Acinetobacter baumannii is an opportunistic bacterium and a globally recognized threat to human health, especially within hospital settings. Often labeled a “superbug,” this organism causes serious, life-threatening infections that are increasingly challenging to treat. Its designation as a high-priority pathogen stems from its unique combination of environmental resilience and its ability to resist multiple classes of antibiotics. Understanding this bacterium—where it lives, how it spreads, and why it is difficult to eliminate—is necessary to combat its growing impact on vulnerable patient populations.
The Organism and Its Environment
Acinetobacter baumannii is a Gram-negative coccobacillus, appearing as a short, slightly rounded rod shape. It is non-motile, relying on external factors for movement, and is an obligate aerobe, requiring oxygen to thrive. These characteristics contribute to its behavior as an opportunistic pathogen primarily found in healthcare environments.
A defining feature of this organism is its ability to survive outside a host for extended periods, sometimes for up to a year on inanimate surfaces. This resilience is partly due to its resistance to desiccation, allowing it to persist on dry hospital equipment and surfaces (fomites). The bacterium is also adept at forming biofilms, which are complex, protective communities encased in a sticky matrix. This biofilm acts as a shield, protecting the bacteria from disinfectants and the body’s immune responses.
While other Acinetobacter species exist naturally in soil and water, A. baumannii is almost exclusively isolated from hospital environments, making it a classic nosocomial (hospital-acquired) pathogen. It thrives particularly well in Intensive Care Units (ICUs) where moisture is often present on medical equipment. This adaptation allows it to colonize equipment like ventilators and catheters, which become reservoirs for infection.
Transmission and Risk Factors
The spread of A. baumannii within healthcare settings occurs through direct and indirect contact with contaminated sources. Direct transmission involves contact with the skin or respiratory secretions of an infected or colonized patient. Indirect transmission, which is more common and difficult to control, happens when the organism moves from a contaminated surface or equipment to a patient.
Healthcare workers’ hands are the primary vector for indirect transmission, especially after touching contaminated items like bed rails, monitors, or intravenous pumps. Strict adherence to hand hygiene protocols is essential for managing outbreaks. Specific activities, such as performing a wound dressing or interacting with an endotracheal tube, carry a greater risk of contaminating a worker’s hands or gloves. Preventing the spread requires constant vigilance and meticulous sterilization of all shared equipment.
The risk factors for developing an A. baumannii infection are linked to a patient’s medical condition and exposure to the hospital environment. Patients who have prolonged hospital stays, particularly those in the ICU, are at the highest risk. The use of invasive medical devices, such as mechanical ventilators, urinary catheters, and central venous lines, provides the bacterium with direct access to the body’s internal systems. Patients with compromised immune systems, chronic lung disease, or open wounds from surgery or trauma are also susceptible.
Types of Infections Caused
Once exposed, A. baumannii can cause severe infections, often starting as colonization on a patient’s skin or mucosal membranes before progressing to invasive disease. Hospital-acquired pneumonia is the most frequently reported manifestation, particularly ventilator-associated pneumonia (VAP) in patients on breathing machines. VAP caused by this organism carries a high mortality rate due to treatment difficulty and the patient’s already fragile state.
The bacterium is also a common cause of bloodstream infections (bacteremia), which can rapidly lead to sepsis. These infections are particularly dangerous and are associated with a poor prognosis, especially in critically ill individuals. Case fatality rates for severe infections can approach 70% when the causative strain is resistant to multiple drugs.
Other serious infections include urinary tract infections (UTIs), especially in patients with indwelling urinary catheters, and wound or surgical site infections. A. baumannii can colonize open wounds, particularly those sustained from major trauma, impeding healing and potentially spreading to deeper tissues. Less common but serious manifestations include meningitis, an infection of the membranes surrounding the brain and spinal cord, which typically occurs following neurosurgical procedures.
The Challenge of Antibiotic Resistance
The primary reason Acinetobacter baumannii is considered a “superbug” is its ability to resist multiple classes of antibiotics, classifying it as a Multi-Drug Resistant (MDR) organism. This resistance is dynamic; the bacterium readily acquires new resistance genes from its environment and other bacteria, often clustering them on mobile genetic elements. Its inclusion in the ESKAPE group of pathogens highlights its threat due to its ability to “escape” the effects of antibacterial drugs.
The most concerning resistance is against carbapenems, a class of broad-spectrum antibiotics often reserved as a last line of defense against severe Gram-negative infections. Carbapenem-resistant A. baumannii (CRAB) is designated by the World Health Organization as a critical priority pathogen requiring urgent research and development of new treatments. This resistance is often mediated by carbapenemase enzymes, which chemically break down the antibiotic molecule, rendering it ineffective.
With resistance to common antibiotics, treatment often relies on older, more toxic drugs, such as polymyxins (colistin and polymyxin B). These drugs were previously avoided due to side effects like nephrotoxicity (damage to the kidneys), but have been reintroduced out of necessity. Treatment failure rates for polymyxin monotherapy can be high, leading clinicians to use combination therapy, pairing a polymyxin with another agent. Resistance to polymyxins is also emerging, complicating patient management and underscoring the limited pipeline of effective new agents.