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Updated: Dec 8, 2025
Separating Bacteria by Capsule Amount Using a Discontinuous Density Gradient
Published on: January 7, 2019
Rapid identification of capsulated Acinetobacter baumannii using a density-dependent gradient test
Hadas Kon1, David Schwartz1, Elizabeth Temkin1
1National Institute for Antibiotic Resistance and Infection Control, Ministry of Health, Tel-Aviv Sourasky Medical Center, 6 Weizmann St, 6423906, Tel-Aviv, Israel.
Background:
Gram-negative bacterial capsules are associated with production of carbohydrates, frequently resulting in a mucoid phenotype. Infections caused by capsulated or mucoid A. baumannii are associated with increased clinical severity. Therefore, it is clinically and epidemiologically important to identify capsulated A. baumannii. Here, we describe a density-dependent gradient test to distinguish between capsulated and thin/non-capsulated A. baumannii.
Results:
Thirty-one of 57 A. baumannii isolates displayed a mucoid phenotype. The density-dependent gradient test was comprised of two phases, with silica concentrations of 30% (top phase) and 50% (bottom phase). Twenty-three isolates migrated to the bottom phase, indicating thin or non-capsulated strains, and 34 migrated to the top phase, suggesting strains suspected to be capsulated. There was agreement between the mucoid and the non-mucoid phenotypes and the density-dependent gradient test for all but three isolates. Total carbohydrates extracted from strains suspected to be capsulated were significantly higher. Transmission electron microscopy confirmed the presence of a capsule in the six representative strains suspected to be capsulated.
Conclusions:
The density-dependent gradient test can be used to verify capsule presence in mucoid-appearing A. baumannii strains. Identifying capsulated strains can be useful for directing infection control measures to reduce the spread of hypervirulent strains.
Insights
A novel density-dependent gradient test effectively identifies capsulated Acinetobacter baumannii, differentiating them from non-capsulated strains. This method aids in controlling infections caused by these hypervirulent bacteria.
Area of Science:
- Microbiology
- Bacterial Pathogenesis
Background:
- Gram-negative bacterial capsules, rich in carbohydrates, often cause a mucoid phenotype.
- Mucoid or capsulated Acinetobacter baumannii infections are linked to increased clinical severity.
- Accurate identification of capsulated A. baumannii is crucial for clinical and epidemiological management.
Purpose of the Study:
- To develop and validate a density-dependent gradient test for distinguishing capsulated from non-capsulated A. baumannii.
- To correlate the test results with observed mucoid phenotypes and carbohydrate content.
Main Methods:
- A two-phase density gradient test using 30% and 50% silica concentrations was employed.
- Acinetobacter baumannii isolates were assessed for migration patterns within the gradient.
- Carbohydrate content analysis and transmission electron microscopy were used for verification.
Main Results:
- Out of 57 A. baumannii isolates, 31 exhibited a mucoid phenotype.
- The density-dependent gradient test showed high agreement with mucoid/non-mucoid phenotypes.
- Strains migrating to the top phase (suggesting capsulation) had significantly higher carbohydrate content and confirmed capsules via electron microscopy.
Conclusions:
- The density-dependent gradient test is a reliable method for verifying capsule presence in mucoid A. baumannii.
- This test can aid in identifying hypervirulent strains for targeted infection control measures.
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