Rapid identification of capsulated Acinetobacter baumannii using a density-dependent gradient test
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METHODOLOGY ARTICLE
Open Access
Rapid identification of capsulated Acinetobacter baumannii using a densitydependent gradient test Hadas Kon1, David Schwartz1, Elizabeth Temkin1, Yehuda Carmeli1,2 and Jonathan Lellouche1*
Abstract 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/noncapsulated 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. Keywords: Carbapenem-resistant Acinetobacter baumannii, Capsule, Mucoid phenotype, Hypervirulence
Background Acinetobacter baumannii is a major cause of hospitalacquired infections [1]. 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]. Surface * Correspondence: [email protected] 1 National Institute for Antibiotic Resistance and Infection Control, Ministry of Health, Tel-Aviv Sourasky Medical Center, 6 Weizmann St, 6423906 Tel-Aviv, Israel Full list of author information is available at the end of the article
carbohydrates are known to influence virulence and fitness of A. baumannii. These carbohydrates include capsular polysaccharides (capsule), lipooligosaccharide (LOS) and the exopolysaccharide poly-β-(1–6)-N-acetylglucosamine (PNAG). While surface carbohydrates influence pathogenicity, studies show that the capsule is the predominant virulence factor in A. baumannii [4, 5]. Capsules are highmolecular-weight hydrophilic polymers that form a layer enveloping the bacterial cell. In A. baumannii, capsules are composed
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