The structure of the polysaccharide isolated from Acinetobacter baumannii strain LAC-4

Revised structure of the polysaccharide from Acinetobacter baumannii LUH5551 assigned as the K63 type capsular polysaccharide

K63 capsular polysaccharide produced by Acinetobacter baumannii isolate LUH5551 (previously designated isolate O24) was re-examined using sugar analysis, Smith degradation, and one- and two-dimensional 1H and 13C NMR spectroscopy. Though previously reported as O24 consisting of linear tetrasaccharide units that include a 7-acetamido-5-acylamino form of 8-epilegionaminic acid [8eLeg5R7Ac, acylated at C5 with (S)-3-hydroxybutanoyl or acetyl (1:1)], the elucidated structure of the K63 type capsule was found to include a derivative of 5,7-diamino-3,5,7,9-tetradeoxy-d-glycero-d-galacto-non-2-ulosonic (legionaminic) acid, Leg5Ac7R, where R is either (S)-3-hydroxybutanoyl or an acetyl group (∼1:1 ratio). This finding is consistent with the presence of the lgaABCHIFG gene module for Leg5Ac7R biosynthesis in the KL63 gene cluster at the capsular polysaccharide (CPS) biosynthesis K locus in the LUH5551 genome. The glycosyltransferases (Gtrs) and Wzy polymerase encoded by KL63 were assigned to linkages in the linear K63 tetrasaccharide unit and linkage of the K63 units.

Exploring Roles of the Polysaccharide Capsule in Pathogenesis of Hypervirulent Acinetobacter baumannii Clinical Isolate Lac-4

The frequently multidrug-resistant bacterial pathogen Acinetobacter baumannii is a leading cause of nosocomial infections, including ventilator-associated pneumonia, such that the World Health Organization and US Centers for Disease Control and Prevention have declared it a top priority candidate for novel drug development. Nearly all clinical A. baumannii strains express a thick surface polysaccharide capsule that protects against desiccation, host defenses, and disinfectants. In this study, we investigated the contribution of the polysaccharide capsule to virulence caused by the A. baumannii clinical isolate Ab Lac-4, which is rare in its ability to cause pneumonia and disseminated sepsis in healthy mice. We assessed the role of the capsule in wildtype Lac-4 (WT) by generating a premature stop codon in wza, which codes for the polysaccharide export protein. The wza# mutant was hypersensitive to killing by complement, whole blood, and healthy human neutrophils compared to WT and a revertant mutant (wza-Rev). Furthermore, the wza# mutant was highly attenuated in murine sepsis and unable to disseminate from the lungs during pneumonia. This study reinforces the capsule as a key contributor to Ab Lac-4 hypervirulence.

Development of a Bispecific Antibody Targeting Clinical Isolates of Acinetobacter baumannii

BACKGROUND

We previously reported developing 2 anticapsular monoclonal antibodies (mAbs) as a novel therapy for Acinetobacter baumannii infections. We sought to determine whether a bispecific mAb (bsAb) could improve avidity and efficacy while maximizing strain coverage in one molecule.

METHODS

Humanized mAb 65 was cloned into a single-chain variable fragment and attached to humanized mAb C8, combining their paratopes into a single bsAb (C73). We tested bsAb C73's strain coverage, binding affinity, ex vivo opsonic activity, and in vivo efficacy compared to each mAb alone and combined.

RESULTS

The bsAb demonstrated strain coverage, binding affinity, opsonization, and in vivo efficacy superior to either original mAb alone or combined.

CONCLUSIONS

A humanized bsAb targeting distinct A. baumannii capsule moieties enabled potent and effective coverage of disparate A. baumannii clinical isolates. The bsAb enhances feasibility of development by minimizing the number of components of a promising novel therapeutic for these difficult-to-treat infections.

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.

An update to the database for Acinetobacter baumannii capsular polysaccharide locus typing extends the extensive and diverse repertoire of genes found at and outside the K locus

Several novel non-antibiotic therapeutics for the critical priority bacterial pathogen, Acinetobacter baumannii, rely on specificity to the cell-surface capsular polysaccharide (CPS). Hence, prediction of CPS type deduced from genes in whole genome sequence data underpins the development and application of these therapies. In this study, we provide a comprehensive update to the A. baumannii K locus reference sequence database for CPS typing (available in Kaptive v. 2.0.1) to include 145 new KL, providing a total of 237 KL reference sequences. The database was also reconfigured for compatibility with the updated Kaptive v. 2.0.0 code that enables prediction of 'K type' from special logic parameters defined by detected combinations of KL and additional genes outside the K locus. Validation of the database against 8994 publicly available A. baumannii genome assemblies from NCBI databases identified the specific KL in 73.45 % of genomes with perfect, very high or high confidence. Poor sequence quality or the presence of insertion sequences were the main reasons for lower confidence levels. Overall, 17 KL were overrepresented in available genomes, with KL2 the most common followed by the related KL3 and KL22. Substantial variation in gene content of the central portion of the K locus, that usually includes genes specific to the CPS type, included 34 distinct groups of genes for synthesis of various complex sugars and >400 genes for forming linkages between sugars or adding non-sugar substituents. A repertoire of 681 gene types were found across the 237 KL, with 88.4 % found in <5 % of KL.

NoteIdentification of 5,7-diacetamido-3,5,7,9-tetradeoxy-d-glycero-l-manno-non-2-ulosonic acid (di-N-acetyl-8-epipseudaminic acid) in the capsular polysaccharide of Acinetobacter baumannii Res546

A structurally diverse capsular polysaccharide that surrounds the bacterial cell plays an important role in virulence of Acinetobacter baumannii, a cause of nosocomial infections worldwide. Various isomers of 5,7-diacylamido-3,5,7,9-tetradeoxynon-2-ulosonic acid have been identified as components of bacterial polysaccharides. In this work, we report on the identification of a new isomer having the d-glycero-l-manno configuration (8-epipseudaminic acid) in the capsular polysaccharide of A. baumannii Res546. The higher sugar was isolated by Smith degradation of the polysaccharide followed by mild acid hydrolysis and identified by a comparison with all isomers using NMR spectroscopy and optical rotation.

Monoclonal Antibody Therapy against Acinetobacter baumannii

Background: Extremely drug-resistant (XDR) Acinetobacter baumannii is a notorious and frequently encountered pathogen demanding novel therapeutic interventions. An initial monoclonal antibody (MAb), C8, raised against A. baumannii capsule proved a highly effective treatment against a minority of clinical isolates. To overcome this limitation, we broadened coverage by developing a second antibody for use in a combination regimen. Methods: We sought to develop an additional anti-A. baumannii MAb through hybridoma technology by immunizing mice with sublethal inocula of virulent, XDR clinical isolates not bound by MAb C8. Results: We identified a new antibacterial MAb, 65, which bound to strains in a pattern distinct from and complementary to MAb C8. MAb 65 enhanced macrophage opsonophagocytosis of targeted strains and markedly improved survival in lethal bacteremic sepsis and aspiration pneumonia murine models of A. baumannii infection. MAb 65 was also synergistic with colistin, substantially enhancing protection compared to monotherapy. Treatment with MAb 65 significantly reduced blood bacterial density, ameliorated cytokine production (IL-1β, IL-6, IL-10, and TNF), and sepsis biomarkers. Conclusions: We describe a novel MAb targeting A. baumannii that broadens immunotherapeutic strain coverage, is highly potent and effective, and synergistically improves outcomes in combination with antibiotics.

Capsule carbohydrate structure determines virulence in Acinetobacter baumannii

Acinetobacter baumannii is a highly antibiotic-resistant bacterial pathogen for which novel therapeutic approaches are needed. Unfortunately, the drivers of virulence in A. baumannii remain uncertain. By comparing genomes among a panel of A. baumannii strains we identified a specific gene variation in the capsule locus that correlated with altered virulence. While less virulent strains possessed the intact gene gtr6, a hypervirulent clinical isolate contained a spontaneous transposon insertion in the same gene, resulting in the loss of a branchpoint in capsular carbohydrate structure. By constructing isogenic gtr6 mutants, we confirmed that gtr6-disrupted strains were protected from phagocytosis in vitro and displayed higher bacterial burden and lethality in vivo. Gtr6+ strains were phagocytized more readily and caused lower bacterial burden and no clinical illness in vivo. We found that the CR3 receptor mediated phagocytosis of gtr6+, but not gtr6-, strains in a complement-dependent manner. Furthermore, hypovirulent gtr6+ strains demonstrated increased virulence in vivo when CR3 function was abrogated. In summary, loss-of-function in a single capsule assembly gene dramatically altered virulence by inhibiting complement deposition and recognition by phagocytes across multiple A. baumannii strains. Thus, capsular structure can determine virulence among A. baumannii strains by altering bacterial interactions with host complement-mediated opsonophagocytosis.

Structural and Biosynthetic Diversity of Nonulosonic Acids (NulOs) That Decorate Surface Structures in Bacteria

Nonulosonic acids (NulOs) are a diverse family of 9-carbon α-keto acid sugars that are involved in a wide range of functions across all branches of life. The family of NulOs includes the sialic acids as well as the prokaryote-specific NulOs. Select bacteria biosynthesize the sialic acid N-acetylneuraminic acid (Neu5Ac), and the ability to produce this sugar and its subsequent incorporation into cell-surface structures is implicated in a variety of bacteria-host interactions. Furthermore, scavenging of sialic acid from the environment for energy has been characterized across a diverse group of bacteria, mainly human commensals and pathogens. In addition to sialic acid, bacteria have the ability to biosynthesize prokaryote-specific NulOs, of which there are several known isomers characterized. These prokaryotic NulOs are similar in structure to Neu5Ac but little is known regarding their role in bacterial physiology. Here, we discuss the diversity in structure, the biosynthesis pathways, and the functions of bacteria-specific NulOs. These carbohydrates are phylogenetically widespread among bacteria, with numerous structurally unique modifications recognized. Despite the diversity in structure, the NulOs are involved in similar functions such as motility, biofilm formation, host colonization, and immune evasion.

Rapid Identification of KL49 Acinetobacter baumannii Associated with Clinical Mortality

Objective

We aimed to establish a tool for rapid identification of KL49 Acinetobacter baumannii.

Methods

Based on the capsular polysaccharide (CPS) synthesis genes database, we investigated the distribution of K locus type 49 (KL49) genes in other KL types and established a rapid identification method for KL49. We collected 61 clinical carbapenem-resistant A. baumannii (CRAB) strains, identified KL49 by gtr100 detection, and used whole genome sequencing (WGS) for verification. A mouse pneumonia model was used to confirm the hypervirulence phenotype. We tested the presence of gtr100 gene in 165 CRAB strains from three provinces in China and evaluated the correlation of gtr100 carrying CRAB infection with mortality.

Results

The gtr100 gene is the CPS synthesis gene found only in KL49. We screened out nine WGS-validated KL49 strains from 61 CRAB clinical strains using polymerase chain reaction (PCR) to detect the gtr100 gene. The survival rates of KL49 strains were significantly lower than nonKL49 strains in a mouse pneumonia model. The survival rates of LAC-4 gtr100 knockout strain decreased significantly. Analysis of phylogenetics showed the worldwide spread of KL49 A. baumannii. Infection of gtr100 carrying CRAB is an independent risk for mortality (OR, 10.76; 95%CI: 3.08–37.55; p<0.001).

Conclusion

The hypervirulence phenotype of KL49 CRAB and the association with mortality highlight the urgent need for implementing control measures. The rapid identification assay has the potential to facilitate early medical intervention and worldwide surveillance.

Host Innate Immune Responses to Acinetobacter baumannii Infection

Acinetobacter baumannii has emerged as a major threat to global public health and is one of the key human pathogens in healthcare (nosocomial and community-acquired)-associated infections. Moreover, A. baumannii rapidly develops resistance to multiple antibiotics and is now globally regarded as a serious multidrug resistant pathogen. There is an urgent need to develop novel vaccines and immunotherapeutics as alternatives to antibiotics for clinical management of A. baumannii infection. However, our knowledge of host immune responses to A. baumannii infection and the identification of novel therapeutic targets are significantly lacking. This review highlights the recent advances and critical gaps in our understanding how A. baumannii interacts with the host innate pattern-recognition receptors, induces a cascade of inflammatory cytokine and chemokine responses, and recruits innate immune effectors (such as neutrophils and macrophages) to the site of infection for effective control of the infection. Such knowledge will facilitate the identification of new targets for the design and development of effective therapeutics and vaccines to fight this emerging threat.

Genomic and Phenotypic Analyses of Acinetobacter baumannii Isolates From Three Tertiary Care Hospitals in Thailand

Antibiotic resistant strains of Acinetobacter baumannii are responsible for a large and increasing burden of nosocomial infections in Thailand and other countries of Southeast Asia. New approaches to their control and treatment are urgently needed and an attractive strategy is to remove the bacterial polysaccharide capsule, and thus the protection from the host's immune system. To examine phylogenetic relationships, distribution of capsule chemotypes, acquired antibiotic resistance determinants, susceptibility to complement and other traits associated with systemic infection, we sequenced 191 isolates from three tertiary referral hospitals in Thailand and used phenotypic assays to characterize key aspects of infectivity. Several distinct lineages were circulating in three hospitals and the majority belonged to global clonal group 2 (GC2). Very high levels of resistance to carbapenems and other front-line antibiotics were found, as were a number of widespread plasmid replicons. A high diversity of capsule genotypes was encountered, with only three of these (KL6, KL10, and KL47) showing more than 10% frequency. Almost 90% of GC2 isolates belonged to the most common capsule genotypes and were fully resistant to the bactericidal action of human serum complement, most likely protected by their polysaccharide capsule, which represents a key determinant of virulence for systemic infection. Our study further highlights the importance to develop therapeutic strategies to remove the polysaccharide capsule from extensively drug-resistant A. baumanii during the course of systemic infection.

K units of the K8 and K54 capsular polysaccharides produced by Acinetobacter baumannii BAL 097 and RCH52 have the same structure but contain different di-N-acyl derivatives of legionaminic acid and are linked differently

The K8 and K54 capsular polysaccharides were isolated from Acinetobacter baumannii BAL 097 and RCH52, respectively, and studied by sugar analysis, partial acid hydrolysis and selective solvolysis with CF₃CO₂H in the presence of 2-methyl-1-propanol, along with 1D and 2D ¹H and ¹³C NMR spectroscopy. The following structures of related branched tetrasaccharide repeats (K units) of the polysaccharides were established: where Leg indicates 5,7-diamino-3,5,7,9-tetradeoxy-d-glycero-d-galacto-non-2-ulosonic (legionaminic) acid and R indicates (R)-3-hydroxybutanoyl or acetyl in the ratio ~2.5:1. The sequences of the KL8 and KL54 capsule gene clusters were closely related. The difference in the acyl group at O-7 on the sidechain legionaminic acid is due to differences in two genes in the legionaminic acid biosynthesis cluster. The wzy genes encoding the K unit polymerases are also different and make different linkages between the K units, allowing the first sugar of both K units to be identified as d-GlcpNAc. The shared Gtr20 glycosyltransferase, also encoded in KL63, forms the α-l-FucpNAc-(1 → 3)-d-GlcpNAc linkage, and Gtr19 was predicted to form α-d-GalpNAc-(1 → 3)-l-FucpNAc. Gtr18 from KL8 is 75% identical to Gtr108 from KL54 and both would link the Leg derivative to d-GalpNAc. Hence the genes present at the K locus were consistent with the composition and structures of the K8 and K54 capsular polysaccharides.

Genomic epidemiology of severe community-onset Acinetobacter baumannii infection

Acinetobacter baumannii causes severe, fulminant, community-acquired pneumonia (CAP) in tropical and subtropical regions. We compared the population structure, virulence and antimicrobial resistance determinants of northern Australian community-onset A. baumannii strains with local and global strains. We performed whole-genome sequencing on 55 clinical and five throat colonization A. baumannii isolates collected in northern Australia between 1994 and 2016. Clinical isolates included CAP (n=41), healthcare-associated pneumonia (n=7) and nosocomial bloodstream (n=7) isolates. We also included 93 publicly available international A. baumannii genome sequences in the analyses. Patients with A. baumannii CAP were almost all critically unwell; 82 % required intensive care unit admission and 18 % died during their inpatient stay. Whole-genome phylogenetic analysis demonstrated that community-onset strains were not phylogenetically distinct from nosocomial strains. Some non-multidrug-resistant local strains were closely related to multidrug-resistant strains from geographically distant locations. Pasteur sequence type (ST)10 was the dominant ST and accounted for 31/60 (52 %) northern Australian strains; the remainder belonged to a diverse range of STs. The most recent common ancestor for ST10 was estimated to have occurred in 1738 (95 % highest posterior density, 1626–1826), with evidence of multiple introduction events between Australia and Southeast Asia between then and the present day. Virulence genes associated with biofilm formation and the type 6 secretion system (T6SS) were absent in many strains, and were not associated with in-hospital mortality. All strains were susceptible to gentamicin and meropenem; none carried an AbaR resistance island. Our results suggest that international dissemination of A. baumannii is occurring in the community on a contemporary timescale. Genes associated with biofilm formation and the T6SS may not be required for survival in community niches. The relative contributions of host and bacterial factors to the clinical severity of community-onset A. baumannii infection require further investigation.

Diversity and Function of Capsular Polysaccharide in Acinetobacter baumannii

The Gram-negative opportunistic bacterium Acinetobacter baumannii is a significant cause of hospital-borne infections worldwide. Alarmingly, the rapid development of antimicrobial resistance coupled with the remarkable ability of isolates to persist on surfaces for extended periods of time has led to infiltration of A. baumannii into our healthcare environments. A major virulence determinant of A. baumannii is the presence of a capsule that surrounds the bacterial surface. This capsule is comprised of tightly packed repeating polysaccharide units which forms a barrier around the bacterial cell wall, providing protection from environmental pressures including desiccation and disinfection regimes as well as host immune responses such as serum complement. Additionally, capsule has been shown to confer resistance to a range of clinically relevant antimicrobial compounds. Distressingly, treatment options for A. baumannii infections are becoming increasingly limited, and the urgency to develop effective infection control strategies and therapies to combat infections is apparent. An increased understanding of the contribution of capsule to the pathobiology of A. baumannii is required to determine its feasibility as a target for new strategies to combat drug resistant infections. Significant variation in capsular polysaccharide structures between A. baumannii isolates has been identified, with over 100 distinct capsule types, incorporating a vast variety of sugars. This review examines the studies undertaken to elucidate capsule diversity and advance our understanding of the role of capsule in A. baumannii pathogenesis.

A Diazido Mannose Analogue as a Chemoenzymatic Synthon for Synthesizing Di-N-acetyllegionaminic Acid-Containing Glycosides

A chemoenzymatic synthon was designed to expand the scope of the chemoenzymatic synthesis of carbohydrates. The synthon was enzymatically converted into carbohydrate analogues, which were readily derivatized chemically to produce the desired targets. The strategy is demonstrated for the synthesis of glycosides containing 7,9-di-N-acetyllegionaminic acid (Leg5,7Ac₂ ), a bacterial nonulosonic acid (NulO) analogue of sialic acid. A versatile library of α2-3/6-linked Leg5,7Ac₂ -glycosides was built by using chemically synthesized 2,4-diazido-2,4,6-trideoxymannose as a chemoenzymatic synthon for highly efficient one-pot multienzyme (OPME) sialylation followed by downstream chemical conversion of the azido groups into acetamido groups. The syntheses required 10 steps from commercially available d-fucose and had an overall yield of 34-52 %, thus representing a significant improvement over previous methods. Free Leg5,7Ac₂ monosaccharide was also synthesized by a sialic acid aldolase-catalyzed reaction.

A new approach to the synthesis of legionaminic acid analogues

Legionaminic acid is a member of the nonulosonic acids, which are a class of sugars considered to be a virulence factor within a wide variety of pathogenic bacteria. We have developed a synthetic pathway towards C-7 analogues of legionaminic acid starting from Neu5Ac, resulting in the complete synthesis of both legionaminic acid, and its C-7 epimer, from a common precurser. Our approach involves the late-stage introduction of the requisite C-7 nitrogen functionality, thus making our strategy amenable to the introduction of a range of different amide groups at C-7 of legionaminic acid.

We report the synthesis of the bacterial nonulosonic acid legionaminic acid, together with its C-7 epimer, from a common precursor derived from N-acetylneuraminic acid.

The KL24 gene cluster and a genomic island encoding a Wzy polymerase contribute genes needed for synthesis of the K24 capsular polysaccharide by the multiply antibiotic resistant Acinetobacter baumannii isolate RCH51

The whole-genome sequence of the multiply antibiotic resistant Acinetobacter baumannii isolate RCH51 belonging to sequence type ST103 (Institut Pasteur scheme) revealed that the set of genes at the capsule locus, KL24, includes four genes predicted to direct the synthesis of 3-acetamido-3,6-dideoxy-d-galactose (d-Fuc3NAc), and this sugar was found in the capsular polysaccharide (CPS). One of these genes, fdtE, encodes a novel bifunctional protein with an N-terminal FdtA 3,4-ketoisomerase domain and a C-terminal acetyltransferase domain. KL24 lacks a gene encoding a Wzy polymerase to link the oligosaccharide K units to form the CPS found associated with isolate RCH51, and a wzy gene was found in a small genomic island (GI) near the cpn60 gene. This GI is in precisely the same location as another GI carrying wzy and atr genes recently found in several A. baumannii isolates, but it does not otherwise resemble it. The CPS isolated from RCH51, studied by sugar analysis and 1D and 2D 1H and 13C NMR spectroscopy, revealed that the K unit has a branched pentasaccharide structure made up of Gal, GalNAc and GlcNAc residues with d-Fuc3NAc as a side branch, and the K units are linked via a β-d-GlcpNAc-(1→3)-β-d-Galp linkage formed by the Wzy encoded by the GI. The functions of the glycosyltransferases encoded by KL24 were assigned to formation of specific bonds. A correspondence between the order of the genes in KL24 and other KL and the order of the linkages they form was noted, and this may be useful in future predictions of glycosyltransferase specificities.

Monoclonal Antibody Protects Against Acinetobacter baumannii Infection by Enhancing Bacterial Clearance and Evading Sepsis

Background

Extremely drug-resistant (XDR) Acinetobacter baumannii is one of the most commonly encountered, highly resistant pathogens requiring novel therapeutic interventions.

Methods

We developed C8, a monoclonal antibody (mAb), by immunizing mice with sublethal inocula of a hypervirulent XDR clinical isolate.

Results

C8 targets capsular carbohydrate on the bacterial surface, enhancing opsonophagocytosis. Treating with a single dose of C8 as low as 0.5 μg/mouse (0.0167 mg/kg) markedly improved survival in lethal bacteremic sepsis and aspiration pneumonia models of XDR A. baumannii infection. C8 was also synergistic with colistin, substantially improving survival compared to monotherapy. Treatment with C8 significantly reduced blood bacterial density, cytokine production (tumor necrosis factor α, interleukin [IL] 6, IL-1β, and IL-10), and sepsis biomarkers. Serial in vitro passaging of A. baumannii in the presence of C8 did not cause loss of mAb binding to the bacteria, but did result in emergence of less-virulent mutants that were more susceptible to macrophage uptake. Finally, we developed a highly humanized variant of C8 that retains opsonophagocytic activity in murine and human macrophages and rescued mice from lethal infection.

Conclusions

We describe a promising and novel mAb as therapy for lethal, XDR A. baumannii infections, and demonstrate that it synergistically improves outcomes in combination with antibiotics.

Clinical and Pathophysiological Overview of Acinetobacter Infections: a Century of Challenges

Acinetobacter is a complex genus, and historically, there has been confusion about the existence of multiple species. The species commonly cause nosocomial infections, predominantly aspiration pneumonia and catheter-associated bacteremia, but can also cause soft tissue and urinary tract infections. Community-acquired infections by Acinetobacter spp. are increasingly reported. Transmission of Acinetobacter and subsequent disease is facilitated by the organism's environmental tenacity, resistance to desiccation, and evasion of host immunity. The virulence properties demonstrated by Acinetobacter spp. primarily stem from evasion of rapid clearance by the innate immune system, effectively enabling high bacterial density that triggers lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4)-mediated sepsis. Capsular polysaccharide is a critical virulence factor that enables immune evasion, while LPS triggers septic shock. However, the primary driver of clinical outcome is antibiotic resistance. Administration of initially effective therapy is key to improving survival, reducing 30-day mortality threefold. Regrettably, due to the high frequency of this organism having an extreme drug resistance (XDR) phenotype, early initiation of effective therapy is a major clinical challenge. Given its high rate of antibiotic resistance and abysmal outcomes (up to 70% mortality rate from infections caused by XDR strains in some case series), new preventative and therapeutic options for Acinetobacter spp. are desperately needed.

Stereoselective Synthesis of the Equatorial Glycosides of Legionaminic Acid

The synthesis of a legionaminic acid donor from N-acetylneuraminic acid in 15 steps and 17% overall yield is described. Activation of the adamantanyl thioglycoside in the donor with N-iodosuccinimide and trifluoromethanesulfonic acid in dichloromethane and acetonitrile at -78 °C in the presence of primary, secondary and tertiary alcohols affords the corresponding glycosides in excellent yield and good to excellent equatorial selectivity. In particular, coupling to the 4-OH of a suitably protected neuraminic acid derivative affords a disaccharide that closely resembles the glycosidic linkage in the polylegionaminic acid from the lipopolysaccharide of the Legionella pneumophila virulence factor. A straightforward deprotection sequence enables conversion of the protected glycosides to the free N,N-diacetyllegionaminic acid glycosides.

Repeated local emergence of carbapenem-resistant Acinetobacter baumannii in a single hospital ward

We recently reported a dramatic increase in the prevalence of carbapenem-resistant Acinetobacter baumannii infections in the intensive care unit (ICU) of a Vietnamese hospital. This upsurge was associated with a specific oxa23-positive clone that was identified by multilocus VNTR analysis. Here, we used whole-genome sequence analysis to dissect the emergence of carbapenem-resistant A. baumannii causing ventilator-associated pneumonia (VAP) in the ICU during 2009–2012. To provide historical context and distinguish microevolution from strain introduction, we compared these genomes with those of A. baumannii asymptomatic carriage and VAP isolates from this same ICU collected during 2003–2007. We identified diverse lineages co-circulating over many years. Carbapenem resistance was associated with the presence of oxa23, oxa40, oxa58 and ndm1 genes in multiple lineages. The majority of resistant isolates were oxa23-positive global clone GC2; fine-scale phylogenomic analysis revealed five distinct GC2 sublineages within the ICU that had evolved locally via independent chromosomal insertions of oxa23 transposons. The increase in infections caused by carbapenem-resistant A. baumannii was associated with transposon-mediated transmission of a carbapenemase gene, rather than clonal expansion or spread of a carbapenemase-harbouring plasmid. Additionally, we found evidence of homologous recombination creating diversity within the local GC2 population, including several events resulting in replacement of the capsule locus. We identified likely donors of the imported capsule locus sequences amongst the A. baumannii isolated on the same ward, suggesting that diversification was largely facilitated via reassortment and sharing of genetic material within the localized A. baumannii population.

Structure of the neutral capsular polysaccharide of Acinetobacter baumannii NIPH146 that carries the KL37 capsule gene cluster

Capsular polysaccharide (CPS) was isolated from Acinetobacter baumannii NIPH146, and the following structure of branched pentasaccharide repeating unit was established by sugar analyses along with 1D and 2D NMR spectroscopy: In comparison to most other known capsular polysaccharides of A. baumannii, the CPS studied is neutral and lacks any specific monosaccharide component. The synthesis, assembly and export of this structure could be attributed to genes in a novel capsule biosynthesis gene cluster, designated KL37, which was found in the NIPH146 genome. The CPS of A. baumannii NIPH146 shares the α-d-Galp-(1→6)-β-d-Glcp-(1→3)-d-GalpNAc-(1→ trisaccharide fragment with the CPS units of several A. baumannii strains, including ATCC 17978 and LUH 5537 that carry the KL3 and KL22 gene clusters, respectively. KL37 contains two genes for glycosyltransferases that are related to two glycosyltransferase genes present in both KL3 and KL22, and the encoded proteins could be tentatively assigned to linkages between sugars in the CPS repeat.

Structure of the K12 capsule containing 5,7-di-N-acetylacinetaminic acid from Acinetobacter baumannii isolate D36

The repeat unit of the K12 capsular polysaccharide isolated from the Acinetobacter baumannii global clone 1 clinical isolate, D36, was elucidated by means of chemical and spectroscopical methods. The structure was shown to contain N-acetyl-D-galactosamine (D-GalpNAc), N-acetyl-D-fucosamine and N-acetyl-L-fucosamine linked together in the main chain, with the novel sugar, 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-altro-non-2-ulosonic acid (5,7-di-N-acetylacinetaminic acid or Aci5Ac7Ac), attached to D-GalpNAc as a side branch. This matched the sugar composition of the K12 capsule and the genetic content of the KL12 capsule gene cluster reported previously. D-FucpNAc was predicted to be the substrate for the initiating transferase, ItrB3, with the Wzy polymerase making a α-D-FucpNAc-(1 → 3)-D-GalpNAc linkage between the repeat units. The three glycosyltransferases encoded by KL12 are all retaining glycosyltransferases and were predicted to form specific linkages between the sugars in the K12 repeat unit.

Complete genome sequence of hypervirulent and outbreak-associated Acinetobacter baumannii strain LAC-4: epidemiology, resistance genetic determinants and potential virulence factors

Acinetobacter baumannii is an important human pathogen due to its multi-drug resistance. In this study, the genome of an ST10 outbreak A. baumannii isolate LAC-4 was completely sequenced to better understand its epidemiology, antibiotic resistance genetic determinants and potential virulence factors. Compared with 20 other complete genomes of A. baumannii, LAC-4 genome harbors at least 12 copies of five distinct insertion sequences. It contains 12 and 14 copies of two novel IS elements, ISAba25 and ISAba26, respectively. Additionally, three novel composite transposons were identified: Tn6250, Tn6251 and Tn6252, two of which contain resistance genes. The antibiotic resistance genetic determinants on the LAC-4 genome correlate well with observed antimicrobial susceptibility patterns. Moreover, twelve genomic islands (GI) were identified in LAC-4 genome. Among them, the 33.4-kb GI12 contains a large number of genes which constitute the K (capsule) locus. LAC-4 harbors several unique putative virulence factor loci. Furthermore, LAC-4 and all 19 other outbreak isolates were found to harbor a heme oxygenase gene (hemO)-containing gene cluster. The sequencing of the first complete genome of an ST10 A. baumannii clinical strain should accelerate our understanding of the epidemiology, mechanisms of resistance and virulence of A. baumannii.

Total synthesis of legionaminic acid as basis for serological studies

Legionaminic acid is a nine-carbon diamino monosaccharide that is found coating the surface of various bacterial human pathogens. Its unique structure makes it a valuable biological probe, but access via isolation is difficult and no practical synthesis has been reported. We describe a stereoselective synthesis that yields a legionaminic acid building block as well as linker-equipped conjugation-ready legionaminic acid starting from cheap d-threonine. To set the desired amino and hydroxyl group pattern of the target, we designed a concise sequence of stereoselective reactions. The key transformations rely on chelation-controlled organometallic additions and a Petasis multicomponent reaction. The legionaminic acid was synthesized in a form that enables attachment to surfaces. Glycan microarray containing legionaminic acid revealed that human antibodies bind the synthetic glycoside. The synthetic bacterial monosaccharide is a valuable probe to detect an immune response to bacterial pathogens such as Legionella pneumophila, the causative agent of Legionnaire's disease.

5,7-di-N-acetyl-acinetaminic acid: A novel non-2-ulosonic acid found in the capsule of an Acinetobacter baumannii isolate

An Acinetobacter baumannii global clone 1 (GC1) isolate was found to carry a novel capsule biosynthesis gene cluster, designated KL12. KL12 contains genes predicted to be involved in the synthesis of simple sugars, as well as ones for N-acetyl-L-fucosamine (L-FucpNAc) and N-acetyl-D-fucosamine (D-FucpNAc). It also contains a module of 10 genes, 6 of which are required for 5,7-di-N-acetyl-legionaminic acid synthesis. Analysis of the composition of the capsule revealed the presence of N-acetyl-D-galactosamine, L-FucpNAc and D-FucpNAc, confirming the role of fnlABC and fnr/gdr genes in the synthesis of L-FucpNAc and D-FucpNAc, respectively. A non-2-ulosonic acid, shown to be 5,7-diacetamido-3,5,7,9-tetradeoxy-L-glycero-L-altro-non-2-ulosonic acid, was also detected. This sugar has not previously been recovered from biological source, and was designated 5,7-di-N-acetyl-acinetaminic acid (Aci5Ac7Ac). Proteins encoded by novel genes, named aciABCD, were predicted to be involved in the conversion of 5,7-di-N-acetyl-legionaminic acid to Aci5Ac7Ac. A pathway for 5,7-di-N-acetyl-8-epilegionaminic acid biosynthesis was also proposed. In available A. baumannii genomes, genes for the synthesis of 5,7-di-N-acetyl-acinetaminic acid were only detected in two closely related capsule gene clusters, KL12 and KL13, which differ only in the wzy gene. KL12 and KL13 are carried by isolates belonging to clinically important clonal groups, GC1, GC2 and ST25. Genes for the synthesis of N-acyl derivatives of legionaminic acid were also found in 10 further A. baumannii capsule gene clusters, and three carried additional genes for production of 5,7-di-N-acetyl-8-epilegionaminic acid.