Acinetobacter baumannii

Overview

Acinetobacter baumannii is a gram-negative, oxidase-negative, nonmotile, non-fermenting coccobacillus. First described in 1911 as Micrococcus calcoaceticus, the genus has been known as Acinetobacter since 1950. The species is catalase-positive and can resemble Neisseria as diplococci on smears — differentiated by its oxidase-negative reaction. Species-level identification often requires MALDI-TOF mass spectrometry or qPCR, as phenotypic methods alone are insufficient.

Microbiology

Property	Detail
Gram stain	Gram-negative coccobacillus
Oxidase	Negative
Catalase	Positive
Motility	None
Growth temp	35–37°C; some strains grow at 44°C
Key feature	Survives environmental desiccation for weeks

Other clinically relevant species include A. nosocomialis, A. pittii, and A. ursingii. A. lwoffii colonizes human skin and can infect immunocompromised patients.

Epidemiology

Natural habitat: water, soil, fruits, vegetables; colonizes human skin, respiratory and GI tracts.
Hospital survival: ability to persist on dry surfaces and equipment for weeks is a critical infection-control concern.
US burden: CDC estimates ~12,000 Acinetobacter infections/year; 7,300 caused by multidrug-resistant (MDR) strains; ~500 deaths annually.
International Clonal Lineages (ICL): ICL I and ICL II are predominantly MDR and drive global outbreaks.
Pangenome structure: small core genome + large accessory genome → high genetic plasticity; antimicrobial resistance (AMR) genes reside in both, often in alien genomic islands acquired via horizontal gene transfer.
COVID-19 setback: pandemic-era increases in carbapenem-resistant A. baumannii (CRAB) reported worldwide.

Risk Factors for Infection

Prolonged ICU stay
Nursing home residence
Mechanical ventilation / tracheostomy
Central venous catheters
Enteral feeds
Prior exposure to 3rd-generation cephalosporins, fluoroquinolones, or carbapenems (highest risk for CRAB acquisition)

Clinical Syndromes

Site	Notes
VAP (ventilator-associated pneumonia)	Most common; major cause of ICU mortality
Bloodstream infection	~2% of all BSIs; associated with intravascular devices
Wound infections	Traumatic (military combat injuries) and nosocomial
Meningitis	Post-neurosurgical
UTI	Biofilm on indwelling catheters
Skin/soft tissue	Burns, traumatic wounds

Pitfall: Differentiating colonization vs. true infection (especially from respiratory specimens) is clinically critical but often difficult.

Resistance Mechanisms

Intrinsic β-lactamases: inactivate 1st and 2nd-generation cephalosporins
Acquired ESBLs: extend resistance to 3rd- and 4th-generation cephalosporins
OXA-type carbapenemases (class D β-lactamases): primary mechanism of carbapenem resistance (OXA-23, OXA-24, OXA-58, OXA-51)
Biofilm formation: impedes antibiotic penetration
Efflux pumps, outer membrane protein loss, altered PBPs

Treatment

Susceptible isolates

Ampicillin-sulbactam (preferred — sulbactam has intrinsic activity against Acinetobacter)
Alternatives: cefepime, meropenem, imipenem-cilastatin (per susceptibility testing)
Susceptible strains may also respond to aminoglycosides (gentamicin, amikacin, tobramycin)

Carbapenem-Resistant A. baumannii (CRAB) — 2024 IDSA Guidance

Regimen	Dose	Notes
Sulbactam-durlobactam + carbapenem	1 g/1 g q6h (3-h infusion)	Preferred; FDA-approved 2023
High-dose ampicillin-sulbactam	3 g q4h (30 min) or 9 g q8h (4-h infusion) or 27 g/24h continuous	Alternative; combined with polymyxin B, minocycline, tigecycline, or cefiderocol
Cefiderocol	2 g q8h (3-h infusion)	Use in combination; siderophore cephalosporin
Colistin / Polymyxin B	—	Last resort; significant nephrotoxicity
Minocycline / Tigecycline	—	Combination use

Sulbactam-durlobactam rationale: Durlobactam is a novel diazabicyclooctane (non-β-lactam) β-lactamase inhibitor that protects sulbactam against Acinetobacter-derived cephalosporinases and OXA-family carbapenemases. High-dose ampicillin-sulbactam saturates PBP2 and PBP3 to optimize cell-wall synthesis inhibition.

Key clinical trial: In 125 CRAB patients (pneumonia or BSI), sulbactam-durlobactam + imipenem-cilastatin achieved 28-day mortality of 19% vs 32% with colistin + imipenem-cilastatin.

Important: No regimen has been proven superior for CRAB in large RCTs. Combination therapy is recommended because monotherapy is prone to failure due to high bacterial loads and rapid resistance emergence.

Infection Control

Strict hand hygiene and environmental decontamination (organism survives weeks on dry surfaces)
Contact precautions for colonized/infected patients
Antibiotic stewardship to limit carbapenem exposure

Recent Evidence Update

A 2025 systematic review in Clinical Microbiology Reviews (PMID 40787933) specifically addresses drug-resistant CRAB mortality and emerging treatments, noting sulbactam-durlobactam and cefiderocol as the most promising agents, with phage therapy and novel β-lactam/β-lactamase inhibitor combinations as future targets.

Sources: Harrison's Principles of Internal Medicine 22e (2025), Jawetz Melnick & Adelbergs Medical Microbiology 28e, Medical Microbiology 9e; 2024 IDSA Guidance on AMR Gram-Negative Infections.

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