What is Acinetobacter baumannii?

Acinetobacter baumannii: A Major Nosocomial Pathogen

Acinetobacter baumannii is a highly problematic Gram-negative, multidrug-resistant pathogen that primarily causes nosocomial infections in intensive care units (ICUs), characterized by its extraordinary environmental persistence and ability to develop resistance to virtually all conventional antimicrobials. 1

Microbiological Characteristics

• A. baumannii is a Gram-negative, strictly aerobic, non-fermentative coccobacillus that is widely distributed in nature 2
• It is characterized by its remarkable ability to persist in the environment for extended periods, enabling rapid spread within healthcare settings 1
• The pathogen exhibits an extraordinary capability to develop resistance to all conventional antimicrobials and some biocides 1
• Accurate identification to species level is clinically important, as A. baumannii has higher mortality rates compared to other Acinetobacter species 1

Epidemiology and Clinical Significance

• A. baumannii is a major cause of nosocomial infections worldwide, particularly in ICUs 1
• In the EPIC II prevalence study, A. baumannii was the fifth most common pathogen in ICUs globally, with significant regional variations 1
• It ranks as the third most common pathogen in ventilator-associated pneumonia (VAP) in Europe, after S. aureus and P. aeruginosa 1
• In a multicenter study across 24 countries, A. baumannii was the most frequently identified pathogen in hospital-acquired bloodstream infections 1

Clinical Manifestations

• A. baumannii causes a wide spectrum of infections, including:
  • Ventilator-associated pneumonia (most common) 3, 4
  • Bloodstream infections and bacteremia 3, 4
  • Skin and soft tissue infections, particularly in trauma patients 3
  • Surgical site infections 5
  • Urinary tract infections 5
  • Meningitis (less common but reported) 3

Risk Factors

• Previous antibiotic use (primary risk factor) 3, 4
• Mechanical ventilation 3, 4
• Prolonged ICU or hospital stay 3, 4
• Severity of underlying illness, particularly hematological malignancies 2
• Presence of invasive devices (central venous catheters, endotracheal tubes, nasogastric tubes) 2
• Immunosuppression, including corticosteroid use for treating graft versus host disease in transplant recipients 2

Antimicrobial Resistance

• A. baumannii exhibits multiple resistance mechanisms, including:
  • Production of β-lactamases, including carbapenemases 6
  • Alteration of antibiotic target sites 6
  • Regulation of antibiotic transport through bacterial membranes 6
  • Enzymatic modifications resulting in antibiotic neutralization 6
• Multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains are increasingly common 3
• Alarmingly increasing resistance to last-resort antibiotics such as carbapenems, colistin, and polymyxins 2
• Geographic variations in resistance patterns exist, with globally and regionally resistant strains described 2

Treatment Approaches

• Empirical coverage of A. baumannii is recommended in severe infections (severe sepsis or septic shock) occurring during outbreaks, in endemic settings, or in previously colonized patients 1
• For carbapenem-susceptible strains, carbapenems remain the drugs of choice 7
• For carbapenem-resistant strains, polymyxins (colistin or polymyxin B) are active agents 7
  • Colistin dosing: loading dose of 9 million IU followed by maintenance of 4.5 million IU every 12 hours (adjusted for renal function) 7
  • Polymyxin B may be preferred due to less nephrotoxicity at 1.5-3 mg/kg/day with a loading dose of 2-2.5 mg/kg 7
• Tigecycline is an alternative for skin/soft tissue or intra-abdominal infections, but should not be used for bacteremia 7
• For severe infections, combination therapy may be required 7

Infection Control Measures

• Strict contact precautions for all patients with A. baumannii 7
• Alert systems to promptly identify colonized or infected patients 7
• Thorough environmental cleaning with appropriate disinfectants 7
• Surveillance cultures during outbreaks 7
• Implementation of antibiotic stewardship programs 7

Prognosis

• Mortality rates in patients with bacteremia or septic shock may reach 70% 2
• Good prognosis factors include:
  • Localized infection 2
  • Absence of multidrug-resistant strain 2
  • Uncomplicated infection 2
• Poor outcome is associated with:
  • Severe underlying medical illness 2
  • Bacteremia and septic shock 2
  • Multi-organ failure 2
  • Healthcare-associated infections 2
  • ICU admission requiring higher levels of care 2

Emerging Therapeutic Approaches

• Novel approaches being investigated include bacteriophage therapy and artilysin-based treatments 3
• Ongoing research focuses on identifying new drug targets and designing novel antimicrobials to combat A. baumannii infections 5