Acinetobacter is a group of bacteria found naturally in soil and water that has become one of the most concerning causes of hospital-acquired infections worldwide. The species that matters most to human health, Acinetobacter baumannii, is classified by the CDC as an urgent public health threat because many strains resist nearly all available antibiotics. Outside of healthcare settings, these bacteria rarely cause disease.
Basic Biology of Acinetobacter
Under a microscope, Acinetobacter appears as a small, rod-shaped bacterium that stains Gram-negative, meaning it has a thin cell wall surrounded by an outer membrane. It doesn’t move on its own, it requires oxygen to grow, and it can’t ferment sugars the way many other bacteria do. These might sound like limitations, but Acinetobacter compensates with a remarkable ability to survive in harsh conditions.
The genus includes many species, but Acinetobacter baumannii (sometimes called the “A. baumannii complex”) is responsible for the vast majority of human infections. Other species exist in the environment and occasionally show up in clinical samples, but they’re far less likely to cause illness.
Why It Thrives in Hospitals
What makes A. baumannii particularly dangerous is its ability to persist on dry hospital surfaces for weeks. Strains that form biofilms, a protective slime layer that bacteria build around themselves, survive an average of 36 days on dry surfaces. Strains that don’t form biofilms last about 15 days. That difference matters because biofilm-forming bacteria are harder to wipe out with routine cleaning and can linger on bedrails, ventilator tubing, IV poles, and other equipment long enough to spread from patient to patient.
The bacterium has also been found colonizing intravenous fluids, which gives it a direct route into the bloodstream. Combined with its surface durability, this makes hospital intensive care units an ideal environment for outbreaks.
Who Gets Infected
Acinetobacter infections overwhelmingly affect people who are already seriously ill and receiving intensive medical care. The strongest risk factors include being on a mechanical ventilator, having a weakened immune system, and having cancer or other conditions that compromise the body’s defenses. Patients in septic shock face dramatically higher mortality risk if they develop an Acinetobacter infection.
The bacterium causes several types of infections. Pneumonia, particularly ventilator-associated pneumonia, is one of the most common and serious. It also causes bloodstream infections, urinary tract infections, and wound infections. A. baumannii accounts for roughly 8% of ventilator-associated pneumonia cases and about 2% of catheter-related bloodstream infections reported through the CDC’s national surveillance system.
Healthy people in the community almost never develop Acinetobacter infections. If you’ve encountered this term, it’s most likely because you or someone you know is hospitalized.
How It Evades the Immune System
A. baumannii carries an arsenal of tools that help it survive inside the human body. Its most abundant surface protein helps the bacterium resist killing by human blood serum and can trigger programmed cell death in human cells. Another surface protein binds to a blood clotting protein called fibronectin, helping the bacterium attach to tissues. Yet another protein interferes with the complement system, a key part of the immune response that normally tags bacteria for destruction.
The bacterium also releases small bubble-like packages from its outer membrane that carry toxins and immune-evading proteins into surrounding tissue. These packages can damage host cells even at a distance from the bacterium itself. On top of all this, A. baumannii has specialized systems for scavenging iron from the host, an essential nutrient that the body normally keeps locked away from invading microbes.
The Antibiotic Resistance Problem
Carbapenem-resistant A. baumannii, known as CRAB, is classified by the CDC as an urgent public health threat. Carbapenems are powerful, broad-spectrum antibiotics typically reserved as a last resort for serious infections. When A. baumannii resists them, treatment options shrink dramatically. Some CRAB strains resist every available antibiotic.
Most CRAB strains in the United States produce enzymes called carbapenemases that break down carbapenem antibiotics before they can work. The most commonly identified types are OXA-23 and OXA-24/40. Multi-drug resistant A. baumannii causes approximately 7,000 infections and 500 deaths per year in the United States, according to CDC estimates.
In hospitalized patients with confirmed A. baumannii infections, the numbers are sobering. One study of 321 symptomatic inpatients found an all-cause in-hospital mortality rate of 44%. The 14-day mortality rate was 12%, and the 28-day mortality rate was 13%. For ventilator-associated pneumonia specifically, mortality rates range from 13% to 30% depending on the patient population and resistance profile of the strain.
Treatment Challenges
Treating CRAB infections is one of the hardest problems in infectious disease. Standard carbapenems alone don’t work against resistant strains. Current guidance from the Infectious Diseases Society of America (2024) points toward combination therapies, sometimes involving three antibiotics used simultaneously, to achieve effectiveness against CRAB. Even then, clinical trials have shown mixed results. Two randomized trials comparing a common last-resort antibiotic alone versus in combination with a carbapenem found no benefit from adding the carbapenem.
The limited options mean that doctors treating CRAB infections often work with infectious disease specialists to tailor therapy based on the specific resistance pattern of each patient’s isolate. For patients and families, this translates to longer hospital stays, more complex treatment regimens, and a higher risk of poor outcomes compared to infections caused by more treatable bacteria.
How Hospitals Prevent Spread
Controlling Acinetobacter requires aggressive, layered infection prevention. Hospitals dealing with outbreaks implement contact precautions, meaning healthcare workers wear gowns and gloves when entering the room of an infected or colonized patient. Patients with CRAB are isolated, ideally in single rooms, and healthcare worker assignments are sometimes dedicated so staff caring for CRAB patients don’t simultaneously care for uninfected patients.
Cleaning intensity increases substantially during outbreaks. In one documented ICU outbreak, patient areas were cleaned three times daily using both manual wiping and automated systems like vaporized hydrogen peroxide. The facility also switched its standard surface disinfectant from a quaternary ammonium compound to an alcohol-based agent, which proved effective in halting transmission. Environmental cleaning was supervised with checklists, and staff-to-patient ratios were tightened from 3:1 to 2:1.
These measures work, but they require sustained effort and resources. The ability of A. baumannii to survive for weeks on surfaces means that even a brief lapse in cleaning protocols can allow the bacterium to persist in a unit and seed new infections.