Saprophytes; parasites and occasional pathogens of man and other animals
Good growth on nutrient media at 20-30°C
G+C content 39-47 mol%
Ubiquitous, free-living saprophytes found in soil, water, foods and the clinical environment
Dry or moist inanimate surfaces and as commensals on the skin of man and animals
Short, plump, gram-negative bacilli
1.0-1.5 mm x 1.5-2.5 mm (log phase)
0.6-0.8 mm x 1- 1.5 mm (coccoid in stationary phase)
May retain crystal violet (Gram’s stain)
No flagella; however, may show twitching motility
Most strains are capsulate
Most clinical isolates (37°C) and many environmental isolates are at lower temperatures.
Grow well on ordinary solid media
Nutrient agar – Smooth, mucoid, greyish white colonies, 2-3 mm in diameter
MacConkey agar – Non-lactose fermenting
Blood agar– usu. Non-hemolytic
May show surface spreading associated with twitching motility
Nutrient broth – uniform turbidity
At least 33 Genomic species of which few have been given formal species name, e.g.,
Acinetobacter calcoaceticus baumannii complex
The infection has been seen as more common in summer. (McDonald LC, Banerjee SN, Jarvis WR. Seasonal variation of Acinetobacter infections: 1987-1996. Clin Infect Dis 1999;29:1133-7)
Adhesion to human epithelial cells (fimbriae, capsule)
Biofilms enhance the Pathogenicity
A. baumannii forms biofilms with enhanced antibiotic resistance and, more recently, that a chaperone-usher secretion system involved in Pilus assembly affects biofilm formation.
Factors Promoting Transmission of Acinetobacter in the ICU
Long survival time on inanimate surfaces
In vitro survival time 329 days (Wagenvoort JHT, Joosten EJAJ. J Hosp Infect 2002;52:226-229)
11 days survival on Formica, 12 days on stainless steel (Webster C et al. Infect Control Hosp Epidemiol 2000;21:246)
Up to 4 months on dry surfaces(Wendt C et al. J Clin Microbiol 1997;35:1394-1397)
Extensive environmental contamination
A high proportion of colonized patients
Frequent contamination of the hands of healthcare workers
ACB complex has been associated with various nosocomial infections including:
Pneumonia (esp. ventilator-associated)
Septicemia (true Acinetobacter bacteremia should be distinguished from pseudobacteremia).
Meningitis (esp. post-trauma or surgery)
Wound infections (in association with an indwelling venous catheter)
Since Operation Iraqi Freedom began in 2003, more than 700 US soldiers have been infected or colonized with Acinetobacter baumannii. A significant number of additional cases have been found in the Canadian and British armed forces and among wounded Iraqi civilians.
Origin of Iraqibacter
Where the Iraqibacter came from remains something of a mystery. Soil samples taken by researchers in Iraq and Kuwait came back negative. However, it was found thriving in the hospitals. When Iraqibacter was compared to MDRAB samples taken in Europe before the war, they were found to be identical (Silberman, 2007). Thus, scientists believe that the current outbreak originated from European sources.
( So MDRAB did exist before the Iraq War.)
Most strains are resistant to ampicillin, first-generation cephalosporins, chloramphenicol
Resistance mechanisms that are expressed frequently in nosocomial strains include
alterations in cell-wall channels (porins),
A. baumannii can become resistant to quinolones through mutations in the genes gyrA and parC. Resistant to aminoglycosides by expressing aminoglycoside-modifying enzymes.
Laboratory isolation and identification
Leeds acinetobacter medium
Unique browning effect on blood agar in presence of D-Glucose by glucose oxidizing strains
CHROMagarAcinetobacter agar is the latest addition to the clinical range of chromogenic media developed by Dr.AlainRambach.
Most A. baumannii are now resistant to ampicillin, Carbenicillin, Cefotaxime, and Chloramphenicol. Resistance to Gentamycin, tobramycin, and amikacin is increasing. Fluoroquinolones, ceftazidime, Trimethoprim-Sulphmethoxazole, Doxycycline, Polymyxin B, colistin, imipenem, and meropenem may retain activity against Nosocomial Acinetobacter.
Carbapenems (Imipenem and Meropenem) are the mainstay of treatment for antimicrobial-resistant gram-negative infections, though Carbapenems-resistant Acinetobacter is increasingly reported. Resistance to the Carbapenems class of antibiotics makes multidrug-resistant Acinetobacter infections difficult, if not impossible, to treat.
Multidrug-Resistant strains a Global Concern
Multidrug-resistant A. baumannii is a common problem in many hospitals in the US and Europe. First-line treatment is with a Carbapenems antibiotic such as imipenem, but carbapenem resistance is increasingly common. Other treatment options include Polymyxin, Tigecycline, and Aminoglycosides.
Treating the Resistant Infections
Colistin and Polymyxin B have been used to treat highly resistant Acinetobacter infections. The choice of appropriate therapy is further complicated by the toxicity of colistin which is mainly renal. Acinetobacter isolates resistant to colistin and Polymyxin B has also been reported.
Preventing Acinetobacter Transmission in the ICU Outbreak Interventions
Environmental cultures following terminal disinfection to document cleaning efficacy
Ask laboratory to save all isolates for molecular typing
Healthcare worker education
If transmission continues despite the above interventions, closure of unit to new admissions
Hand Hygiene is an Important Preventive option
Acinetobacter can live on the skin and may survive in the environment for several days. Careful attention to infection control procedures such as hand hygiene and environmental cleaning can reduce the risk of transmission.
Topley and Wilson’s microbiology and microbial infection – Bacteriology-2-10th Edn.
Manual of Clinical Microbiology-Patrick R. Murray -8th Edn.
Bailey and Scott’s Diagnostic Microbiology -13th Edn.
Mackie & Mc Cartney Practical Medical Microbiology – 14th Edn.
Diagnostic Microbiology -Connie R. Mahon & George Manuselis
Cowan and Steel’s, manual for the identification of medical bacteria
Koneman Color Atlas and Textbook of Diagnostic Microbiology-6th Edn.
Jawetz Melnick and Adelberg’s Medical Microbiology- 25th Edn.