Acinetobacter: Introduction, Morphology, Pathogenecity, Lab Diagnosis and Treatment

Acinetobacter in Gram stain of sputum

Acinetobacter in Gram stain of sputum

Acinetobacter encapsulated strain in Gram stain of sputum from ICU patient as shown above picture.

Scientific classification

  • Domain: Bacteria
  • Phylum: Proteobacteria
  • Class: Gammaproteobacteria
  • Order: Pseudomonadales
  • Family: Moraxellaceae
  • Genus: Acinetobacter
  • Species: A. baumannii


Gr. a, not;

Gr. kineô, to set in motion, move;

acinetus, unable to move;

bacter, a rod;

Acinetobacter, nonmotile rod.

baumannii, of Baumann (named in honor of Paul and Linda Baumann)

L .calxcis, limestone, chalk;

L. acidum aceticum, acetic acid; calcoaceticus, pertaining to calcium acetate, which was used by Beijerinck in the enrichment medium from which he isolated the organism.


Genus definition


  • 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
  • Fimbriate
  • Most strains are capsulate

Cultural characteristics

  • 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


Previously……….A. calcoaceticus

  • var anitratus
  • var lwoffii

At least 33 Genomic species of which few have been given formal species name, e.g.,

  • A. calcoaceticus
  • A. baumannii
  • A. pittii
  • A. nosocomialis
  • A. haemolyticus
  • A. junii
  • A. johnsonii
  • A. lwoffii
  • A. radioresistens

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)


  1. Polysaccharide capsule
  2. Adhesion to human epithelial cells (fimbriae, capsule)
  3. Lipases
  4. Lipopolysaccharide
  5. Siderophores

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.

Risk factors


  • Hospitalization
  • Significant co-morbidity
  • Mechanical ventilation
  • Cardiorespiratory failure
  • Previous infection
  • Antimicrobial therapy
  • CVP lines
  • Urinary catheters

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
  • Highly antibiotic-resistant
  • 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.)

Antibiotic resistance

  • 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),

efflux pumps

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

  • Selective media
  • Herellea agar
  • Leeds acinetobacter medium

Unique browning effect on blood agar in presence of D-Glucose by glucose oxidizing strains

CHROMagar Acinetobacter agar is the latest addition to the clinical range of chromogenic media developed by Dr.Alain Rambach.

TSI – Alk/No change

SIM – H2S Negative, Indole Negative, Motility- Non-motile

Simmon’s Citrate – Citrate Utilized

Carbohydrate breakdown – Oxidative

Current trends in Antibiograms

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.

Typing methods

  1. Biotyping
  2. Phage-typing
  3. Serotyping
  4. Bacteriocin typing
  5. Molecular fingerprinting
  6. Plasmid profiling

Preventing Acinetobacter Transmission in the ICU
Outbreak Interventions

  • Hand cultures
  • Surveillance cultures
  • Environmental cultures following terminal disinfection to document cleaning efficacy
  • Cohorting
  • 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.

Further Readings

  • 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.
  • Lippincott’s –Illustrated- review-Microbiology-3rd Edn.
  • Mandell’s Infectious Disease-7th Edn.
  • Bergey’s Manual of Systemic Bacteriology- 2nd  Edn.
  • The Practice of Medical Microbiology Vol-12th Edn. –Robert Cruickshank
  • District Laboratory Practice in  Tropical Countries –  Part-2-   Monica Cheesebrough-   2nd Edn Update
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