Treatment of Co-infection with Acinetobacter baumannii
For co-infections involving A. baumannii, initiate empirical therapy with a polymyxin (colistin or polymyxin B) combined with high-dose ampicillin-sulbactam (9-12 g/day in 3-4 divided doses), then tailor therapy based on susceptibility testing and the specific co-infecting pathogen. 1, 2
Initial Empirical Approach
When A. baumannii co-infection is suspected (during an outbreak, endemic setting, or in previously colonized patients), you must cover both A. baumannii and the co-pathogen immediately:
For Carbapenem-Resistant A. baumannii (Most Common Scenario)
- Primary regimen: Intravenous polymyxin (colistin 6-9 million IU loading dose, then 9 million IU/day in 2-3 doses OR polymyxin B 2-2.5 mg/kg loading dose, then 1.5-3 mg/kg/day) 2
- Add: High-dose ampicillin-sulbactam 9-12 g/day in 3 divided doses (given as 4-hour infusions) 2
- Consider adjunctive inhaled colistin for pneumonia 1
For Carbapenem-Susceptible A. baumannii (Low Resistance Areas)
- Use carbapenem monotherapy (imipenem 0.5-1 g q6h or meropenem 2 g q8h) if susceptible 2
- Adjust coverage for the co-pathogen based on local resistance patterns
Critical Pitfalls to Avoid
Never use tigecycline or sulbactam as monotherapy for empirical coverage 2. While sulbactam has intrinsic activity against A. baumannii, monotherapy is inadequate for severe infections until susceptibilities are confirmed.
Avoid tigecycline for HAP/VAP caused by Acinetobacter - the guidelines explicitly recommend against it due to poor lung penetration and inferior outcomes 1. If you must use tigecycline (only for approved indications like intra-abdominal infections with MIC ≤1 mg/L), use high-dose regimen (200 mg loading, then 100 mg q12h) and always combine with another active agent 2.
Definitive Therapy Based on Susceptibilities
Once susceptibility results return, de-escalate or optimize:
If Susceptible to Carbapenems
- Switch to carbapenem monotherapy (imipenem or meropenem at standard doses) 2
If Susceptible to Sulbactam (MIC ≤4 mg/L)
- Prefer ampicillin-sulbactam 9-12 g/day over polymyxins when both are active - better safety profile with comparable efficacy and significantly lower nephrotoxicity (15.3% vs 33%) and mortality 2
- Recent evidence shows sulbactam-based therapy achieves higher microbial clearance in polymicrobial infections compared to monomicrobial A. baumannii infections 3
If Only Polymyxin-Susceptible (Carbapenem-Resistant)
- Continue IV polymyxin (colistin or polymyxin B) 1, 2
- Add adjunctive inhaled colistin for pneumonia 1
- Do NOT add rifampicin - multiple RCTs show no clinical benefit despite microbiological eradication, plus increased hepatotoxicity 1, 2
Combination vs Monotherapy
The evidence is nuanced here:
- For non-septic shock patients with known susceptibilities: Use monotherapy with an active agent 1
- For septic shock or high mortality risk (>25%): Consider combination therapy with two active agents until shock resolves 1
- For polymicrobial infections: Recent data suggests sulbactam combinations (particularly sulbactam + polymyxins) achieve better microbiological and clinical outcomes than monotherapy, with optimal sulbactam dose ≥8 g/day 3
Dosing Specifics for Normal Renal Function
| Agent | Loading Dose | Maintenance |
|---|---|---|
| Colistin | 6-9 million IU | 9 million IU/day (q12h or q8h) |
| Polymyxin B | 2-2.5 mg/kg | 1.5-3 mg/kg/day in 2 doses |
| Sulbactam | None | 9-12 g/day in 3-4 doses (4-hour infusions) |
| Imipenem | None | 0.5-1 g q6h |
| Meropenem | None | 2 g q8h |
Special Considerations for Co-infections
When A. baumannii co-exists with other pathogens (commonly P. aeruginosa or K. pneumoniae in ICU settings):
- Ensure your regimen covers both organisms - polymyxins have activity against most gram-negatives, but verify susceptibilities
- Sulbactam-based combinations show superior outcomes in polymicrobial vs monomicrobial infections, particularly when combined with polymyxins 3
- For P. aeruginosa co-infection: Add an anti-pseudomonal beta-lactam if not already covered
- Monitor for antagonism: Some combinations may be antagonistic; avoid empirically combining agents with overlapping mechanisms
Monitoring and Adjustment
- Obtain susceptibility testing immediately - this is non-negotiable for optimizing therapy 1
- Monitor renal function closely - polymyxins cause nephrotoxicity in 30-50% of patients; polymyxin B may have slightly better safety profile than colistin 2
- Reassess at 48-72 hours - if septic shock resolves and susceptibilities are known, de-escalate to monotherapy 1
- Consider source control - no antibiotic regimen compensates for inadequate drainage or device removal
The key principle: Start broad with polymyxin-based combination therapy when carbapenem resistance is suspected, then narrow based on susceptibilities, always ensuring coverage of co-pathogens while minimizing toxicity.