Infectious Disease and Microbiology - Acinetobacter Species

Basics

Acinetobacter is a genus of aerobic gram-negative bacteria that includes several species capable of causing human disease. Clinically important members include Acinetobacter baumannii, A. calcoaceticus, A. haemolyticus, A. johnsonii, A. junii, A. lwoffii, and A. radioresistens, together with several less commonly identified or unnamed species.

Among these organisms, A. baumannii is the most important cause of serious healthcare-associated infection and is also the species most strongly associated with multidrug resistance.

Microbiologic Characteristics

Acinetobacter species are aerobic, nonmotile, encapsulated gram-negative organisms. During rapid growth they may appear more rod-shaped, whereas in the stationary phase they often have a coccobacillary appearance.

They are capable of colonizing human skin and mucosal surfaces and may therefore be recovered from patients without necessarily causing invasive disease. This ability to colonize patients and survive in the healthcare environment contributes substantially to their importance as nosocomial pathogens.

Epidemiology

Acinetobacter species have a worldwide distribution. They are particularly important in hospitals, where they can survive on environmental surfaces and medical equipment for prolonged periods.

Healthcare-associated infections occur most frequently in:

• Intensive care units

• Patients receiving mechanical ventilation

• Patients with prolonged hospitalization

• Patients with invasive devices

• Severely ill or immunocompromised individuals

A. baumannii has become especially important because hospital strains may acquire resistance to multiple antimicrobial classes.

Major Risk Factors

Important factors that predispose to Acinetobacter infection include prolonged hospitalization, ICU admission, mechanical ventilation, endotracheal intubation, urinary catheterization, vascular catheters, major surgery, severe trauma, burns, and previous exposure to broad-spectrum antibiotics.

Alcohol use disorder has historically been associated with community-acquired Acinetobacter pneumonia, although most serious contemporary infections are healthcare-associated.

Combat injuries and heavily contaminated traumatic wounds can also become infected with A. baumannii and related species.

Pneumonia

Acinetobacter can cause severe pneumonia, particularly in hospitalized and mechanically ventilated patients.

Ventilator-associated pneumonia is one of the most important clinical syndromes associated with A. baumannii. Patients may present with fever, increasing respiratory secretions, hypoxemia, leukocytosis, and new or progressive pulmonary infiltrates.

Because Acinetobacter can colonize respiratory secretions, isolation from tracheal aspirates does not always prove infection. The microbiologic result must therefore be interpreted together with the clinical and radiographic findings.

Community-acquired pneumonia caused by Acinetobacter is much less common but has historically been described in individuals with heavy alcohol use or major underlying illness.

Bacteremia

Acinetobacter bacteremia usually occurs in hospitalized patients and is frequently associated with:

• Central venous catheters

• Pneumonia

• Wound infection

• Urinary tract infection

• Severe systemic illness

Patients may progress to septic shock and multiorgan dysfunction, particularly when infection is caused by a highly resistant strain or when appropriate therapy is delayed.

Tracheobronchitis

Acinetobacter may cause tracheobronchitis, especially in patients with artificial airways or prolonged tracheal intubation.

Children with tracheostomies or other airway devices may also develop infection or colonization.

The distinction between airway colonization and true lower respiratory infection is clinically important.

Skin, Soft-Tissue, and Wound Infection

Acinetobacter can cause cellulitis and other skin and soft-tissue infections, particularly when associated with:

• Vascular catheters

• Major trauma

• Burns

• Surgical wounds

• Combat-related wounds

Burn wounds and traumatic wounds provide an especially favorable environment for colonization and invasive infection.

Local infection around an intravascular catheter may occasionally resolve only after the catheter is removed.

Urinary Tract Infection

Acinetobacter urinary tract infections usually occur in patients with complicated urinary systems rather than in otherwise healthy individuals.

Major risk factors include:

• Urinary catheterization

• Structural urinary tract disease

• Prolonged hospitalization

• Critical illness

• Previous antimicrobial therapy

Urinary isolates should be interpreted carefully because catheterized patients may have asymptomatic bacteriuria or colonization.

Meningitis

Acinetobacter can cause meningitis, particularly following:

• Neurosurgical procedures

• Cranial trauma

• Ventricular drains

• Other intracranial devices

Post-neurosurgical meningitis caused by multidrug-resistant A. baumannii can be particularly difficult to treat because antimicrobial penetration into cerebrospinal fluid may be limited.

Diagnosis

Diagnosis is based primarily on microbiologic culture from the clinically involved site.

Possible specimens include:

• Blood

• Respiratory secretions

• Bronchoalveolar lavage

• Urine

• Wound material

• Cerebrospinal fluid

• Catheter-tip cultures when clinically appropriate

Because Acinetobacter frequently colonizes hospitalized patients, a positive culture does not automatically establish infection.

The clinician must determine whether the isolate represents:

True infection or colonization.

Antimicrobial Susceptibility Testing

Susceptibility testing is essential because Acinetobacter, particularly A. baumannii, can display highly unpredictable resistance patterns.

Resistance may involve:

• Carbapenems

• Cephalosporins

• Fluoroquinolones

• Aminoglycosides

• Multiple β-lactams

For serious infection, definitive treatment should therefore be based on the susceptibility profile whenever possible.

Treatment

Treatment depends on the site and severity of infection and, most importantly, on antimicrobial susceptibility results.

Historically, carbapenems such as imipenem or meropenem were among the most reliable agents for susceptible Acinetobacter infections.

However, carbapenem-resistant A. baumannii has become a major worldwide problem, so carbapenems should not be assumed to be active without susceptibility confirmation.

Sulbactam

Sulbactam is unusual among β-lactamase inhibitors because it has intrinsic antibacterial activity against Acinetobacter.

This characteristic makes sulbactam-containing regimens particularly important in treatment.

Sulbactam generally has greater direct activity against Acinetobacter than:

• Clavulanic acid

• Tazobactam

Modern treatment of difficult A. baumannii infection often incorporates sulbactam-based therapy when the organism is susceptible or when high-dose sulbactam strategies are appropriate.

Other Potentially Active Agents

Depending on susceptibility results, active agents may include:

• Third- or fourth-generation cephalosporins

• Aminoglycosides

• Fluoroquinolones

• Doxycycline

• Minocycline

• Tigecycline

• Polymyxins

• Sulbactam-containing regimens

No single drug should be assumed effective against a resistant hospital strain.

Aminoglycosides

Amikacin or another active aminoglycoside may sometimes be added to treatment of severe systemic infection when susceptibility is demonstrated.

Combination therapy may be considered for:

• Septic shock

• Highly resistant infection

• Severe pneumonia

• Bacteremia

However, aminoglycoside toxicity and limited penetration into some tissues must be considered.

Polymyxins

Polymyxins such as colistin or polymyxin B may retain activity against extensively drug-resistant Acinetobacter.

They have historically been used as salvage agents, but their use is limited by toxicity, particularly nephrotoxicity and neurotoxicity.

They should therefore be used selectively and with careful monitoring.

Tigecycline and Tetracycline Derivatives

Tigecycline and certain tetracycline derivatives may show activity against Acinetobacter.

These agents may be useful in selected infections, but they are not ideal for all sites.

For example, tigecycline produces relatively low serum concentrations and therefore may be less appropriate as sole therapy for bloodstream infection.

Drug choice should always take infection site into account.

Catheter-Associated Infection

When a vascular or urinary catheter is clearly the source of infection, source control is important.

Management may include:

• Removal or replacement of the infected catheter

• Appropriate systemic antimicrobial therapy

Catheter removal alone is generally insufficient when bacteremia or invasive infection is present.

Antimicrobial Resistance

A. baumannii is generally more resistant than many other Acinetobacter species.

Clinically important resistance patterns include:

Multidrug-resistant Acinetobacter:

→ Resistance to multiple major antimicrobial classes

Carbapenem-resistant Acinetobacter:

→ Resistance to imipenem, meropenem, or related carbapenems

Extensively drug-resistant strains:

→ Susceptibility remains to very few agents

Resistance mechanisms may include β-lactamases, carbapenemases, altered membrane permeability, efflux pumps, and target modifications.

Infection Control

Strict infection-control practices are essential because Acinetobacter can persist on dry hospital surfaces and spread between patients.

Important measures include:

• Careful hand hygiene

• Contact precautions when indicated

• Proper cleaning and disinfection of equipment

• Environmental cleaning

• Appropriate management of ventilators and respiratory equipment

• Minimizing unnecessary invasive devices

• Antimicrobial stewardship

• Surveillance during outbreaks

ICUs require particular vigilance because vulnerable patients and frequent device use facilitate transmission.

Prevention

Prevention depends primarily on reducing healthcare-associated transmission.

Important strategies include minimizing unnecessary catheterization, shortening the duration of mechanical ventilation when possible, adhering to ventilator-associated pneumonia prevention measures, maintaining proper wound care, and following strict infection-control protocols.

Judicious antibiotic use is also important because excessive broad-spectrum antimicrobial exposure promotes selection of resistant Acinetobacter strains.

High-Yield Clinical Pattern

ICU patient

• ●

Mechanical ventilation

• ●

New pneumonia

• ●

Multidrug-resistant gram-negative coccobacillus

→ Think Acinetobacter baumannii.

High-Yield Resistance Pattern

Hospital-acquired infection

• ●

Carbapenem resistance

• ●

Few remaining antimicrobial options

→ Consider multidrug-resistant or carbapenem-resistant A. baumannii.

High-Yield Drug Feature

Sulbactam is not merely a β-lactamase inhibitor in Acinetobacter infection.

It also has:

Direct antibacterial activity against Acinetobacter.

High-Yield Infection Sites

Common Acinetobacter infections include:

• Ventilator-associated pneumonia

• Bacteremia

• Wound and burn infections

• Catheter-associated infections

• Complicated urinary tract infection

• Post-neurosurgical meningitis

Exam Essentials

Major pathogenic species:

→ Acinetobacter baumannii

Organism:

→ Aerobic gram-negative coccobacillus

Motility:

→ Nonmotile

Normal colonization:

→ Skin and mucosal surfaces

Major setting:

→ Healthcare-associated infection, especially ICU

Classic pulmonary syndrome:

→ Ventilator-associated pneumonia

Important wound association:

→ Burns and traumatic or combat wounds

Important CNS association:

→ Post-neurosurgical meningitis

Diagnosis:

→ Culture with susceptibility testing

Historically important susceptible-drug class:

→ Carbapenems

Important problem:

→ Carbapenem-resistant A. baumannii

β-lactamase inhibitor with intrinsic Acinetobacter activity:

→ Sulbactam

Potential agents for resistant infection:

→ Sulbactam-based therapy, selected tetracyclines, aminoglycosides, polymyxins, or other active agents according to susceptibility

Important non-drug treatment:

→ Source control, including removal of infected catheters when appropriate

Major prevention:

→ Strict hospital infection-control practices, especially in ICUs