Acinetobacter

Article Author:
Mark Brady
Article Editor:
Najwa Pervin
Updated:
12/28/2018 11:30:28 AM
PubMed Link:
Acinetobacter

Introduction

Acinetobacter is a gram-negative, aerobic, non-fermentative, oxidase negative and nonmotile organism.Acinteobacter has several different species but A.baumanii has the greatest known clinical significance. Acinetobacter can be found in soil and water.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.[1][2]

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 nosocomial spread and 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, mechanical ventilation and even peritoneal dialysis. In most cases, the presence of Acinetobacter in respiratory secretions of ventilated patients represents colonization.

Etiology

Even though Acinetobacter is mostly a nosocomial pathogen and is 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.[3][4]

Epidemiology

Acinetobacter gained clinical significance in the 1960s with the increasing growth of Intensive care units at hospitals. [5]Though Acinetobacter is an organism of low virulence, its ability to survive dessication and persist in the environment for extended duration of time makes it easily transmissible in the healthcare setting. Nosocomial spread by healthcare personnel, respiratory equipment like ventilators and other devices has been reported frequently. 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.[6]

Acinetobacter has also been recognized in wounds sustained by combat injuries in returning soldiers from Iraq and Afghanistan.

Mortality/Morbidity

The morbidity of this organism is related to the patient's underlying medical condition and immune status. The organism is not very virulent but due to various innate mechanisms it has the capacity to acquire resistance. Multidrug-resistant , extensively drug resistant and pan-drug resistant isolates of Acinetobacter have been described as non-susceptible to at least one agent in three or more antibiotic classes, non-susceptible to at least one agent in all but two or fewer antibiotic classes and non-susceptible to all antibiotic classes. Multi-drug resistant Acinetobacter was designated a threat category of ‘Serious’ Center of disease Control Antimicrobial resistance report published in 2013. Though not virulent by itself, the morbidity and mortality of Acinetobacter are high in sick patients with multiorgan disease. In the case of infections being caused by resistant strains of A.baumanii, mortality rates of upto 70% have been reported. [5]

Pathophysiology

Various mechanisms contribute to the pathogenesis and virulence of Acinetobacter. Acinetobacter elaborate Lipopolysaccharide (LPS) or Lipooligosaccharide (LOS) in their outer membranes. Modification in the synthesis of these structures impart antibiotic resistance and increased resistance dessication. Acinetobacter species also have Capsules that protect them from complement mediated killing. Pili on the surface on Acinetobacter contribute to its twitching motility, biofilm formation and adherence to environmental surfaces. Acinetobacter also secrete various proteins that lead to antibiotic resistance by efflux or degradation. [7]

Histopathology

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.

Pneumonia, wound infection, catheter-associated bacteremia or nosocomial meningitis have all been described by Acinetobacter.

There are no pathognomic findings in 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.

Laboratory Studies

There may be leucocytosis, with a left shift. However, the findings are nonspecific and do not always indicate the presence of a bacterial 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 analyzed and cultured.

Histologic Findings

There are no specific histopathological features of an Acinetobacter infection that can differentiate it from any other gram-negative bacilli

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. [6][7][8][9]

Any external device, infected line, shunt or drain must be removed to obtain a cure. If the patient has a collection of an abscess or necrotic tissue, it needs to be debrided thoroughly.

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

Over the last few years, drug resistance has become a common problem in the United States. Anti-microbials which may be used in Acinetobacter infections include colistin, carbapenems, tigecycline, polymyxin, amikacin, and beta-lactam/beta-lactamase combinations. But, there have been more recent outbreaks with extensively drug resistant Acinetobacter which makes the management of these infections much more complicated. The duration of therapy is from 7 to 10 days, depending on the patient illness.

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.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 tend to have a poor outcome.

Enhancing Healthcare Team Outcomes

Over the past decade, Acinetobacter infections have become common in hospitalized patients. Since this organism can infect almost any organ system a multidisciplinary approach to diagnosis and management is essential. Often the organism is difficult to isolate because of colonization and hence an infectious disease consult should be made. Infection control teams should be involved earlier on to prevent out-breaks and utilize ways to prevent nosocomial transmission by eradication of bacteria and strict isolation techniques. The outcomes for most patients with Acinetobacter infection is excellent as long as there is no other comorbidity. However, those with immunosuppression may have a guarded prognosis.

Management should involve a coordinated effort of nurses, pharmacists, and clinicians.