Importance of the polysaccharide capsule

The outer envelope of the A. baumannii cell is coated by a packed layer of polysaccharides, known as the capsule. At the immediate interface between the cell and the environment (Fig. 1), the capsule (otherwise known as capsular polysaccharide or CPS) plays an important role in many biological processes of A. baumannii and is a major determinant of virulence (Talyansky et al., 2021). Capsule fundamentally acts as a protective barrier providing resistance to complement- and phagocyte-mediated killing driving host innate immune evasion (Russo et al., 2010, Lees-Miller et al., 2013; Talyansky et al., 2021), but also to external stresses including desiccation, disinfectants, and certain antimicrobials (Tipton et al., 2018, Geisinger & Isberg, 2015). As one of the outer-most features of the cell, the capsule is also a primary receptor for many A. baumannii-specific bacteriophage (Gordillo Altamirano et al., 2021, Knirel et al., 2020, Oliveira et al., 2017) and a target for specific vaccines (Yang et al., 2017) and immunotherapies (Nielsen et al., 2021). These non-antibiotic therapeutic strategies rely on recognition and specificity for epitopes in the capsule structure, hence identifying and characterising the many diverse forms that strains may produce is important.

Structural diversity

Most A. baumannii isolates produce one of more than 100 distinct capsule types (Cahill et al., 2022, Wyres et al., 2020). These diverse structural formats exist as complex heteropolymers of repeating oligosaccharide glycan units, known as K-units, that can include between 2-8 sugars. The number, combination and order of sugars or non-carbohydrate constituents in the K-unit may differ between capsule types, as well as the glycosidic linkages between sugars or the K-units themselves (e.g. Kenyon et al., 2014, Kenyon et al., 2016, Arbatsky et al., 2022, Vinogradov et al., 2014). Interestingly, many capsular types have been found to include one of the nine known isomers of bacterial non-2-ulosonic acids. Three of these complex nine-carbon acidic sugars, acinetaminic acid (Kenyon et al., 2015), 8-epiacinetaminic acid (Kenyon et al., 2017) and 8-epipseudaminnic acid (Shashkov et al., 2022), are so far found exclusively in A. baumannii. While the role of different capsule types and relative pathogenicity has not been clearly established, some types have been associated with increased virulence in vivo (Talyansky et al., 2021) and/or poorer patient outcomes (Hsieh et al., 2021, Yang et al., 2022).

Genetics of capsule biosynthesis

Extensive diversity in A. baumannii capsular polysaccharides is primarily driven by the many possible combinations of genes that strains may carry at the chromosomal 'K locus' (Kenyon & Hall, 2013, Cahill et al., 2022, Wyres et al., 2020). Most genes required for capsule biosynthesis are clustered together at this location between the conserved fkpA and lldP genes (Kenyon & Hall, 2013). However, genes that influence the determination of the type of capsule produced have also been found in integrated genomic islands (Kenyon et al., 2016) or prophage regions (Arbatsky et al., 2022) elsewhere in the chromosome. Therefore, to simplify the ability to detect genetic differences at the K locus, and hence predict the type of capsule structure produced by an isolate, any new combination of genes found at this genomic location is assigned a unique KL identifier (e.g. KL1) and annotated using a standardised nomenclature system (Kenyon & Hall, 2013). More than 240 distinct KL have been described to date (Cahill et al., 2022), and are included in an international database of KL reference sequences (Cahill et al., 2022, Wyres et al., 2020). This database can be used to screen queried genome sequences for specific KL and other capsule biosynthesis genes in order to predict K type. It is available for public use and can be accessed via github, Kaptive-web and PathogenWatch.

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