Variation in the Complex Carbohydrate Biosynthesis Loci of Acinetobacter baumannii Genomes

Abstract

Extracellular polysaccharides are major immunogenic components of the bacterial cell envelope. However, little is known about their biosynthesis in the genus Acinetobacter, which includes A. baumannii, an important nosocomial pathogen. Whether Acinetobacter sp. produce a capsule or a lipopolysaccharide carrying an O antigen or both is not resolved. To explore these issues, genes involved in the synthesis of complex polysaccharides were located in 10 complete A. baumannii genome sequences, and the function of each of their products was predicted via comparison to enzymes with a known function. The absence of a gene encoding a WaaL ligase, required to link the carbohydrate polymer to the lipid A-core oligosaccharide (lipooligosaccharide) forming lipopolysaccharide, suggests that only a capsule is produced. Nine distinct arrangements of a large capsule biosynthesis locus, designated KL1 to KL9, were found in the genomes. Three forms of a second, smaller variable locus, likely to be required for synthesis of the outer core of the lipid A-core moiety, were designated OCL1 to OCL3 and also annotated. Each K locus includes genes for capsule export as well as genes for synthesis of activated sugar precursors, and for glycosyltransfer, glycan modification and oligosaccharide repeat-unit processing. The K loci all include the export genes at one end and genes for synthesis of common sugar precursors at the other, with a highly variable region that includes the remaining genes in between. Five different capsule loci, KL2, KL6, KL7, KL8 and KL9 were detected in multiply antibiotic resistant isolates belonging to global clone 2, and two other loci, KL1 and KL4, in global clone 1. This indicates that this region is being substituted repeatedly in multiply antibiotic resistant isolates from these clones.

The Capsular Polysaccharide of Acinetobacter baumannii Is an Obstacle for Therapeutic Passive Immunization Strategies

“…Importantly, however, OmpA expression was confirmed in all tested strains using Western blot analysis of bacterial cell lysates, which was demonstrated as positive binding bands at 38 kDa ( (21), we hypothesized that the capsular polysaccharide produced by clinical isolates might be responsible for the observed decrease in binding of MAbs to OmpA. To test this hypothesis, MAb binding to AB307.30, an isogenic capsule-negative derivative of AB307-0294 (K1 capsular serotype, or KL1 in the classification system designed by Kenyon and Hall [21]), was assessed by flow cytometry (22). In contrast to the lack of binding observed for AB307-0294, significant binding of the MAbs was observed for AB307.30 ( Fig.…”

OXA β-Lactamases

“…A. Evans, unpublished data). This may be due to its genetic location within the capsule locus in A. baumannii (153), a region of the genome that is likely to be under selection, which may result in deletions and recombination. This is supported by evidence that the gpi locus as well as the gyrB locus (another gene used in the Oxford scheme), which is located next to the capsule locus, have undergone horizontal gene transfer (154).…”

Pathogenic Acinetobacter: from the Cell Surface to Infinity and Beyond

“…Intriguingly, Acinetobacter spp. possess either one or two genes, depending on the strain, that encode proteins with domains similar to those found in WaaL ligase orthologs (29)(30)(31)(32); however, those domains are also found in PglL, the enzyme responsible for Olinked protein glycosylation (see below) (29). Bioinformatic analysis alone is not sufficient to distinguish between WaaL and PglL orthologs, and careful experimental analysis is required to determine the true function of the proteins possessing these domains.…”

“…How A. baumannii partitions a given carbohydrate repeat unit to the protein glycosylation pathway or capsule production remains to be determined. Although the carbohydrate structures produced by different strains are highly variable, functional studies have shown that capsule production is essential for Acinetobacter survival during infection and growth in serum (32,46,50,51). It was recently reported that capsule production could be increased by the presence of subinhibitory levels of antibiotics, which increased resistance to complement-mediated killing and led to a hypervirulent phenotype in a mouse model of systemic infection (52).…”