Acinetobacter: Genus definition, Habitat, Morphology, Pathogenecity, Lab Diagnosis and Treatment
Acinetobacter
Acinetobacter calcoaceticus -baumanii complex on MacConkey agar is resembling late lactose fermenter colonies as above image.
Genus definition
- Short, stout gram-negative coccobacilli
- Strictly aerobic
- non-motile
- Catalase-positive, oxidase-negative
- Frequently capsulated
- Nitrates are reduced only rarely
- 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%
Habitats
- 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
Morphology
- 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
Pathogenicity
- Polysaccharide capsule
- Adhesion to human epithelial cells (fimbriae, capsule)
- Lipases
- Lipopolysaccharide
- 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
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
Treatment
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.
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