Acinetobacter is a group of bacteria commonly found in soil, water, and dry environments. Acinetobacter poses very little risk to immune-competent people and the infections are mainly confined in healthcare settings housing very ill patients. People with a weakened immune system are susceptible to infections with Acinetobacter. They acquire Acinetobacter infections by person-to-person contact or contact with contaminated surfaces.
Immunocompromised patients i.e. people who have weakened immune systems, chronic lung disease, or diabetes are susceptible to this infection. Very ill patients on a ventilator, those with a prolonged hospital stay, persons having invasive devices like urinary catheters are at greater risk of Acinetobacter infections. Outbreaks of Acinetobacter infections typically occur in intensive care units (ICU).
Acinetobacter can live on the skin and may survive in the environment/inanimate surfaces for several days. Acinetobacter is associated with skin colonization of hospital personnel and may also “colonize” or live in a patient without causing infection or symptoms, especially in tracheostomy sites or open wounds.
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Diseases caused by Acinetobacter
While there are many species of Acinetobacter and all can cause human disease, Acinetobacter baumannii for about 80% of reported infections. Acinetobacter causes a variety of diseases, ranging from pneumonia to serious blood or wound infections, and the symptoms vary depending on the disease. It is an important cause of ventilator-associated pneumonia and catheter-related bacteremia.
Biochemical Properties of Acinetobacter
Acinetobacter was identified by Gram staining, cell and colony morphology, positive catalase test, negative oxidase test, and absence of motility.
| Biochemical Tests | Results |
| Gram Staining | Gram-negative cocci or coccobacilli |
| Catalase Test | Positive (+ve) |
| Oxidase Test: | Negative (-ve) |
| Oxidative fermentative (O/F) test | Non-fermentative |
| Motility | Negative |
| Oxygen requirement | Strictly aerobic |
| Growth requirements | Non-fastidious |
| Indole Test | Negative |
| Methyl Red Test | Negative |
| Voges Proskauer (VP) test | Negative |
| Citrate Test | Positive (+ve) some species may not give a positive citrate utilization test |
| Urease Test | Negative |
| Nitrate Reduction Test | Negative (-ve) |
| Hemolysis | Negative except. A.haemolyticus |
| Chloramphenicol | Resistant |
| Arginine Hydrolysis | +ve |
| H2S production | Negative |
Drug Resistance and Antibiotics in Use
Acinetobacter is often resistant to many commonly prescribed antibiotics. Multiple Drug Resistance (MDR) patterns observed in Acinetobacter baumannii (MDR-AB) currently pose significant challenges for the management and treatment of infections. CDC has categorized Multidrug-resistant Acinetobacter as a serious threats to public health.
There are few antimicrobial agents that are commonly used for the treatment of infections with Acinetobacter baumannii .
These antibiotics are:
- Amikacin: Among Aminoglycosides, amikacin and tobramycin still shows activity against many A. baumannii isolates. Resistance with these drugs are increasing and drug susceptibility is needed to find out susceptibility pattern of a particular isolate.
- Carbapenems: Carbapenems (imipenem, meropenem, or doripenem) is one of the most important therapeutic options available to treat infections with multidrug-resistant A.baumannii. Antibiotics resistance (resistance ranging from 10% to as high as 70%) with carbapenems is increasing and the strains which are resistant to carbapenems are also resistant to majority of other antibiotics (except polymyxins or tigecycline).
- Colistin: An antibiotic of the Polymyxins group, colistin (polymyxin E) still shows excellent activity against Acinetobacter baumannii. Polymyxin B is also used in clinical practice. Colistin is effective for various Gram-negative rod-shaped bacteria, including multidrug-resistant Pseudomonas aeruginosa and Klebsiella pneumoniae.
- Tigecycline: A member of the glycylcylines antibiotics, tigecycline is a semisynthetic derivative of minocycline and inhibits the 30S ribosomal subunit. It has broader specturm of activity. Tigecycline is being used to treat MDR-Acinetobacter baumannii infections but clinical data is not sufficient to ascertain its accuracy.
- Sulbactam: Sulbactam possesses greatest intrinsic bactericidal activity against A.baumannii isolates. It is one of the commonly used β-lactamase inhibitors and commercially available in combination with ampicillin.
