Pφ

A breach in the skin may be microscopic or obvious. Pathogenic organisms that either are part of normal skin flora or are colonizing the skin, or organisms introduced from an exogenous source, may then be introduced and cause infection. Multiplication of organisms in the skin and upper dermis elicit an inflammatory response that leads to the cardinal clinical features of cellulitis. The infection may spread via the lymphatics to the bloodstream, resulting in concomitant bacteremia. S. pyogenes is a normal part of the skin flora and is the most common cause of cellulitis. Although not considered part of the normal flora, S. aureus may colonize the skin in individuals who use needles such as injection drug users, diabetics, and hemodialysis patients. Other streptococci (e.g., group C, G, or F) may cause cellulitis. The incidence of infections due to Streptococcus agalactiae (group B streptococcus) is increasing among nonpregnant adults; immunosuppression, diabetes, chronic liver or kidney disease, malignancy, and older age are risk factors. Gram-negative organisms are an uncommon cause of cellulitis, although they may occur in wound infections that are associated with health care. S. aureus and S. pyogenes are potential pathogens when cellulitis occurs following a bite; other pathogens that must be considered are organisms that compose the oral flora of the biter. Pasteurella multocida is associated with cat and dog bites, Capnocytophaga canimorsus is associated with dog bites, and Eikenella corrodens is associated with human bites. In addition, oral anaerobes must always be considered in the microbiology of bite wounds.

TP

As discussed previously, pain, swelling, erythema, and warmth are the typical clinical features of cellulitis. Patients may have signs of systemic toxicity including fever and chills.

Dx

A diagnosis of cellulitis can be made based on the presentation and clinical findings; a microbiologic diagnosis cannot be confirmed unless there is purulent drainage from a wound, abscess formation, or bacteremia.

Pφ

Erysipelas is caused by S. pyogenes (group A streptococcus). Similar to that outlined above for cellulitis, erysipelas is a more superficial form of cellulitis that exhibits prominent involvement of the lymphatics.

TP

Erysipelas presents as a bright-red or salmon-colored area with a sharp demarcation, raised above the surrounding skin. Erysipelas is more common in older adults and typically occurs on the face (usually the cheek) or the lower extremity. Because of the prominent lymphatic involvement, the skin may have a “peau d’orange” appearance.

Dx

The diagnosis is made on the basis of the clinical presentation and examination findings.

Pφ

Any infection of the skin and soft tissue may form an area of induration and suppuration that leads to a localized collection of pus. Both S. aureus and S. pyogenes (along with other streptococci) can cause skin abscess. In the last several years there has been a dramatic increase in skin and soft-tissue infections due to CA-MRSA. These isolates are clearly distinct from health care–associated MRSA infections, and many belong to a single clone, the USA 300 clone. Although CA-MRSA organisms are resistant to β-lactam antibiotics, unlike health care–associated MRSA, they are susceptible to many other antimicrobials. Skin abscesses that occur in injection drug users and those that complicate diabetic foot infections are frequently polymicrobial, including the Enterobacteriaceae and anaerobes in addition to S. aureus and streptococci.

TP

A soft-tissue abscess may form in an area of cellulitis. An abscess should be suspected if there is an area of more intense edema or fluctuance, or if the cellulitis is failing to respond to appropriate antibiotic therapy. Frequently development of an abscess is apparent on examination; occasionally CT scan or MRI is required to evaluate for an abscess in the deeper soft tissues that may not be readily apparent clinically. CA-MRSA SSTIs most commonly present as multiple abscesses, although a single abscess may occur. Patients will frequently report that they have “spider bites” or give a history of a household member with “spider bites.” Outbreaks of CA-MRSA skin infection have been described among prisoners, men who have sex with men, and athletes (wrestlers, football players); however, many patients do not have a specific risk factor for CA-MRSA infection, so the diagnosis must be suspected based on the clinical presentation.

Dx

As discussed above, a skin and soft-tissue abscess can frequently be diagnosed based on the clinical examination. In situations where abscess is suspected clinically but is not clearly present on exam, CT or MRI can confirm the presence of a collection of pus. Abscess drainage should always be cultured to confirm the microbiologic diagnosis and direct appropriate antibiotic therapy. A Gram stain can provide rapid initial information regarding the microbiology of the infection.

Pφ

Necrotizing fasciitis is an SSTI that has spread beyond the dermis to the deeper subcutaneous tissues and fascia. Necrotizing fasciitis type I is a polymicrobial infection that can include gram-positive, gram-negative, and anaerobic organisms; type II is caused by S. pyogenes. The elaboration of toxins (e.g., streptococcal pyrogenic exotoxin) is important in the pathogenesis of this rapidly progressive and fulminant infection.

TP

Necrotizing fasciitis must be suspected when a patient with cellulitis has a rapidly spreading infection, especially when associated with signs of severe systemic toxicity or severe pain that seems out of proportion to the clinical findings. Rapidly forming bullae, ecchymosis, cutaneous anesthesia, and evidence of skin necrosis may occur; palpable crepitance is strongly suggestive of necrotizing fasciitis. Occasionally, patients may present just with severe pain in the involved area and signs of systemic toxicity.

Dx

The possibility of necrotizing fasciitis is a medical and surgical emergency, and surgical consultation should be obtained promptly. The diagnosis can be suggested by findings on imaging with CT or MRI, but surgical evaluation should not be delayed to obtain these studies; the diagnosis can only be confirmed with certainty at the time of surgery. Blood cultures should always be obtained in patients with suspected necrotizing fasciitis.

Pφ

Foot ulcers may occur in patients with diabetes mellitus as a result of unrecognized trauma due to sensory neuropathy. These infections are generally considered to be polymicrobial, including staphylococci, streptococci, anaerobes, and Enterobacteriaceae; long-standing ulcers that have been treated with previous courses of antimicrobial therapy for secondary infection may be colonized with Pseudomonas and other non–lactose-fermenting gram-negative pathogens. The pathogenesis of these ulcers also involves vascular insufficiency due to peripheral vascular disease. Nonhealing ulcers become colonized with bacteria; infection of the ulcer base may occur, or the ulcer may serve as a portal of entry for bacteria resulting in cellulitis of the surrounding intact skin and soft tissues. As the ulcer base deepens, contiguous spread to the underlying bone may occur, resulting in osteomyelitis. Polymorphonuclear cell dysfunction in the setting of poorly controlled blood glucose also contributes to the pathogenesis of infection.

TP

As described above, patients may present with a malodorous, draining, infected ulcer with or without concomitant cellulitis and osteomyelitis.

Dx

Ulcers must be carefully inspected; after cleansing and debridement of the ulcer base, a sterile cotton-tipped swab can be utilized to determine if the ulcer extends to the bone. The positive predictive value for osteomyelitis if the ulcer extends to bone is >90%, although the negative predictive value is only ~50%, so further investigation with plain films or MRI will be required to exclude a diagnosis of osteomyelitis. Cultures may then be obtained from the ulcer base to guide microbiologic therapy; if there is evidence of osteomyelitis, long-term antibiotic therapy will be required. Thus, the confirmation of a microbiologic diagnosis with cultures obtained from a bone biopsy is essential.