The K129 capsular polysaccharide produced by Acinetobacter baumannii MAR 15-4076 has the same composition as K84 but differs in the linkage between units altering the overall branching topology.

Capsular polysaccharide (CPS) is a heteroglycan that coats the cell surface of most isolates of the important Gram-negative bacterial pathogen, Acinetobacter baumannii. Strain MAR 15-4076, a clinical isolate recovered in Russia in 2015, was found to carry the KL129 sequence at the CPS biosynthesis K locus. The CPS was isolated from the strain and studied by sugar analysis, Smith degradation, one- and two-dimensional 1H and 13C NMR spectroscopy. It was composed of branched pentasaccharide units that include a →3)-α-l-Rhap-(1 â†’ 3)-α-l-Rhap-(1 â†’ 3)-ß-d-GlcpNAc-(1→ mainchain and α-d-ManpNAc-(1 â†’ 3)-l-Rhap side branch. Though the pentasaccharide units are identical to those that make up the K84 CPS produced by A. baumannii LUH5540, the units are linked differently via the substitution of an alternate l-Rhap residue, resulting in a difference in the overall topology of the CPS. This was due to the replacement of the Wzy polymerase gene encoded at the K locus.

New Obolenskvirus Phages Brutus and Scipio: Biology, Evolution, and Phage-Host Interaction.

Two novel virulent phages of the genus Obolenskvirus infecting Acinetobacter baumannii, a significant nosocomial pathogen, have been isolated and studied. Phages Brutus and Scipio were able to infect A. baumannii strains belonging to the K116 and K82 capsular types, respectively. The biological properties and genomic organization of the phages were characterized. Comparative genomic, phylogenetic, and pangenomic analyses were performed to investigate the relationship of Brutus and Scipio to other bacterial viruses and to trace the possible origin and evolutionary history of these phages and other representatives of the genus Obolenskvirus. The investigation of enzymatic activity of the tailspike depolymerase encoded in the genome of phage Scipio, the first reported virus infecting A. baumannii of the K82 capsular type, was performed. The study of new representatives of the genus Obolenskvirus and mechanisms of action of depolymerases encoded in their genomes expands knowledge about the diversity of viruses within this taxonomic group and strategies of Obolenskvirus-host bacteria interaction.

Friunavirus Phage-Encoded Depolymerases Specific to Different Capsular Types of Acinetobacter baumannii.

Acinetobacter baumannii is a critical priority nosocomial pathogen that produces a variety of capsular polysaccharides (CPSs), the primary receptors for specific depolymerase-carrying phages. In this study, the tailspike depolymerases (TSDs) encoded in genomes of six novel Friunaviruses, APK09, APK14, APK16, APK86, APK127v, APK128, and one previously described Friunavirus phage, APK37.1, were characterized. For all TSDs, the mechanism of specific cleavage of corresponding A. baumannii capsular polysaccharides (CPSs) was established. The structures of oligosaccharide fragments derived from K9, K14, K16, K37/K3-v1, K86, K127, and K128 CPSs degradation by the recombinant depolymerases have been determined. The crystal structures of three of the studied TSDs were obtained. A significant reduction in mortality of Galleria mellonella larvae infected with A. baumannii of K9 capsular type was shown in the example of recombinant TSD APK09_gp48. The data obtained will provide a better understanding of the interaction of phage-bacterial host systems and will contribute to the formation of principles of rational usage of lytic phages and phage-derived enzymes as antibacterial agents.

5,7-Diamino-3,5,7,9-tetradeoxynon-2-ulosonic Acids in the Capsular Polysaccharides of Acinetobacter baumannii.

The polysaccharide capsule surrounding bacterial cell plays an important role in pathogenesis of infections caused by the opportunistic pathogen Acinetobacter baumannii by providing protection from external factors. The structures of the capsular polysaccharide (CPS) produced by A. baumannii isolates and the corresponding CPS biosynthesis gene clusters are highly diverse, although many of them are related. Many types of A. baumannii CPSs contain isomers of 5,7-diamino-3,5,7,9-tetradeoxynon-2-ulosonic acid (DTNA). Three of these isomers, namely acinetaminic acid (l-glycero-l-altro isomer), 8-epiacinetaminic acid (d-glycero-l-altro isomer), and 8-epipseudaminic acid (d-glycero-l-manno isomer), have not been found so far in naturally occurring carbohydrates from other species. In A. baumannii CPSs, DTNAs carry N-acyl substituents at positions 5 and 7; in some CPSs, both N-acetyl and N-(3-hydroxybutanoyl) groups are present. Remarkably, pseudaminic acid carries the (R)-isomer and legionaminic acid carries the (S)-isomer of the 3-hydroxybutanoyl group. The review addresses the structure and genetics of biosynthesis of A. baumannii CPSs containing di-N-acyl derivatives of DTNA.

Capsule-Targeting Depolymerases Derived from Acinetobacter baumannii Prophage Regions.

In this study, several different depolymerases encoded in the prophage regions of Acinetobacter baumannii genomes have been bioinformatically predicted and recombinantly produced. The identified depolymerases possessed multi-domain structures and were identical or closely homologous to various proteins encoded in other A. baumannii genomes. This means that prophage-derived depolymerases are widespread, and different bacterial genomes can be the source of proteins with polysaccharide-degrading activities. For two depolymerases, the specificity to capsular polysaccharides (CPSs) of A. baumannii belonging to K1 and K92 capsular types (K types) was determined. The data obtained showed that the prophage-derived depolymerases were glycosidases that cleaved the A. baumannii CPSs by the hydrolytic mechanism to yield monomers and oligomers of the K units. The recombinant proteins with established enzymatic activity significantly reduced the mortality of Galleria mellonella larvae infected with A. baumannii of K1 and K92 capsular types. Therefore, these enzymes can be considered as suitable candidates for the development of new antibacterials against corresponding A. baumannii K types.

Novel Acinetobacter baumannii Bacteriophage Aristophanes Encoding Structural Polysaccharide Deacetylase.

Acinetobacter baumannii appears to be one of the most crucial nosocomial pathogens. A possible component of antimicrobial therapy for infections caused by extremely drug-resistant A. baumannii strains may be specific lytic bacteriophages or phage-derived enzymes. In the present study, we observe the biological features, genomic organization, and phage-host interaction strategy of novel virulent bacteriophage Aristophanes isolated on A. baumannii strain having K26 capsular polysaccharide structure. According to phylogenetic analysis phage Aristophanes can be classified as a representative of a new distinct genus of the subfamily Beijerinckvirinae of the family Autographiviridae. This is the first reported A. baumannii phage carrying tailspike deacetylase, which caused O-acetylation of one of the K26 sugar residues.