone, a draining fistulous tract, failure to heal, or continued drainage.
• S. aureus is the most common cause of septic arthritis in native joints of both adults and children. S. aureus septic arthritis in adults may result from trauma, surgery, or hematogenous dissemination.
  • - The joints most commonly affected are the knees, shoulders, hips, and phalanges.
  • - Examination of synovial fluid reveals >50,000 PMNs/µL and gram-positive cocci in clusters on Gram's stain.
• Pyomyositis, an infection of skeletal muscles that is seen in tropical climates and in immunocompromised pts (e.g., HIV-infected pts), causes fever, swelling, and pain overlying involved muscle and is usually due to S. aureus.

Respiratory Tract Infections

• Newborns and infants can develop serious infections characterized by fever, dyspnea, and respiratory failure; pneumatoceles (shaggy, thin-walled cavities), pneumothorax, and empyema are known complications.
• Community-acquired pneumonia usually follows viral infections (e.g., influenza) and manifests as fever, bloody sputum production, and midlung-field pneumatoceles or multiple patchy pulmonary infiltrates.
  • - Blood cultures are usually negative.
• Nosocomial pneumonia is common in intubated pts and is indistinguishable from pneumonia of other bacterial etiologies.
  • - Pts produce an increased volume of purulent sputum and develop fever, new pulmonary infiltrates, and respiratory distress.

Bacteremia and Sepsis The incidence of metastatic seeding during bacteremia has been estimated to be as high as 31%, with bones, joints, kidneys, and lungs most commonly infected.

• Diabetes, HIV infection, and renal insufficiency are often seen in association with S. aureus bacteremia and increase the risk of complications.

Infective Endocarditis See Chap. 80. Infective Endocarditis for additional details.

• S. aureus is the leading cause of endocarditis worldwide and accounts for 25-35% of cases.
• The incidence is increasing as a result of injection drug use, hemodialysis, intravascular prosthetic devices, and immunosuppression.
• Mortality rates range from 20% to 40% despite the availability of effective antibiotics.
• The four clinical settings in which S. aureus endocarditis is encountered are (1) right-sided endocarditis in association with injection drug use, (2) left-sided native-valve endocarditis, (3) prosthetic-valve endocarditis, and (4) nosocomial endocarditis.

Urinary Tract Infections UTIs due to S. aureus are uncommon and suggest hematogenous dissemination.

Prosthetic Device-Related Infections Compared with coagulase-negative staphylococci (CoNS), S. aureus causes more acute disease, with localized and systemic manifestations that tend to be rapidly progressive. Successful treatment usually involves removal of the prosthetic device.

CA-MRSA Infections While the skin and soft tissues are the most common sites of infection associated with CA-MRSA, 5-10% of these infections are invasive and potentially life threatening (e.g., necrotizing fasciitis, necrotic pneumonia, sepsis, purpura fulminans).

Toxin-Mediated Disease Each class of toxin produced by S. aureus results in a characteristic syndrome.

• Food poisoning: results from inoculation of toxin-producing S. aureus into food by colonized food handlers, with subsequent toxin elaboration in growth-promoting foods (e.g., custard, potato salad, processed meat)
  • - The heat-stable toxin is not destroyed even if heating kills the bacteria.
  • - Because the disease is caused by preformed toxins, its onset is rapid and explosive, occurring within 1-6 h of ingestion of contaminated food.
  • - The chief signs and symptoms are nausea and vomiting, but diarrhea, hypotension, and dehydration may occur. Fever is absent.
  • - Symptoms resolve within 8-10 h; treatment is entirely supportive.
• Toxic shock syndrome (TSS): results from elaboration of an enterotoxin (many nonmenstrual TSS cases) or TSST-1 (some nonmenstrual cases and >90% of menstrual cases)
  • - Although the specific toxin may differ, the clinical presentation is similar in menstrual and nonmenstrual cases.
  • - Diagnosis is based on a constellation of clinical findings. Table 86-1 summarizes the case definition for staphylococcal TSS.
  • - Menstrual cases occur 2-3 days after menses begin.
  • - Illness occurs only in people who lack antibody to the toxin.
• Staphylococcal scalded-skin syndrome (SSSS): most often affects newborns and children. Fragility of the skin, with tender, thick-walled, fluid-filled bullae, can lead to exfoliation of most of the skin surface. Nikolsky's sign is positive when gentle pressure on bullae causes rupture of lesions and leaves denuded underlying skin.