This patient has necrotizing fasciitis and should receive empiric treatment with vancomycin plus piperacillin-tazobactam. Clues to a potential necrotizing skin infection include systemic toxicity (abnormal liver and kidney function, metabolic acidosis) with fever, chills, and hypotension. The patient's pain may be disproportionate to the physical examination findings. Skin changes can evolve rapidly and become ecchymotic, vesiculobullous, and gangrenous in appearance. “Woody” induration is also characteristic. Prompt surgical intervention is indicated as the primary treatment, with concurrent antibiotic therapy. The microbiologic cause can be monomicrobial or polymicrobial. Until the microbiology is determined, empiric therapy should be broad and consist of coverage against mixed aerobic and anaerobic gram-positive and gram-negative organisms, including methicillin-resistant Staphylococcus aureus (MRSA). Recommended regimens include vancomycin, linezolid, or daptomycin plus one of the following: piperacillin-tazobactam, a carbapenem (such as imipenem or meropenem), or metronidazole with either ceftriaxone or a fluoroquinolone. Polymicrobial infections are generally seen in patients with gastrointestinal and genitourinary infections, pressure ulcers, or at injection sites in patients using illicit drugs.

Ceftriaxone plus metronidazole alone provides broad-spectrum coverage against many organisms, but lacks MRSA activity. The addition of vancomycin or linezolid to this regimen would be needed until the microbiologic causes of necrotizing fasciitis are determined.

Doxycycline plus ciprofloxacin or ceftriaxone is the recommended regimen for patients with monomicrobial Aeromonas hydrophila–associated necrotizing skin infection. Patients who are immunocompromised, including those with liver disease and cancer, are at increased risk for serious skin infections and bacteremia/sepsis with this gram-negative bacillus. Wound infection usually occurs by inoculation through the skin. A. hydrophila is found in freshwater environments, but may also be present in brackish water. This regimen would not provide reliable empiric coverage against anaerobic bacteria or MRSA.

If Streptococcus pyogenes is confirmed by Gram stain and culture as the cause of necrotizing fasciitis, then penicillin plus clindamycin are recommended, particularly with associated toxic shock syndrome. Clindamycin is included because it inhibits toxin production and remains effective even in the presence of a high inoculum of bacteria. This regimen also would not provide adequate empiric coverage against gram-negative aerobic bacteria.