Updated: Jun 06, 2023
  • Author: Shirin A Mazumder, MD, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
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Acinetobacter baumannii is a pleomorphic aerobic gram-negative bacillus (similar in appearance to Haemophilus influenzae on Gram stain) commonly isolated from the hospital environment and hospitalized patients. A baumannii is a water organism and preferentially colonizes aquatic environments. It is also a natural inhabitant of soil. This organism is often cultured from hospitalized patients' sputum or respiratory secretions, wounds, and urine. In a hospital setting, Acinetobacter commonly colonizes irrigating solutions and intravenous solutions.

The Acinetobacter has more than 50 species, most of which are nonpathogenic environmental organisms. The most common infection-causing species is A baumannii, followed by Acinetobactercalcoaceticus and Acinetobacterlwoffii. [1]

The organism has several features which contribute to its virulence. Acinetobacter contains fimbriae that can assist in attaching to different surfaces in the environment. [2]  The formation of a biofilm is critical in allowing the organism to survive in dry environmental conditions (ref94}.

Most Acinetobacter isolates recovered from hospitalized patients, particularly those recovered from respiratory secretions and urine, represent colonization rather than infection. Care must be exercised in determining whether the isolate is due to colonization or is truly causing infection. Multiple factors tend to increase the risk of acquiring an Acinetobacter infection, including prior antibiotic exposure, intensive care unit admission, use of a central venous catheter, and mechanical ventilation or hemodialysis use. Acinetobacter species can be transmitted to patients because of their persistence on environmental surfaces and because of colonization of the hands of healthcare workers. [1]

When Acinetobacter infections occur, they usually involve organ systems that have a high fluid content (eg, respiratory tract, CSF, peritoneal fluid, urinary tract). These infections may occur as outbreaks rather than isolated cases of nosocomial pneumonia. Infections may complicate continuous ambulatory peritoneal dialysis (CAPD) or cause catheter-associated bacteruria. The presence of Acinetobacter isolates in respiratory secretions of intubated patients may represent colonization. Acinetobacter pneumonias occur in outbreaks and are usually associated with colonized respiratory-support equipment or fluids.

The 2005 IDSA guidelines for hospital-acquired pneumonia discuss the role of Acinetobacter as a cause of nosocomial pneumonia. Nosocomial meningitis may occur in colonized neurosurgical patients with external ventricular drainage tubes. [3, 4]

A baumannii is inherently resistant to multiple antibiotics.




When Acinetobacter causes actual infection, the pathological changes that occur depend on the organ system involved. The pathological changes, as observed in patients with pneumonia, are indistinguishable from those caused by other noncavitating aerobic gram-negative bacilli that cause nosocomial pneumonias. Similarly, Acinetobacter urinary tract infections are clinically indistinguishable from catheter-associated bacteremias caused by other aerobic gram-negative bacilli.

The predominant predispositions to infection include colonization pressure, exposure to broad-spectrum antibiotics, and disruption of anatomic barriers. [1]






Acinetobacter commonly colonizes patients in the intensive care setting. Acinetobacter colonization is particularly common in patients who are intubated and in those who have multiple intravenous lines or monitoring devices, surgical drains, or indwelling urinary catheters. Acinetobacter infections are uncommon and occur almost exclusively in hospitalized patients.


Although Acinetobacter is primarily a colonizer in the hospital environment, it occasionally causes infection. Mortality and morbidity resulting from A baumannii infection relate to the underlying cardiopulmonary immune status of the host rather than the inherent virulence of the organism.

Mortality and morbidity rates in patients who are very ill with multisystem disease are increased because of their underlying illness rather than the superimposed infection with Acinetobacter.


Acinetobacter infection has no known racial predilection.


Acinetobacter infection has no known sexual predilection.


Acinetobacter infection has no known predilection for age.



The prognosis of Acinetobacter infection depends on the underlying health of the host and the extent of organ involvement; it is the same as for other aerobic gram-negative bacillary infections.