What is the appropriate treatment for a diabetic patient with a wound infection on the 2nd toe of the right foot, with moderate growth of Enterococcus faecalis and Acinetobacter baumannii, and known antimicrobial susceptibility results?

Treatment of Polymicrobial Diabetic Foot Infection with Enterococcus faecalis and Acinetobacter baumannii

For this diabetic foot infection with moderate growth of both Enterococcus faecalis (susceptible to penicillin, vancomycin, and linezolid) and Acinetobacter baumannii (susceptible to multiple agents including carbapenems and fluoroquinolones), initiate combination therapy with either vancomycin plus a carbapenem (imipenem or meropenem) OR linezolid plus a carbapenem, along with urgent surgical debridement and metabolic stabilization. 1, 2

Infection Severity Assessment

• This represents a moderate-to-severe diabetic foot infection requiring immediate attention based on the polymicrobial nature with both gram-positive cocci and gram-negative bacilli, even though the Gram stain shows few WBCs. 1

• The absence of WBCs on Gram stain does not exclude significant infection in diabetic patients, who often have impaired inflammatory responses. 1

• Polymicrobial infections in diabetic foot ulcers, particularly those involving Enterococcus faecalis, are associated with higher rates of limb loss (60% in polymicrobial vs. monomicrobial infections). 3

Recommended Antibiotic Regimen

Primary Option: Vancomycin + Carbapenem

• Vancomycin 15-20 mg/kg IV every 12 hours (target trough 15-20 mcg/mL) provides excellent coverage for Enterococcus faecalis (MIC 0.5, susceptible) and covers potential MRSA. 1

• Plus imipenem 500 mg IV every 6 hours OR meropenem 1 g IV every 8 hours for Acinetobacter baumannii coverage (imipenem MIC ≤1, susceptible; meropenem MIC 4, intermediate). 4

• Carbapenems remain the mainstay of treatment for Acinetobacter baumannii infections, with imipenem showing full susceptibility in this case. 4

Alternative Option: Linezolid + Carbapenem

• Linezolid 600 mg IV or PO every 12 hours is an excellent alternative for Enterococcus faecalis (MIC 2, susceptible) with the advantage of oral bioavailability for transition. 5

• In the FDA-approved diabetic foot infection trial, linezolid achieved 83% cure rates in clinically evaluable patients and 78% cure rate specifically for Staphylococcus aureus. 5

• Plus the same carbapenem regimen as above for Acinetobacter coverage. 4

Why Not Monotherapy?

• Penicillin alone is inadequate despite E. faecalis susceptibility (MIC 2) because it provides no coverage for Acinetobacter baumannii. 1

• Fluoroquinolones alone are insufficient despite Acinetobacter susceptibility to ciprofloxacin and levofloxacin, as they have poor activity against Enterococcus faecalis in diabetic foot infections. 1

• The IDSA guidelines explicitly recommend broad-spectrum coverage for moderate-to-severe infections with gram-positive, gram-negative, and potentially anaerobic organisms. 1, 2

Critical Concurrent Interventions

Surgical Management

• Urgent sharp debridement of all necrotic tissue, callus, and non-viable material is essential and should not be delayed. 1

• The presence of moderate bacterial growth indicates established infection requiring source control beyond antibiotics alone. 1

• If bone is exposed or osteomyelitis suspected (probe-to-bone test), send debrided bone specimens for culture and histology. 1

Metabolic Stabilization

• Optimize glycemic control immediately as hyperglycemia impairs wound healing and immune function in diabetic foot infections. 1

• Correct fluid and electrolyte imbalances, acidosis, and azotemia before or concurrent with surgical intervention. 1

Vascular Assessment

• Evaluate for peripheral arterial disease as PAD is strongly associated with limb loss (p=0.001) in diabetic foot infections. 3

• Consider vascular surgery consultation if pulses are diminished or absent, as revascularization may be necessary. 1, 6

Wound Care and Off-loading

• Complete pressure off-loading of the affected toe is mandatory to allow healing. 1

• Daily wound inspection and appropriate dressing changes are required, though no specific dressing type has proven superiority. 6

Duration of Therapy

• For soft tissue infection alone: 2-3 weeks of antibiotic therapy is typically sufficient for moderate infections. 1

• If osteomyelitis is present: minimum 4-6 weeks of therapy is required, or shorter if all infected bone is surgically removed. 1, 6

• Antibiotics should be continued until clinical resolution of infection (no erythema, warmth, purulent drainage), but not necessarily until complete wound healing. 1

• Reassess daily and narrow therapy based on clinical response once improvement is documented. 2

De-escalation Strategy

• After 48-72 hours of clinical improvement, consider narrowing to targeted therapy based on these susceptibility results. 1

• Potential de-escalation options include:

  • Ampicillin 2 g IV every 4-6 hours for E. faecalis (predictably susceptible based on penicillin susceptibility) 1
  • Plus ciprofloxacin 400 mg IV every 12 hours for A. baumannii (MIC ≤0.25, highly susceptible) 4

• Transition to oral therapy when systemically stable: amoxicillin 875 mg PO every 8 hours plus ciprofloxacin 750 mg PO every 12 hours. 1

Common Pitfalls to Avoid

• Do not treat based on culture results alone without clinical signs of infection, though this case clearly shows infection with moderate bacterial growth. 7

• Do not add empiric anaerobic coverage unless there is necrotic tissue, foul odor, or gas in tissues—none of which are indicated here, and anaerobic culture was negative. 1

• Do not use daptomycin as monotherapy despite intermediate susceptibility (MIC 4) for E. faecalis, as it is not FDA-approved for diabetic foot infections and requires combination therapy. 1

• Avoid trimethoprim-sulfamethoxazole as A. baumannii shows resistance (MIC >2/38). 1

• Do not rely on tigecycline alone despite susceptibility, as it has poor tissue penetration and is not recommended as monotherapy for serious infections. 1

Monitoring and Follow-up

• Daily clinical assessment for the first 3-5 days to ensure response to therapy, including reduction in erythema, warmth, and drainage. 6

• If no improvement after 3-5 days, consider imaging (MRI preferred) to evaluate for deeper abscess, osteomyelitis, or need for additional surgical intervention. 1

• Weekly follow-up after initial improvement until complete resolution of infection signs. 6