Acinetobacter baumannii complex

Background

Microbiology

• Contains Acinetobacter baumannii, Acinetobacter nosocomialis, and Acinetobacter pittii
• Non-motile, non-fermenting Gram-negative bacillus

Antimicrobial Resistance

• A number of mechanisms
• Carbapenem resistance is usually mediated by acquisition of OXA-type class D carbapenemase
  • Less common mechanisms include acquisition of class B (VIM, IMP, and NDM) carbapenemases, loss of outer membrane CarO protein, and modification of AdeABC efflux pump

Management

• Choice of antibiotic depends on susceptibility testing
• Possible options include:
  • Cefepime 2 g IV every 8 hours (infused over 3 to 4 hours)
  • Ceftazidime 2 g IV every 8 hours (infused over 3 to 4 hours)
  • Piperacillin-tazobactam 4.5 g every 8 hours (infused over 4 hours)
  • Ampicillin-sulbactam 3 g IV every 3 to 4 hours (mild, for mild) or 9 g IV every 8 hours (infused over 4 hours, for severe)
  • Meropenem 2 g IV every 8 hours (infused over 3 hours), though can do 1 g for cystitis
  • Imipenem 500 mg IV every 6 hours
  • Ciprofloxacin 400 mg IV every 8 hours or 750 mg p.o. every 12 hours
  • Levofloxacin 750 mg p.o./IV daily
  • Minocycline 200 mg p.o./IV every 12 hours or doxycycline 100 mg p.o./IV every 12 hours
  • Cefiderocol
  • Tigecycline
  • Colistin and polymyxin B (though Acinetobacter junii has inherent resistance)
• Aminoglycosides may not penetrate well into lungs and brain, so are usually avoided
• Bacteriophages are promising

Carbapenem-Resistant Acinetobacter baumannii

• Infection must be distinguished from colonization of the airway or wound
• Resistance may be mediated by a number of β-lactamases, including OXA-24/40-like carbapenemases, OXA-23-like carbapenemases, and metallo-β-lactamases, and often has sulbactam resistance
• Often have concurrent aminoglycoside-modifying enzymes or 16S rRNA methyltransferases, which confer resistance to aminoglycosides including plazomicin
• Refer to ESCMID guidelines1
• Single-agent treatment may be sufficient for mild infections
  • High-dose ampicillin-sulbactam is preferred, at a dose of either 9 g IV q8h infused over 4 hours, or 27 g IV q24h continuous infusion
  • If susceptible, polymixin B or high-dose tigecycline
• Combination treatment with at least two agents that have in vitro activity for most other infections
  • Options include minocycline, tigecycline, aminoglycosides, or polymyxin B
  • Ampicillin-sulbactam may remain effective in non-susceptible isolates when used at high doses
  • Fosfomycin and rifampin are not recommended
  • After clinical improvement, step down to single-agent therapy
• Cefiderocol should generally be avoided