Acinobacter
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Acinetobacter — A Comprehensive Overview
Note: The genus name is spelled Acinetobacter (not "Acinobacter").
1. Microbiology & Classification
Acinetobacter species are:
- Gram-negative, aerobic coccobacilli (rod-shaped in early log phase, coccobacillary in stationary phase)
- Oxidase-negative, catalase-positive, nonmotile, non-lactose-fermenting
- Morphologically resemble Neisseria (appear as diplococci on smears) — distinguished by being oxidase-negative (Neisseria is oxidase-positive)
- Grow well on most standard agar media; optimum temperature 35–37°C; some A. baumannii strains grow at 44°C
Clinically relevant species:
| Species | Clinical relevance |
|---|---|
| A. baumannii | Most common; dominant nosocomial pathogen |
| A. nosocomialis | Healthcare-associated infections |
| A. pittii | Healthcare-associated infections |
| A. ursingii | Healthcare infections |
| A. lwoffii | Skin colonizer; occasional infections in immunocompromised |
| A. radioresistens | Skin colonizer |
Species-level identification requires MALDI-TOF MS or real-time PCR; phenotypic methods alone are unreliable. — Harrison's Principles of Internal Medicine 22E, p. 1338; Jawetz, Melnick & Adelberg's Medical Microbiology 28E, p. 264
2. Epidemiology
- Ubiquitous in nature (water, soil, fruits, vegetables); colonizes human skin, respiratory tract, and GI tract
- Hospital pathogen: predominantly ICU patients, especially those on mechanical ventilation
- A. baumannii can survive desiccation on surfaces for weeks — a major driver of nosocomial spread
- CDC estimate: ~12,000 Acinetobacter infections/year in the US; 7,300 due to multidrug-resistant (MDR) strains; ~500 deaths annually
- Three international clonal lineages (ICL); ICL I and ICL II are MDR
- COVID-19 pandemic worsened control efforts: 35% increase in hospital-based carbapenem-resistant Acinetobacter, 78% increase in hospital-onset cases (CDC data)
- Historically associated with tropical/hot climates; now worldwide, including temperate regions
- Notorious in military conflict wounds (e.g., Iraq war); "Iraqibacter"
— Harrison's 22E, p. 1338; Goldman-Cecil Medicine, p. 3141
3. Pathogenesis & Virulence Factors
| Mechanism | Details |
|---|---|
| Biofilm formation | Adhesion to surfaces, medical devices, and human cells; mediated by Acinetobacter trimeric autotransporter (Ata) |
| Outer-membrane protein A (OmpA) | Induces apoptosis in host cells; packaged in outer-membrane vesicles (OMVs) |
| Outer-membrane vesicles (OMVs) | Contain proteases, phospholipases, OmpA; trigger potent innate immune responses |
| Capsule | Antiphagocytic; essential for virulence |
| Iron acquisition systems | Siderophores (acinetobactin) and surface receptors enable iron scavenging from host |
| Phospholipases | Disrupt host cell membranes |
| Type VI secretion system | Kills competing bacteria; enables niche establishment |
— Harrison's 22E, p. 1339
4. Antimicrobial Resistance Mechanisms
A. baumannii is one of the most resistant clinical pathogens (part of the ESKAPE group):
| Resistance mechanism | Details |
|---|---|
| β-lactamases | Intrinsic AmpC (class C); acquired ESBLs; OXA-type carbapenemases |
| Carbapenem resistance | OXA-23-like, OXA-24/40-like, OXA-58-like, OXA-143-like, OXA-235-like (Ambler class D oxacillinases); also metallo-β-lactamases |
| Porin loss | Reduced expression of outer-membrane porins → decreased β-lactam entry |
| Efflux pumps | Active extrusion of β-lactams, quinolones, tetracyclines, chloramphenicol, tigecycline, disinfectants |
| Horizontal gene transfer | Alien islands flanked by integrases/transposases in the accessory genome |
ISAba1 insertion sequence next to the ampC gene dramatically upregulates AmpC expression → resistance to most cephalosporins. — Harrison's 22E, p. 1339
5. Risk Factors for Infection
- Prolonged ICU stay
- Mechanical ventilation / tracheostomy
- Central venous catheterization
- Nursing home residence
- Enteral feedings
- Prior antibiotic exposure (3rd-generation cephalosporins, fluoroquinolones, carbapenems — especially for carbapenem-resistant acquisition)
- Immunocompromised state
- Severe trauma or burn wounds
6. Clinical Manifestations
Pneumonia
- Most common clinical syndrome
- Ventilator-associated pneumonia (VAP): late-onset; fever, ↑ sputum production; non-specific CXR (lobar consolidation, pleural effusion; cavitation rare)
- Crude mortality: up to 65%
- Community-acquired pneumonia (rare): severe; fever, purulent cough, multi-organ dysfunction; mortality >50%
- Key challenge: distinguishing colonization from true infection in respiratory specimens
Bloodstream Infections
- Usually ICU patients with central venous catheters or secondary to VAP
- Polymicrobial in 20–36% of episodes
- Treatment with inappropriate antibiotics is an independent predictor of mortality
Other Sites
| Site | Notes |
|---|---|
| Meningitis | Post-neurosurgical; healthcare-associated |
| Wound infections | Traumatic (war wounds) and nosocomial burn wounds |
| Urinary tract infections | Associated with indwelling urinary catheters and biofilm production |
| Skin/soft tissue | Uncommon; seen in immunocompromised |
— Harrison's 22E, p. 1339; Goldman-Cecil Medicine, p. 3142
7. Diagnosis
- Culture: grows on standard media (blood agar, MacConkey agar); oxidase-negative oxidase test key differentiator
- MALDI-TOF MS: now standard for species-level identification
- Antimicrobial susceptibility testing: mandatory before therapy
- Molecular methods (PCR, WGS) for resistance gene detection and outbreak investigation
8. Treatment
Treatment depends on susceptibility profile. The organism is classified as:
- MDR: resistant to ≥3 antimicrobial classes
- XDR (Extensively Drug-Resistant): susceptible to only 1–2 classes
- PDR (Pan-Drug-Resistant): resistant to all tested agents
| Susceptibility | Preferred agents |
|---|---|
| Susceptible strains | Aminoglycosides (gentamicin, amikacin, tobramycin); extended-spectrum penicillins; ampicillin-sulbactam |
| Carbapenem-susceptible | Meropenem or imipenem (drug of choice) |
| Carbapenem-resistant | Sulbactam (as ampicillin-sulbactam); tigecycline; polymyxins (colistin, polymyxin B); minocycline; cefiderocol |
| XDR/PDR | Combination therapy (e.g., polymyxin + carbapenem, or polymyxin + tigecycline); cefiderocol |
Sulbactam (given as ampicillin-sulbactam in the US) is a cornerstone of carbapenem-resistant Acinetobacter therapy due to sulbactam's intrinsic activity against Acinetobacter (independent of its β-lactamase inhibitor role). Cefiderocol, a novel siderophore cephalosporin, has activity against many XDR strains. — Goldman-Cecil Medicine, p. 3141–3142
9. Infection Control
Given A. baumannii's environmental persistence:
- Contact precautions (gown and gloves)
- Strict hand hygiene — soap & water or alcohol-based hand rub
- Environmental cleaning and disinfection of surfaces and equipment
- Judicious antibiotic stewardship
- Surveillance cultures in high-risk units
- Cohorting of infected/colonized patients during outbreaks
10. Summary Table
| Feature | Acinetobacter baumannii |
|---|---|
| Gram stain | Gram-negative coccobacilli |
| Oxidase | Negative |
| Catalase | Positive |
| Motility | Nonmotile |
| Key habitat | Soil, water, hospital surfaces |
| Environmental survival | Weeks (desiccation-resistant) |
| Key virulence | Biofilm, OmpA, OMVs, capsule |
| Key resistance | OXA carbapenemases, efflux pumps, porin loss |
| Primary syndrome | VAP, bloodstream infection |
| Treatment (CR strains) | Sulbactam, cefiderocol, polymyxins, tigecycline |
| Classification | ESKAPE pathogen; WHO Critical Priority |
Sources: Harrison's Principles of Internal Medicine 22E, p. 1338–1340; Goldman-Cecil Medicine, p. 3141–3142; Jawetz, Melnick & Adelberg's Medical Microbiology 28E, p. 264; Medical Microbiology 9E
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