Mechanistic and structural studies into the biosynthesis of the bacterial sugar pseudaminic acid (Pse5Ac7Ac)

KpsS1 Mediates the Glycosylation of Pseudaminic Acid in Acinetobacter Baumannii

Pseudaminic acid (Pse) is found in the polysaccharide structures of the cell surface of various Gram-negative pathogenic bacteria including Acinetobacter baumannii and considered as an important component of cell surface glycans including oligosaccharides and glycoproteins. However, the glycosyltransferase that is responsible for the Pse glycosylation in A. baumannii remains unknown yet. In this study, through comparative genomics analysis of Pse-positive and negative A. baumannii clinical isolates, we identified a potential glycosyltransferase, KpsS1, located right downstream of the Pse biosynthesis genetic locus. Deletion of this gene in an Pse-positive A. baumannii strain, Ab8, impaired the glycosylation of Pse to the surface CPS and proteins, while the gene knockout strain, Ab8ΔkpsS1, could still produce Pse with 2.86 folds higher amount than that of Ab8. Furthermore, impairment of Pse glycosylation affected the morphology and virulence potential of A. baumannii, suggesting the important role of this protein. This study will provide insights into the further understanding of Pse in bacterial physiology and pathogenesis.

The Retaining Pse5Ac7Ac Pseudaminyltransferase KpsS1 Defines a Previously Unreported glycosyltransferase family (GT118)

Cell surface sugar 5,7-diacetyl pseudaminic acid (Pse5Ac7Ac) is a bacterial analogue of the ubiquitous sialic acid, Neu5Ac, and contributes to the virulence of a number of multidrug resistant bacteria, including ESKAPE pathogens Pseudomonas aeruginosa, and Acinetobacter baumannii. Despite its discovery in the surface glycans of bacteria over thirty years ago, to date no glycosyltransferase enzymes (GTs) dedicated to the synthesis of a pseudaminic acid glycosidic linkage have been unequivocally characterised in vitro. Herein we demonstrate that A. baumannii KpsS1 is a dedicated pseudaminyltransferase enzyme (PseT) which constructs a Pse5Ac7Ac-α(2,6)-Glcp linkage, and proceeds with retention of anomeric configuration. We utilise this PseT activity in tandem with the biosynthetic enzymes required for CMP-Pse5Ac7Ac assembly, in a two-pot, seven enzyme synthesis of an α-linked Pse5Ac7Ac glycoside. Due to its unique activity and protein sequence, we also assign KpsS1 as the prototypical member of a previously unreported GT family (GT118).

Co-factor prosthesis facilitates biosynthesis of azido-pseudaminic acid probes for use as glycosyltransferase reporters

Truncated thioester N,S-diacetylcysteamine (SNAc) was utilised as a co-factor mimic for PseH, an acetyl-coA dependent aminoglycoside N-acetyltransferase, in the biosynthesis of the bacterial sugar, pseudaminic acid. Additionally, an azido-SNAc analogue was used to smuggle N7-azide functionality into the pseudaminic acid backbone, facilitating its use as a reporter of pseudaminyltransferase activity.

Revised Model for the Type A Glycan Biosynthetic Pathway in Clostridioides difficile Strain 630Δerm Based on Quantitative Proteomics of cd0241-cd0244 Mutant Strains

The bacterial flagellum is involved in a variety of processes including motility, adherence, and immunomodulation. In the Clostridioides difficile strain 630Δerm, the main filamentous component, FliC, is post-translationally modified with an O-linked Type A glycan structure. This modification is essential for flagellar function, since motility is seriously impaired in gene mutants with improper biosynthesis of the Type A glycan. The cd0240-cd0244 gene cluster encodes the Type A biosynthetic proteins, but the role of each gene, and the corresponding enzymatic activity, have not been fully elucidated. Using quantitative mass spectrometry-based proteomics analyses, we determined the relative abundance of the observed glycan variations of the Type A structure in cd0241, cd0242, cd0243, and cd0244 mutant strains. Our data not only confirm the importance of CD0241, CD0242, and CD0243 but, in contrast to previous data, also show that CD0244 is essential for the biosynthesis of the Type A modification. Combined with additional bioinformatic analyses, we propose a revised model for Type A glycan biosynthesis.

Stereoisomer-specific reprogramming of a bacterial flagellin sialyltransferase

Glycosylation of surface structures diversifies cells chemically and physically. Nucleotide-activated sialic acids commonly serve as glycosyl donors, particularly pseudaminic acid (Pse) and its stereoisomer legionaminic acid (Leg), which decorate eubacterial and archaeal surface layers or protein appendages. FlmG, a recently identified protein sialyltransferase, O-glycosylates flagellins, the subunits of the flagellar filament. We show that flagellin glycosylation and motility in Caulobacter crescentus and Brevundimonas subvibrioides is conferred by functionally insulated Pse and Leg biosynthesis pathways, respectively, and by specialized FlmG orthologs. We established a genetic glyco-profiling platform for the classification of Pse or Leg biosynthesis pathways, discovered a signature determinant of eubacterial and archaeal Leg biosynthesis, and validated it by reconstitution experiments in a heterologous host. Finally, by rewiring FlmG glycosylation using chimeras, we defined two modular determinants that govern flagellin glycosyltransferase specificity: a glycosyltransferase domain that either donates Leg or Pse and a specialized flagellin-binding domain that identifies the acceptor.

The Colorectal Cancer Microbiota Alter Their Transcriptome To Adapt to the Acidity, Reactive Oxygen Species, and Metabolite Availability of Gut Microenvironments

The gut microbiome is implicated in the pathology of colorectal cancer (CRC). However, the mechanisms by which the microbiota actively contribute to disease onset and progression remain elusive. In this pilot study, we sequenced fecal metatranscriptomes of 10 non-CRC and 10 CRC patient gut microbiomes and conducted differential gene expression analyses to assess any changed functionality in disease. We report that oxidative stress responses were the dominant activity across cohorts, an overlooked protective housekeeping role of the human gut microbiome. However, expression of hydrogen peroxide and nitric oxide-scavenging genes was diminished and augmented, respectively, positing that these regulated microbial responses have implications for CRC pathology. CRC microbes enhanced expression of genes for host colonization, biofilm formation, genetic exchange, virulence determinants, antibiotic, and acid resistances. Moreover, microbes promoted transcription of genes involved in metabolism of several beneficial metabolites, suggesting their contribution to patient metabolite deficiencies previously solely attributed to tumor cells. We showed in vitro that expression of genes involved in amino acid-dependent acid resistance mechanisms of meta-gut Escherichia coli responded differently to acid, salt, and oxidative pressures under aerobic conditions. These responses were mostly dictated by the host health status of origin of the microbiota, suggesting their exposure to fundamentally different gut conditions. These findings for the first time highlight mechanisms by which the gut microbiota can either protect against or drive colorectal cancer and provide insights into the cancerous gut environment that drives functional characteristics of the microbiome. IMPORTANCE The human gut microbiota has the genetic potential to drive colorectal cancer onset and progression; however, the expression of this genetic potential during the disease has not been investigated. We found that microbial expression of genes that detoxify DNA-damaging reactive oxygen species, which drive colorectal cancer, is compromised in cancer. We observed a greater activation of expression of genes involved in virulence, host colonization, exchange of genetic material, metabolite utilization, defense against antibiotics, and environmental pressures. Culturing gut Escherichia coli of cancerous and noncancerous metamicrobiota revealed different regulatory responses of amino acid-dependent acid resistance mechanisms in a health-dependent manner under environmental acid, oxidative, and osmotic pressures. Here, for the first time, we demonstrate that the activity of microbial genomes is regulated by the health status of the gut in vivo and in vitro and provides new insights for shifts in microbial gene expression in colorectal cancer.

Risk assessment of three sheep stocking modes via identification of bacterial genomes carrying antibiotic resistance genes and virulence factor genes

In order to protect the prairie ecological environment, intensive farming has become a prevalent method of sheep stocking. However, the link between captivity stocking mode and ecological risk of sheep feces is still poorly understood. In this study, metagenomics was used to identify the environmental risk of sheep feces among three stocking modes. Our results showed that captivity mode (C) elevated antibiotic resistance in feces, with the abundance of antibiotic resistance genes (ARGs) (5.381 copies/cell) higher than that of half-pen stocking (Fh) (1.093 copies/cell) and grazing mode (Fr) (0.315 copies/cell) (Duncan's test, P < 0.05). Virulence factor genes (VFGs) analysis showed offensive virulence factors had the highest abundance in captivity feces (C: 3.826 copies/cell, Fh: 0.342 copies/cell, Fr: 0.163 copies/cell) (Duncan's test, P < 0.05). 15 metagenome-assembled genomes (MAGs) were identified as potential pathogenic antibiotic resistant bacteria (PARB) and revealed that Escherichia, Klebsiella may be the main host of ARGs and VFGs in sheep feces. Furthermore, the minimal inhibition concentrations (MIC) of tetracycline of E. coli in the captivity feces was 8.6 times and 4.7 times than that of grazing and half-pen stocking samples, respectively. The Non-metric multidimensional scaling (NMDS) revealed that high stocking density leads to feces causing increased harm to the environment. Although feces from sheep raised in captivity and half-pen stocking modes are easier to collect, they are more harmful to the environment and aerobic composting should be done before their application to farmland. This work provides a guideline for better control of the environmental risk of sheep feces from different stocking modes.

Nucleotide sugar dehydratases: Structure, mechanism, substrate specificity, and application potential

Nucleotide sugar (NS) dehydratases play a central role in the biosynthesis of deoxy and amino sugars, which are involved in a variety of biological functions in all domains of life. Bacteria are true masters of deoxy sugar biosynthesis as they can produce a wide range of highly specialized monosaccharides. Indeed, deoxy and amino sugars play important roles in the virulence of gram-positive and gram-negative pathogenic species and are additionally involved in the biosynthesis of diverse macrolide antibiotics. The biosynthesis of deoxy sugars relies on the activity of NS dehydratases, which can be subdivided into three groups based on their structure and reaction mechanism. The best-characterized NS dehydratases are the 4,6-dehydratases that, together with the 5,6-dehydratases, belong to the NS-short-chain dehydrogenase/reductase superfamily. The other two groups are the less abundant 2,3-dehydratases that belong to the Nudix hydrolase superfamily and 3-dehydratases, which are related to aspartame aminotransferases. 4,6-Dehydratases catalyze the first step in all deoxy sugar biosynthesis pathways, converting nucleoside diphosphate hexoses to nucleoside diphosphate-4-keto-6-deoxy hexoses, which in turn are further deoxygenated by the 2,3- and 3-dehydratases to form dideoxy and trideoxy sugars. In this review, we give an overview of the NS dehydratases focusing on the comparison of their structure and reaction mechanisms, thereby highlighting common features, and investigating differences between closely related members of the same superfamilies.

Metagenomic Analysis Reveals a Changing Microbiome Associated With the Depth of Invasion of Oral Squamous Cell Carcinoma

The relationship between oral squamous cell carcinoma (OSCC) development and the microbiome has attracted increasing attention. The depth of invasion (DOI) is an important indicator of tumor progression, staging and prognosis, and the change in the oral microbiome based on the DOI is unclear. This report describes the use of metagenomic analyses to investigate the relationship between the oral microbiome and the DOI. Forty patients in different DOI categories were recruited; 10 healthy people served as the control group. Swab samples collected from the participants were subjected to metagenomic analyses, and the oral microbial communities and their functions were investigated. The abundances of Fusobacterium nucleatum, Capnocytophaga sputigena, Porphyromonas endodontalis, and Gemella haemolysans were significantly increased in the patients compared with the controls. The abundances of some bacteria exhibited a stage-related trend. The abundances of P. endodontalis, Gemella morbillorum and G. haemolysans increased with increasing DOI. In contrast, the abundances of Prevotella melaninogenica, Haemophilus parainfluenzae and Neisseria flavescens decreased with increasing DOI. Based on receiver operating characteristic (ROC) curve analysis, eight species were found to have predictive value: Rothia mucilaginosa, P. melaninogenica, H. parainfluenzae, and N. flavescens in the healthy control group and P. endodontalis, G. morbillorum, G. haemolysans and Fusobacterium periodonticum in the high DOI group. In the functional analysis, several metabolic pathways were decreased, whereas flagellar assembly and bacterial chemotaxis showed an increasing trend as the disease progressed. Biofilm formation, flagella, lipopolysaccharide (LPS) and other virulence factors exhibited staging-related changes. These pathogenic pathways and factors had a clear correlation with specific pathogens. In particular, when OSCC progressed to the late stage, microbial diversity and functional potential changed greatly.

Identifying the targets and functions of N-linked protein glycosylation in Campylobacter jejuni

Campylobacter jejuni is a major cause of bacterial gastroenteritis in humans that is primarily associated with the consumption of inadequately prepared poultry products, since the organism is generally thought to be asymptomatic in avian species. Unlike many other microorganisms, C. jejuni is capable of performing extensive post-translational modification (PTM) of proteins by N- and O-linked glycosylation, both of which are required for optimal chicken colonization and human virulence. The biosynthesis and attachment of N-glycans to C. jejuni proteins is encoded by the pgl (protein glycosylation) locus, with the PglB oligosaccharyltransferase (OST) enabling en bloc transfer of a heptasaccharide N-glycan from a lipid carrier in the inner membrane to proteins exposed within the periplasm. Seventy-eight C. jejuni glycoproteins (represented by 134 sites of experimentally verified N-glycosylation) have now been identified, and include inner and outer membrane proteins, periplasmic proteins and lipoproteins, which are generally of poorly defined or unknown function. Despite our extensive knowledge of the targets of this apparently widespread process, we still do not fully understand the role N-glycosylation plays biologically, although several phenotypes, including wild-type stress resistance, biofilm formation, motility and chemotaxis have been related to a functional pgl system. Recent work has described enzymatic processes (nitrate reductase NapAB) and antibiotic efflux (CmeABC) as major targets requiring N-glycan attachment for optimal function, and experimental evidence also points to roles in cell binding via glycan-glycan interactions, protein complex formation and protein stability by conferring protection against host and bacterial proteolytic activity. Here we examine the biochemistry of the N-linked glycosylation system, define its currently known protein targets and discuss evidence for the structural and functional roles of this PTM in individual proteins and globally in C. jejuni pathogenesis.

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.

Genomic Epidemiology and Evolution of Diverse Lineages of Clinical Campylobacter jejuni Cocirculating in New Hampshire, USA, 2017

Campylobacter jejuni is one of the leading causes of bacterial gastroenteritis worldwide. In the United States, New Hampshire was one of the 18 states that reported cases in the 2016 to 2018 multistate outbreak of multidrug-resistant C. jejuni. Here, we aimed to elucidate the baseline diversity of the wider New Hampshire C. jejuni population during the outbreak. We used genome sequences of 52 clinical isolates sampled in New Hampshire in 2017, including 1 of the 2 isolates from the outbreak.

Campylobacter jejuni is one of the leading causes of bacterial gastroenteritis worldwide. In the United States, New Hampshire was one of the 18 states that reported cases in the 2016 to 2018 multistate outbreak of multidrug-resistant C. jejuni. Here, we aimed to elucidate the baseline diversity of the wider New Hampshire C. jejuni population during the outbreak. We used genome sequences of 52 clinical isolates sampled in New Hampshire in 2017, including 1 of the 2 isolates from the outbreak. Results revealed a remarkably diverse population composed of at least 28 sequence types, which are mostly represented by 1 or a few strains. A comparison of our isolates with 249 clinical C. jejuni from other states showed frequent phylogenetic intermingling, suggesting a lack of geographical structure and minimal local diversification within the state. Multiple independent acquisitions of resistance genes from 5 classes of antibiotics characterize the population, with 47/52 (90.4%) of the genomes carrying at least 1 horizontally acquired resistance gene. Frequently recombining genes include those associated with heptose biosynthesis, colonization, and stress resistance. We conclude that the diversity of clinical C. jejuni in New Hampshire in 2017 was driven mainly by the coexistence of phylogenetically diverse antibiotic-resistant lineages, widespread geographical mixing, and frequent recombination. This study provides an important baseline census of the standing pangenomic variation and drug resistance to aid the development of a statewide database for epidemiological studies and clinical decision making. Continued genomic surveillance will be necessary to accurately assess how the population of C. jejuni changes over the long term.