5.2 Pharmacotherapy of Skin and Soft Tissue Infections

Clinical Classification and Diagnostic Framework of SSTIs

Skin and soft tissue infections (SSTIs) encompass a heterogeneous spectrum of clinical presentations ranging from superficial pyoderma to rapidly lethal fascial necrosis. The Infectious Diseases Society of America (IDSA) classification algorithm stratifies infections into purulent versus non-purulent, followed by severity tiering (mild, moderate, or severe). This clinical paradigm dictates whether surgical incision and drainage, oral outpatient therapy, or emergent broad-spectrum parenteral antimicrobials are required.

                                          SSTI Classification Algorithm
                                                       │
                          ┌────────────────────────────┴────────────────────────────┐
                          ▼                                                         ▼
                  Non-Purulent SSTI                                         Purulent SSTI
        (Erysipelas, Cellulitis, Necrotizing)                    (Furuncles, Carbuncles, Abscesses)
                          │                                                         │
              ┌───────────┼───────────┐                                 ┌───────────┼───────────┐
              ▼           ▼           ▼                                 ▼           ▼           ▼
            Mild       Moderate    Severe                             Mild       Moderate    Severe
          (Oral PO)    (IV Meds)   (Emergent Surgery + IV)           (I&D Alone) (I&D + PO)  (I&D + IV)
          • Cephalexin • Cefazolin • Vanc/Dapto + Pip-Tazo/Carba                 • TMP-SMX   • Vanc/Dapto
          • Penicillin • Ceftriaxone + Clindamycin (Antitoxin)                   • Doxycycline

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Non-Purulent Skin and Soft Tissue Infections

Non-purulent SSTIs comprise erysipelas and diffuse cellulitis, conditions characterized by erythema, warmth, edema, and localized pain without gross purulent drainage or fluctuant collections.

Clinical Distinction: Erysipelas vs. Cellulitis

• Erysipelas: Involves the upper dermis and superficial cutaneous lymphatics. It presents with an abrupt onset, intense fiery erythema, elevated raised plaque-like borders, and a distinct, palpable line of demarcation between involved and uninvolved skin. It occurs characteristically on the lower extremities or face. Etiology is almost exclusively Group A beta-hemolytic Streptococcus (Streptococcus pyogenes), followed rarely by Groups B, C, or G streptococci.
• Cellulitis: Involves the deeper dermis and subcutaneous adipose tissue. It exhibits an indolent onset with diffuse, poorly demarcated borders that blend imperceptibly into surrounding normal skin. Etiology is predominantly driven by beta-hemolytic streptococci (S. pyogenes, S. agalactiae, S. dysgalactiae), alongside methicillin-susceptible Staphylococcus aureus (MSSA).

Severity Stratification and Treatment Paradigm

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Purulent Skin and Soft Tissue Infections

Purulent SSTIs comprise cutaneous abscesses, furuncles (focal suppurative staphylococcal infection of a hair follicle), and carbuncles (coalescent clusters of multiple furuncles with subcutaneous extension).

Microbiology and Surgical Primacy

• Microbiology: Overwhelmingly driven by Staphylococcus aureus, with CA-MRSA (typically pulsed-field gel electrophoresis type USA300 encoding Panton-Valentine leukocidin [PVL]) accounting for 60% to 80% of purulent collections presenting to emergency departments.
• Surgical Incision and Drainage (I&D): Incision, evacuation of purulent loculations, and drainage is the primary, definitive therapeutic intervention. Antibiotics cannot penetrate thick, acidic, necrotic abscess cavities and are entirely ineffective without surgical drainage.

Severity Stratification and Antimicrobial Selection

1. Mild Purulent SSTI:
   • Single small cutaneous abscess or localized furuncle without systemic signs of toxicity.
   • Management: Incision and drainage ALONE.
   • Antibiotic therapy is NOT recommended. Multiple randomized, double-blind trials confirm that adjunctive antibiotics do not improve cure rates or accelerate resolution in uncomplicated mild drained abscesses.
2. Moderate Purulent SSTI:
   • Drained abscess or carbuncle presenting with systemic signs of infection (fever, tachycardia, leukocytosis) OR high-risk clinical characteristics: multiple or recurrent abscesses, marked surrounding cellulitis, extremes of age ($< 1$ year or elderly), immunocompromise, location in difficult-to-drain anatomical sites (face, hands, genitalia), or lack of response to prior I&D alone.
   • Management: I&D PLUS Oral MRSA Antimicrobial Therapy (5–7 days):
     • Trimethoprim-Sulfamethoxazole (TMP-SMX): 1 to 2 Double-Strength (DS) tablets PO BID (first-line; highly active against CA-MRSA).
     • Doxycycline: 100 mg PO BID (first-line alternative).
     • Clindamycin: 300–450 mg PO TID. Caution: Use only if local MRSA clindamycin resistance is $< 10\%$ and the isolate demonstrates a negative erythromycin-induction D-zone test (to rule out inducible MLSb resistance mediated by erm genes).
3. Severe Purulent SSTI:
   • Patients who have failed initial oral therapy and I&D, those presenting with hemodynamic compromise (septic shock), or those with extensive tissue involvement.
   • Management: Emergent I&D PLUS Parenteral MRSA Therapy:
     • Vancomycin 15–20 mg/kg IV q8–12h (AUC-guided),
     • Daptomycin 6–8 mg/kg IV once daily (cell-membrane depolarizing lipopeptide),
     • Linezolid 600 mg IV q12h,
     • Ceftaroline 600 mg IV q12h (binds PBP2a),
     • Long-acting lipoglycopeptides: Dalbavancin (1500 mg IV single dose) or Oritavancin (1200 mg IV single dose).

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Severe and Necrotizing Soft Tissue Infections (NSTIs)

Necrotizing soft tissue infections (necrotizing fasciitis, myonecrosis) represent life- and limb-threatening surgical emergencies characterized by extensive, rapid necrosis of subcutaneous adipose tissue, superficial fascia, and deep muscular compartments.

Diagnostic Hallmarks and Emergency Management

• Clinical Signs: The cardinal physical finding is severe, excruciating pain out of proportion to physical exam findings. Early skin may appear deceptively normal or resemble mild cellulitis. Late ominous findings include "wooden-hard" induration of subcutaneous tissues, cutaneous anesthesia (due to thrombosis of nutrient dermal vessels and necrosis of cutaneous nerve fibers), skin bullae containing purple/dishwater fluid, ecchymosis, crepitus (subcutaneous gas production), and rapid clinical deterioration into septic shock and multi-organ failure.
• Surgical Priority: Emergent surgical exploration and radical operative debridement is mandatory and definitive. Antimicrobial therapy without operative source control is uniformly fatal.

Microbiological Classification

1. Type I NSTI (Polymicrobial, 70%–80% of cases):
   • Mixed synergistic infection comprising obligate anaerobes (Bacteroides fragilis, Peptostreptococcus, Prevotella) alongside facultative aerobic Gram-negative Enterobacterales (E. coli, Klebsiella) and streptococci.
   • Typically encountered in surgical incisions, immunocompromised hosts, diabetics, and the perineum (Fournier gangrene).
2. Type II NSTI (Monomicrobial, 20%–30% of cases):
   • Classically designated as "flesh-eating disease," caused by virulent Group A Streptococcus (Streptococcus pyogenes), alone or in combination with Staphylococcus aureus (CA-MRSA carrying PVL).
   • Occurs frequently in young, previously healthy individuals following minor blunt trauma, lacerations, or varicella infection.
3. Type III NSTI (Marine and Aquatic Zoonoses):
   • Vibrio vulnificus: Transmitted via exposure of open wounds to warm coastal seawater or ingestion of raw oysters; highly fulminant in patients with chronic liver cirrhosis, hemochromatosis, or iron overload. Requires Ceftriaxone PLUS Doxycycline.
   • Aeromonas hydrophila: Acquired via freshwater aquatic exposure, leeches, or soil; treated with Ciprofloxacin or Cefepime.

Empiric Regimen and the Critical Antitoxin Role of Clindamycin

                                     Empiric Regimen for Necrotizing Fasciitis
                                                        │
                    ┌───────────────────────────────────┼───────────────────────────────────┐
                    ▼                                   ▼                                   ▼
          MRSA / Gram(+) Coverage             Gram(-) / Anaerobic Coverage              Antitoxin Therapy
                    │                                   │                                   │
            • Vancomycin (AUC 400-600)          • Piperacillin-Tazobactam           • Clindamycin
            • Daptomycin 6-8 mg/kg                4.5g IV q6h (Extended Infusion)     600-900 mg IV q8h
                                                • Meropenem 1g IV q8h               (Arrests SpeA/SpeC/PVL
                                                                                     ribosomal translation)

• Empiric Regimen: Vancomycin (or Daptomycin) PLUS Piperacillin-tazobactam (or Meropenem) PLUS Clindamycin (600–900 mg IV every 8 hours).

• Intravenous Immunoglobulin (IVIG): Adjunctive administration of high-dose IVIG (1 g/kg on day 1, followed by 0.5 g/kg on days 2 and 3) may be considered in severe streptococcal toxic shock syndrome (STSS) associated with necrotizing fasciitis to bind and neutralize circulating superantigens and exotoxins.

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Diabetic Foot Infections (DFI)

Diabetic foot infections represent a major complication of diabetic peripheral neuropathy, peripheral artery disease (PAD), and impaired neutrophil phagocytosis, constituting the leading non-traumatic indication for lower extremity amputations.

Severity Classification (2023 IWGDF / IDSA Guidelines)

The Pseudomonas aeruginosa Question

Pseudomonas aeruginosa is frequently isolated from diabetic foot wound surface swabs, but in temperate climates it often represents non-pathogenic superficial colonization rather than invasive infection.

• Guidelines Consensus: Routine empiric antipseudomonal coverage is NOT indicated for mild or moderate DFIs.
• Empiric Antipseudomonal Indications: Antipseudomonal agents (piperacillin-tazobactam, cefepime, meropenem) should be prescribed ONLY when specific risk factors are present:
  1. High local prevalence of P. aeruginosa in diabetic ulcer cultures.
  2. Macerated ulcers secondary to continuous wet dressing contact or frequent foot soaking / water immersion.
  3. Warm, humid, tropical geographic climates.
  4. Severe infection presenting with septic shock or after clinical failure of a broad-spectrum regimen lacking pseudomonal activity.

Assessment and Diagnosis of Underlying Diabetic Foot Osteomyelitis (DFO)

• Probe-to-Bone (PTB) Test: In a chronic diabetic foot ulcer, gently palpating the ulcer bed with a sterile, blunt metallic probe. Feeling a hard, gritty, stone-like surface without an intervening soft-tissue cushion constitutes a positive PTB test. In high-prevalence settings, a positive PTB has a Positive Predictive Value (PPV) of $\sim 89\%$, strongly confirming underlying osteomyelitis.
• Imaging Modalities:
  • Plain Radiographs: Initial diagnostic imaging. Detects cortical erosion, periosteal reaction, and osteolysis; however, plain radiographs exhibit low early sensitivity because bone demineralization lags clinical infection by 10 to 21 days.
  • Magnetic Resonance Imaging (MRI): Gold standard imaging modality (sensitivity $\sim 90\%$, specificity $\sim 85\%$). Indicated when osteomyelitis is clinically suspected but plain radiographs are negative or equivocal, or to evaluate deep soft-tissue abscesses, joint effusion, and cortical sequestra.
• Definitive Microbiological Diagnosis: Percutaneous or intraoperative bone biopsy through uninfected skin provides the definitive microbiological standard. Superficial swab cultures correlate poorly with deep bone pathogens and must never be used to guide osteomyelitis therapy.

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Animal and Human Bite Wounds

Bite wounds are heavily inoculated with polymicrobial oral commensal flora, resulting in aggressive soft tissue infections, closed space tenosynovitis, and septic arthritis.

                                        Bite Wound Pathogen Spectra
                                                     │
                        ┌────────────────────────────┴────────────────────────────┐
                        ▼                                                         ▼
              Cat and Dog Bite Wounds                                   Human Bite Wounds
                        │                                                         │
            • Pasteurella multocida (Cats > Dogs)                     • Eikenella corrodens
            • Capnocytophaga canimorsus                               • Streptococcus anginosus group
            • Staphylococcus / Streptococcus                          • Staphylococcus aureus
            • Oral anaerobes (Fusobacterium)                          • Oral anaerobes (Prevotella, Peptostreptococcus)
                        │                                                         │
                        └────────────────────────────┬────────────────────────────┘
                                                     ▼
                                        First-Line Drug of Choice:
                                   Amoxicillin-Clavulanate 875/125 mg PO BID
                                       (IV Ampicillin-Sulbactam 1.5-3g q6h)

Microbiology

• Cat Bites (80% develop infection): Deep, penetrating puncture wounds that introduce organisms into the periosteum and tendon sheaths. Dominated by Pasteurella multocida, a fastidious Gram-negative coccobacillus that causes exceptionally rapid, intense local cellulitis within 12 to 24 hours of injury.
• Dog Bites (15%–20% develop infection): Crushing tissue trauma and lacerations. Polymicrobial flora comprising Pasteurella canis, Pasteurella multocida, Staphylococcus aureus, Streptococcus, and Capnocytophaga canimorsus. In asplenic, cirrhotic, or immunocompromised individuals, C. canimorsus can trigger fulminant, catastrophic septic shock, disseminated intravascular coagulation (DIC), symmetrical peripheral gangrene, and death within 24 to 48 hours.
• Human Bites ("Clenched-Fist" / Fight Bites and Occlusional Bites): Highly virulent polymicrobial inoculations. In clenched-fist injuries, impact against human teeth violates the thin dorsal skin over the metacarpophalangeal (MCP) joint, inoculating bacteria directly into the extensor tendon sheath and joint capsule. Dominated by Eikenella corrodens (a facultative Gram-negative rod that pits agar and produces a bleach-like odor), Streptococcus anginosus group, Staphylococcus aureus, and oral anaerobes (Fusobacterium, Prevotella).

Pharmacotherapy and Contraindicated Regimens

• Drug of Choice (First-Line): Amoxicillin-Clavulanate (875/125 mg PO BID or 1000/62.5 mg ER BID) for 7–10 days (or IV Ampicillin-Sulbactam 1.5–3 g q6h for hospitalized patients). Provides reliable coverage against Pasteurella, Eikenella, MSSA, Streptococcus, and beta-lactamase-producing oral anaerobes.
• Penicillin Allergy Alternatives:
  • Doxycycline (100 mg PO BID): Excellent monotherapy option active against Pasteurella, Eikenella, staphylococci, and streptococci.
  • Fluoroquinolones (Moxifloxacin 400 mg daily, or Ciprofloxacin 500 mg BID PLUS Metronidazole 500 mg TID): Covers Pasteurella and Eikenella; metronidazole adds obligatory anaerobic coverage.
  • TMP-SMX (1–2 DS tablets BID) PLUS Metronidazole (500 mg TID).

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Surgical Site Infections (SSI) and Perioperative Prophylaxis

Surgical site infections represent the most common nosocomial infection among surgical patients, contributing substantially to excess readmissions, intensive care utilization, and healthcare expenditures.

CDC / NHSN Classification of SSIs

• Superficial Incisional SSI: Involves only the skin and subcutaneous tissue of the incision; occurs within 30 days of the operative procedure.
• Deep Incisional SSI: Involves deep soft tissues (fascial and muscle layers) of the incision; occurs within 30 days (or within 90 days if an implant/prosthesis was placed).
• Organ/Space SSI: Involves any anatomical structure deeper than the fascial/muscle layers that was opened or manipulated during the surgical procedure (e.g., intra-abdominal abscess, mediastinitis, joint space infection); occurs within 30 or 90 days.

Evidence-Based Perioperative Prophylaxis (ASHP/IDSA/SIS/SHEA Guidelines)

Perioperative antimicrobial prophylaxis is designed to prevent SSI by reducing the microbial burden at the surgical site during the operative window. It does not sterilize tissue, but prevents viable bacterial inocula from establishing invasive infection.

                                Perioperative Antimicrobial Prophylaxis Rules
                                                      │
         ┌─────────────────────────┬──────────────────┴──────────────────┬─────────────────────────┐
         ▼                         ▼                                     ▼                         ▼
    Drug Selection              Timing                              Weight Dosing             Redosing Window
    • Cefazolin standard       • Within 60 min of incision          • Cefazolin 2g (<120kg)   • Every 4 hours
    • Add Metronidazole        • Within 120 min for Vancomycin      • Cefazolin 3g (>=120kg)    (two half-lives)
      for colorectal             or Fluoroquinolones                                          • Discontinue <= 24h

1. Antimicrobial Selection:
   • Cefazolin is the standard, preferred first-line agent for nearly all clean and clean-contaminated surgical procedures (cardiothoracic, orthopedic, vascular, plastic, and general surgery). It possesses excellent bactericidal activity against MSSA, Streptococcus, and wild-type E. coli, achieves high serum and tissue concentrations, and has an established safety profile.
   • Colorectal and Appendectomy Procedures: Require coverage of Enterobacterales and enteric anaerobes (Bacteroides fragilis). Preferred regimens: Cefazolin PLUS Metronidazole, or Cefoxitin (2 g IV), or Cefotetan (2 g IV).
   • Vancomycin Indications: Routine vancomycin prophylaxis is strictly discouraged. Vancomycin is justified ONLY in patients with documented colonization or prior infection with MRSA, or in institutions with high endemic rates of MRSA undergoing prosthetic implant placement (cardiac valve replacement, total joint arthroplasty).
2. Infusion Timing:
   • Prophylactic antimicrobials must be administered to establish bactericidal tissue concentrations at the exact moment of surgical incision.
   • Standard Infusion Agents (Cefazolin, Cefoxitin, Ampicillin-sulbactam, Metronidazole): Must be initiated within 60 minutes prior to surgical incision (ideally 30–60 minutes prior).
   • Prolonged Infusion Agents (Vancomycin, Fluoroquinolones): Because rapid infusion triggers non-allergic histamine release (vancomycin infusion reaction / "red man syndrome"), vancomycin requires a 60- to 120-minute infusion window. Administration must begin within 120 minutes prior to surgical incision.
3. Weight-Based Dosing of Cefazolin:
   • Patients weighing $< 120$ kg: Cefazolin 2 g IV.
   • Patients weighing $\ge 120$ kg: Cefazolin 3 g IV.
4. Intraoperative Redosing:
   • Prophylactic antibiotics must be redosed intraoperatively if the duration of surgery exceeds two half-lives of the antimicrobial, or in the event of major blood loss ($> 1,500$ mL in adults):
     • Cefazolin (half-life $\sim 1.8$ hours): Redose every 4 hours (measured from the time of the preoperative dose).
     • Cefoxitin (half-life $\sim 0.7$–1 hour): Redose every 2 hours.
     • Ampicillin-sulbactam: Redose every 2 hours.
     • Clindamycin: Redose every 6 hours.
     • Vancomycin: Redosing is rarely required unless the procedure exceeds 12 hours.
5. Postoperative Discontinuation (The 24-Hour Rule):
   • Antimicrobial prophylaxis must be discontinued within 24 hours of surgery completion (and ideally at the time of surgical incision closure).
   • Multiple prospective trials demonstrate that continuing prophylactic antibiotics beyond 24 hours postoperatively does NOT reduce surgical site infections, but significantly increases acute kidney injury, Clostridioides difficile colitis, and selective pressure for multidrug-resistant pathogens.