What is the treatment plan for a patient with a skin infection caused by moderate growth of Prevotella disiens (beta-lactamase-positive) and heavy growth of Pseudomonas aeruginosa and Group C Streptococcus?

Treatment Plan for Polymicrobial Skin Infection with Pseudomonas aeruginosa, Group C Streptococcus, and Beta-Lactamase-Positive Prevotella disiens

This patient requires immediate treatment with piperacillin-tazobactam 4.5g IV every 6 hours to cover all three pathogens, including the beta-lactamase-producing anaerobe and heavy Pseudomonas growth. 1, 2

Initial Antibiotic Selection

Piperacillin-tazobactam is the single best choice because it provides:

• Antipseudomonal activity (critical given heavy Pseudomonas growth) 1, 3
• Beta-lactamase inhibition via tazobactam (essential for beta-lactamase-positive Prevotella) 1, 2
• Streptococcal coverage including Group C streptococcus 1, 2
• FDA-approved for skin and soft tissue infections 2

The IDSA guidelines explicitly recommend piperacillin-tazobactam as the preferred broad-spectrum agent for polymicrobial skin infections requiring coverage of both aerobes (including Pseudomonas) and anaerobes. 1

Why Combination Therapy May Be Needed

Consider adding an aminoglycoside (tobramycin 5-7 mg/kg IV daily) if:

• The patient is critically ill or has signs of systemic toxicity 1, 3
• There is extensive tissue involvement suggesting necrotizing infection 1
• The patient has structural foot disease (diabetic foot, peripheral vascular disease) 4
• Prior antibiotic use within 90 days 3, 5, 4

The IDSA guidelines state that combination therapy with an antipseudomonal β-lactam plus aminoglycoside is recommended for severe polymicrobial necrotizing infections and for documented Pseudomonas when the patient is hemodynamically unstable. 1

Alternative Regimens (If Piperacillin-Tazobactam Unavailable or Allergic)

For severe penicillin allergy:

• Meropenem 1g IV every 8 hours PLUS metronidazole 500mg IV every 8 hours 1, 3, 5
• Meropenem covers Pseudomonas and streptococci but requires metronidazole addition for optimal anaerobic coverage including Prevotella 1, 3

Avoid these common errors:

• Ceftriaxone lacks antipseudomonal activity entirely 3, 5
• Ertapenem has no Pseudomonas coverage despite being a carbapenem 3, 5
• Ampicillin-sulbactam has no clinically relevant Pseudomonas activity 3

Treatment Duration and Monitoring

Standard duration: 7-14 days 1, 3

• Start with 7 days for uncomplicated infections 1
• Extend to 14 days if there is delayed clinical response, extensive tissue involvement, or immunocompromise 1, 4

Monitor for clinical improvement within 48-72 hours:

• Decreasing erythema, pain, and purulent drainage 1, 4
• Defervescence if febrile 1
• If worsening despite therapy, consider surgical debridement and repeat cultures 1, 4

Surgical Considerations

Immediate surgical consultation is mandatory if:

• Rapidly progressive infection despite antibiotics 1
• Gas in soft tissues (crepitus) suggesting necrotizing fasciitis 1
• Extensive necrosis or abscess formation requiring drainage 1

The IDSA guidelines emphasize that surgical intervention for drainage or debridement is essential for progressive polymicrobial necrotizing infections and cannot be replaced by antibiotics alone. 1

Critical Pitfalls to Avoid

Never use fluoroquinolone monotherapy (ciprofloxacin or levofloxacin) for this infection despite Pseudomonas coverage, because:

• No activity against anaerobes like Prevotella 1, 4
• Suboptimal streptococcal coverage 1
• Rapid resistance emergence with Pseudomonas monotherapy 3, 5, 6

Do not underdose antipseudomonal agents:

• Use maximum recommended doses given heavy Pseudomonas growth 3, 5
• Piperacillin-tazobactam 4.5g (not 3.375g) every 6 hours for serious infections 1, 3, 2

Recognize this is NOT simple "athlete's foot":

• True athlete's foot (tinea pedis) is fungal and would not grow these bacteria 4
• This culture represents a polymicrobial bacterial infection of macerated interdigital skin, likely with tissue invasion given the heavy bacterial growth 4
• The wet, macerated interdigital environment is a documented risk factor for Pseudomonas colonization and infection 4

De-escalation Strategy

Once susceptibility results return and clinical improvement is evident:

• Narrow to the most appropriate single agent if all organisms are susceptible 1, 5, 4
• Continue piperacillin-tazobactam if it provides the narrowest spectrum covering all three pathogens 1, 5
• Do not de-escalate if the patient is not improving clinically, regardless of in vitro susceptibilities 4