Acinetobacter

Article Author:
Mark Brady
Article Editor:
Najwa Pervin
Updated:
11/13/2018 8:04:27 PM
PubMed Link:
Acinetobacter

Introduction

Acinetobacter is a gram-negative organism which is very similar to Haemophilus influenzae. Acinetobacter is aerobic, non-fermentative, oxidase negative and nonmotile organisms. Acinetobacter is frequently isolated in hospitalized patients. Some of the species of Acinetobacter colonize water environments. In patients, they are frequently cultured from the urine, saliva, respiratory secretions and open wounds. The organism is also known to colonize intravenous fluids and other irrigation solutions.

In general, Acinetobacter has low virulence but is capable of causing infection in immunocompromised and neutropenic patients. Most of the infections are a result of colonization rather than de novo infections. Thus, great care is required when acinetobacter is isolated - whether it is an actual infection or just colonization. Risk factors for Acinetobacter infection include:

  1. Prolonged stay in the intensive care until 
  2. Prior antibiotic exposure 
  3. Mechanical ventilation 
  4. Use of a central venous catheter
  5. Hemodialysis

Most acinetobacter infections are group outbreaks, and isolated cases are rare. Infections may complicate intravenous (IV) catheter treatment and even peritoneal dialysis. In most cases, the presence of Acinetobacter in respiratory secretions of ventilated patients represents colonization. Acinetobacter baumanniii is now very resistant to numerous antibiotics.

Etiology

Even though Acinetobacter is frequently isolated from hospitalized patients, care should be exercised in determining if the isolate is a cause or simply a result of colonization. Acinetobacter is a water-loving organism and has the propensity to colonize body organs that contain fluid. Thus, in hospitalized patients, Acinetobacter is often found in the peritoneal fluid, cerebrospinal fluid (CSF), saliva, respiratory secretions and urinary tract.

Epidemiology

Acinetobacter is now commonly isolated in hospitalized patients. It can be found in all types of secretions such as wounds, saliva, urine, and blood. The organism has low virulence but is still capable of causing infections in patients with febrile neutropenia and those who have received organ transplants.

Mortality/Morbidity

Even though acinetobacter frequently causes colonization, it can sometimes cause an infection. The morbidity of this organism is related to the patient's underlying medical condition and immune status. The organism is not very virulent. The morbidity and mortality of Acinetobacter are high in sick patients with multiorgan disease. 

Acinetobacter has no racial predilection and has the potential to infect anyone. Both genders are equally susceptible to the organism, and the infection can occur at any age.

Pathophysiology

The infection caused by Acinetobacter is similar in histopathology as any other gram-negative bacilli. Gross or microscopic observation do not provide any clue to the organism and culture is required to confirm the diagnosis.

History and Physical

Prolonged hospitalization or antibiotic therapy predisposes to Acinetobacter colonization, and because colonization is the rule and infection is the exception, colonized patients have no physical findings.

Most patients are in hospital when infected with Acinetobacter. The one organ most commonly involved is the lung, primarily because of colonization of the airways and respiratory equipment used for mechanical ventilation.

Because colonization is very common with Acinetobacter, the majority of patients have no physical findings. However, if an infection is present, it may be pneumonia, wound infection, catheter-associated bacteremia or nosocomial meningitis.

There is nothing specific about Acinetobacter infections, and they need to be differentiated from other gram-negative infections like Enterobacter, Burkholderia, Pseudomonas, and Serratia. Since Acinetobacter is chiefly a colonizing organism, the physician has the onus to prove that it is causing the pathology in any given clinical scenario.

Evaluation

Acinetobacter is a common colonizer of patients in the intensive care unit and those who have multiple comorbid disorders. It is most likely to cause infections in patients who are immunocompromised and those with a compromise of their cardiopulmonary system. The organism can readily be cultured, but the findings need to be correlated to the clinical picture.

Blood Work

There may be leucocytosis, with a left shift. However, the findings are nonspecific and do not always indicate the presence of a bacterial infection. Specimen obtained from the different organs may grow Acinetobacter, but this does not always mean that there is an infection. When there is an outbreak of Acinetobacter, the organism is usually readily isolated and cultured from body fluids. More important, the outbreak usually involves multiple patients.

Imaging Studies

A chest x-ray is required if pneumonia is suspected. Other imaging tests depend on the signs and symptoms.

Procedures

If meningitis is suspected, then CSF needs to be cultured.

Histologic Findings

There are no specific histopathological features of an Acinetobacter infection that can differentiate it from any other gram-negative bacilli, except those caused by Pseudomonas or Klebsiella.

Treatment / Management

Being a gram-negative organism, the drugs used to treat Acinetobacter infections include the aminoglycosides, fourth-generation cephalosporins, tigecycline, and rifampin. The organism will not respond to macrolides, third-generation cephalosporins, and penicillin. When an infection is suspected in the presence of a long-term catheter or a pacemaker, it should be removed.

Any external device, infected line, shunt or drain must be removed to obtain a cure.

One should avoid treating colonization as it only leads to more antibiotic resistance.

In most cases, combination therapy is required, but so far there are no studies showing that this approach is better than a single antibiotic.

Over the last few years, drug resistance has become a common problem in the United States. Most acinetobacter species are resistant to multiple antibiotics. Medications that are usually sensitive to acinetobacter include colistin, meropenem, tigecycline, polymyxin, amikacin, and rifampin. The duration of therapy is from 7 to 10 days, depending on the patient illness.

If the patient has a collection of an abscess or necrotic tissue, it needs to be debrided thoroughly.

Pearls and Other Issues

Patients in the intensive care unit are the most difficult to treat as colonization is common and it is difficult to distinguish this from an infection. All patients who are noted to have colonization with Acinetobacter should be isolated from other patients to prevent further colonization. Unfortunately, this is not always a practical maneuver in the intensive care unit. Once an infection is treated, the patient's clinical course must be followed rather than cultures, because colonization may offer a falsely positive diagnosis.

The prognosis of an isolated Acinetobacter infection is excellent in patients who are otherwise healthy. Patients who are immunosuppressed or neutropenic tend to have a poor outcome.