What is the appropriate antibiotic treatment for a patient with a wound infection showing moderate growth of Enterococcus faecalis and Acinetobacter baumannii, considering the patient's susceptibility results and potential for resistance?

Treatment Recommendation for Polymicrobial Wound Infection

For this wound infection with moderate growth of both Enterococcus faecalis and Acinetobacter baumannii, initiate combination therapy with ampicillin 2g IV every 4 hours PLUS imipenem 500mg IV every 6 hours (or meropenem 1g IV every 8 hours), given both organisms demonstrate susceptibility to these agents and this dual approach provides optimal coverage for both pathogens. 1, 2

Rationale for Dual Therapy

Your susceptibility results show:

• E. faecalis: Penicillin-susceptible (MIC 2, susceptible), with synergy demonstrated for gentamicin and streptomycin 2
• A. baumannii: Carbapenem-susceptible (imipenem MIC ≤1, susceptible; meropenem MIC 4, intermediate) 1

This is a favorable susceptibility profile that allows avoidance of more toxic agents like colistin or tigecycline. 1

Specific Antibiotic Selection

For Enterococcus faecalis Coverage

• Ampicillin 2g IV every 4-6 hours is the gold standard for penicillin-susceptible E. faecalis, as most strains retain ampicillin susceptibility and this provides superior outcomes compared to vancomycin. 2
• Your isolate shows penicillin susceptibility (MIC 2), which predicts susceptibility to ampicillin, amoxicillin, and piperacillin-tazobactam for non-beta-lactamase-producing enterococci. 2
• Do not use vancomycin for this E. faecalis infection despite its susceptibility (MIC 0.5), as ampicillin is superior and vancomycin should be reserved for documented beta-lactam allergy. 2

For Acinetobacter baumannii Coverage

• Carbapenems (imipenem, meropenem, doripenem) are the drugs of choice for carbapenem-susceptible A. baumannii infections. 1, 3
• Your isolate shows imipenem susceptibility (MIC ≤1) and meropenem intermediate susceptibility (MIC 4). 1
• Choose imipenem 500mg IV every 6 hours over meropenem given the fully susceptible MIC, though meropenem 1g IV every 8 hours is acceptable. 1
• Never use ertapenem for A. baumannii as it lacks activity against this pathogen despite being a carbapenem. 1

Why Not Monotherapy?

• Ampicillin alone would not cover A. baumannii, as enterococcal-directed therapy has no activity against gram-negative organisms. 2
• Carbapenem monotherapy would not reliably cover E. faecalis, as enterococci demonstrate intrinsic resistance to cephalosporins and variable carbapenem activity. 2
• The absence of WBCs on Gram stain does not indicate colonization versus infection in the context of moderate growth of two distinct pathogens from a wound culture. 1

Alternative Considerations if Beta-Lactam Allergy

If the patient has a documented severe penicillin allergy:

• For E. faecalis: Linezolid 600mg IV/PO every 12 hours OR daptomycin 6-8mg/kg/day IV (note: daptomycin shows intermediate susceptibility with MIC 4 in your isolate, making linezolid preferable). 2, 4
• For A. baumannii: Continue carbapenem (imipenem/meropenem) as carbapenems have low cross-reactivity with penicillins in true IgE-mediated allergy. 1

Treatment Duration

• Treat for 7-14 days for uncomplicated wound infections, with duration guided by clinical response, source control, and resolution of signs/symptoms of infection. 2
• Extend to 14 days if bacteremia is documented or if there are signs of systemic infection (fever, elevated inflammatory markers, hemodynamic instability). 1
• Ensure adequate wound debridement and source control, as antimicrobial therapy alone is insufficient without mechanical removal of devitalized tissue. 1

Monitoring Parameters

• Monitor renal function closely given dual beta-lactam therapy, particularly if considering addition of aminoglycosides for synergy. 5
• Repeat cultures are not routinely necessary unless clinical failure occurs after 48-72 hours of appropriate therapy. 2
• Assess wound healing clinically with decreased erythema, purulence, and pain as markers of treatment response. 1

Critical Pitfalls to Avoid

• Never use cephalosporins alone for enterococcal coverage, despite your isolate showing ceftriaxone and ceftazidime susceptibility for A. baumannii, as cephalosporins have no intrinsic activity against enterococci. 2
• Do not use tigecycline as monotherapy despite susceptibility, as it achieves suboptimal serum concentrations for bacteremia and has higher treatment failure rates. 1
• Avoid trimethoprim-sulfamethoxazole as your A. baumannii isolate shows resistance (MIC >2/38). 1
• Do not add gentamicin for synergy in this wound infection, as synergy is primarily indicated for enterococcal endocarditis, not soft tissue infections, and adds nephrotoxicity risk. 5, 2

Why Not Other Susceptible Agents?

• Ciprofloxacin/levofloxacin: While your A. baumannii shows susceptibility, fluoroquinolones do not provide adequate enterococcal coverage and should not be used as monotherapy. 1
• Linezolid: Shows susceptibility for E. faecalis (MIC 2) but has no activity against A. baumannii, making it unsuitable as monotherapy. 4
• Daptomycin: Shows intermediate susceptibility for E. faecalis (MIC 4) and has no gram-negative activity, making it a suboptimal choice. 2