Rapid identification of capsulated Acinetobacter baumannii using a density-dependent gradient test

Hadas Kon,David Schwartz,Jonathan Lellouche,Elizabeth Temkin,Yehuda Carmeli

doi:10.1186/s12866-020-01971-9

Hadas Kon, David Schwartz + Show 3 more

Open Access

https://doi.org/10.1186/s12866-020-01971-9

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Abstract

BackgroundGram-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.ResultsThirty-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.ConclusionsThe 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.

Highlights

Gram-negative bacterial capsules are associated with production of carbohydrates, frequently resulting in a mucoid phenotype
A. baumannii virulence factors include siderophore-mediated iron acquisition systems, biofilm formation, motility, and a remarkable capacity to acquire and rearrange genetic determinants [2]. These virulence factors are involved in the pathobiology and infection process, such as binding to host epithelial cells, cellular damage, serum resistance, and invasion [3]
Colonies of mucoid A. baumannii appeared moist, raised, and viscid with irregular margins, while non-mucoid strains displayed a typical phenotype of small, round, convex colonies with distinct margins

Summary

Introduction

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. A. baumannii virulence factors include siderophore-mediated iron acquisition systems, biofilm formation, motility, and a remarkable capacity to acquire and rearrange genetic determinants [2]. These virulence factors are involved in the pathobiology and infection process, such as binding to host epithelial cells, cellular damage, serum resistance, and invasion [3]. Few methods are available to quantify surface carbohydrates and to determine their composition. Clinical laboratories do not incorporate these methods because they are cumbersome and require hazardous materials

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