Functional and topological analysis of the Burkholderia cenocepacia priming glucosyltransferase BceB, involved in the biosynthesis of the cepacian exopolysaccharide

Exopolysaccharides from agriculturally important microorganisms: Conferring soil nutrient status and plant health

A wide variety of microorganisms secretes extracellular polymeric substances or commonly known as exopolysaccharides (EPS), which have been studied to influence plant growth via various mechanisms. EPS-producing microorganisms have been found to have positive effects on plant health such as by facilitating nutrient entrapment in the soil, or by improving soil quality, especially by helping in mitigating various abiotic stress conditions. The various types of microbial polysaccharides allow for the compartmentalization of the microbial community enabling them to endure undressing stress conditions. With the growing population, there is a constant need for developing sustainable agriculture where we could use various PGPR to help the plant cope with various stress conditions and simultaneously enhance the crop yield. These polysaccharides have also found application in various sectors, especially in the biomedical fields, manifesting their potential to act as antitumor drugs, play a significant role in immune evasion, and reveal various therapeutic potentials. These constitute high levels of bioactive polysaccharides which possess a wide range of implementation starting from industrial applications to novel food applications. In this current review, we aim at presenting a comprehensive study of how these microbial extracellular polymeric substances influence agricultural productivity along with their other commercial applications.

The Bep gene cluster in Burkholderia cenocepacia H111 codes for a water-insoluble exopolysaccharide essential for biofilm formation

Burkholderia cenocepacia is an opportunistic pathogen isolated from cystic fibrosis patients where it causes infections that are extremely difficult to treat with antibiotics, and sometimes have a fatal outcome. Biofilm is a virulence trait of B. cenocepacia, and is associated with infection persistence and increased tolerance to antibiotics. In biofilms exopolysaccharides have an important role, conferring mechanical stability and antibiotic tolerance. Two different exopolysaccharides were isolated from B. cenocepacia H111 biofilms: a water-soluble polysaccharide rich in rhamnose and containing an L-Man residue, and a water-insoluble polymer made of glucose, galactose and mannose. In the present work, the product encoded by B. cenocepacia H111 bepA-L gene cluster was identified as the water-insoluble exopolysaccharide, using mutant strains and NMR spectroscopy of the purified polysaccharides. It was also demonstrated that the B. cenocepacia H111 wild type strain produces the water-insoluble exopolysaccharide in pellicles, thus underlining its potential importance in in vivo infections.

LysX2 is a Mycobacterium tuberculosis membrane protein with an extracytoplasmic MprF-like domain

Background

Aminoacyl-phosphatidylglycerol (aaPG) synthases are bacterial enzymes that usually catalyze transfer of aminoacyl residues to the plasma membrane phospholipid phosphatidylglycerol (PG). The result is introduction of positive charges onto the cytoplasmic membrane, yielding reduced affinity towards cationic antimicrobial peptides, and increased resistance to acidic environments. Therefore, these enzymes represent an important defense mechanism for many pathogens, including Staphylococcus aureus and Mycobacterium tuberculosis (Mtb), which are known to encode for lysyl-(Lys)-PG synthase MprF and LysX, respectively. Here, we used a combination of bioinformatic, genetic and bacteriological methods to characterize a protein encoded by the Mtb genome, Rv1619, carrying a domain with high similarity to MprF-like domains, suggesting that this protein could be a new aaPG synthase family member. However, unlike homologous domains of MprF and LysX that are positioned in the cytoplasm, we predicted that the MprF-like domain in LysX2 is in the extracytoplasmic region.

Results

Using genetic fusions to the Escherichia coli proteins PhoA and LacZ of LysX2, we confirmed this unique membrane topology, as well as LysX and MprF as benchmarks. Expression of lysX2 in Mycobacterium smegmatis increased cell resistance to human β-defensin 2 and sodium nitrite, enhanced cell viability and delayed biofilm formation in acidic pH environment. Remarkably, MtLysX2 significantly reduced the negative charge on the bacterial surface upon exposure to an acidic environment. Additionally, we found LysX2 orthologues in major human pathogens and in rapid-growing mycobacteria frequently associated with human infections, but not in environmental and non-pathogenic mycobacteria.

Conclusions

Overall, our data suggest that LysX2 is a prototype of a new class within the MprF-like protein family that likely enhances survival of the pathogenic species through its catalytic domain which is exposed to the extracytoplasmic side of the cell membrane and is required to decrease the negative charge on the bacterial surface through a yet uncharacterized mechanism.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02493-2.

Polysaccharides from Burkholderia species as targets for vaccine development, immunomodulation and chemical synthesis

Covering: up to 2018 Burkholderia species are a vast group of human pathogenic, phytopathogenic, and plant- or environment-associated bacteria. B. pseudomallei, B. mallei, and B. cepacia complex are the causative agents of melioidosis, glanders, and cystic fibrosis-related infections, respectively, which are fatal diseases in humans and animals. Due to their high resistance to antibiotics, high mortality rates, and increased infectivity via the respiratory tract, B. pseudomallei and B. mallei have been listed as potential bioterrorism agents by the Centers for Disease Control and Prevention. Burkholderia species are able to produce a large network of surface-exposed polysaccharides, i.e., lipopolysaccharides, capsular polysaccharides, and exopolysaccharides, which are virulence factors, immunomodulators, major biofilm components, and protective antigens, and have crucial implications in the pathogenicity of Burkholderia-associated diseases. This review provides a comprehensive and up-to-date account regarding the structural elucidation and biological activities of surface polysaccharides produced by Burkholderia species. The chemical synthesis of oligosaccharides mimicking Burkholderia polysaccharides is described in detail. Emphasis is placed on the recent research efforts toward the development of glycoconjugate vaccines against melioidosis and glanders based on synthetic or native Burkholderia oligo/polysaccharides.

Mucoid switch in Burkholderia cepacia complex bacteria: Triggers, molecular mechanisms and implications in pathogenesis

Bacteria produce a vast range of exopolysaccharides (EPSs) to thrive in diverse environmental niches and often display a mucoid phenotype in solid media. One such exopolysaccharide, cepacian, is produced by bacteria of the genus Burkholderia and is of interest due to its role in pathogenesis associated with lung infections in cystic fibrosis (CF) patients. Cepacian is a repeat-unit polymer that has been implicated in biofilm formation, immune system evasion, interaction with host cells, resistance against antimicrobials, and virulence. Its biosynthesis proceeds through the Wzy-dependent polymerization and secretion mechanism, which requires a multienzymatic complex. Key aspects of its structure, genetic organization, and the regulatory network involved in mucoid switch and regulation of cepacian biosynthesis at transcriptional and posttranscriptional levels are reviewed. It is also evaluated the importance of cepacian biosynthesis/regulation key players as evolutionary targets of selection and highlighted the complexity of the regulatory network, which allows cells to coordinate the expression of metabolic functions to the ones of the cell wall, in order to be successful in ever changing environments, including in the interaction with host cells.

Topological analysis of the Escherichia coli WcaJ protein reveals a new conserved configuration for the polyisoprenyl-phosphate hexose-1-phosphate transferase family

WcaJ is an Escherichia coli membrane enzyme catalysing the biosynthesis of undecaprenyl-diphosphate-glucose, the first step in the assembly of colanic acid exopolysaccharide. WcaJ belongs to a large family of polyisoprenyl-phosphate hexose-1-phosphate transferases (PHPTs) sharing a similar predicted topology consisting of an N-terminal domain containing four transmembrane helices (TMHs), a large central periplasmic loop, and a C-terminal domain containing the fifth TMH (TMH-V) and a cytosolic tail. However, the topology of PHPTs has not been experimentally validated. Here, we investigated the topology of WcaJ using a combination of LacZ/PhoA reporter fusions and sulfhydryl labelling by PEGylation of novel cysteine residues introduced into a cysteine-less WcaJ. The results showed that the large central loop and the C-terminal tail both reside in the cytoplasm and are separated by TMH-V, which does not fully span the membrane, likely forming a "hairpin" structure. Modelling of TMH-V revealed that a highly conserved proline might contribute to a helix-break-helix structure in all PHPT members. Bioinformatic analyses show that all of these features are conserved in PHPT homologues from Gram-negative and Gram-positive bacteria. Our data demonstrate a novel topological configuration for PHPTs, which is proposed as a signature for all members of this enzyme family.

Bifidobacterium breve UCC2003 surface exopolysaccharide production is a beneficial trait mediating commensal-host interaction through immune modulation and pathogen protection

Bifidobacteria constitute a substantial proportion of the human gut microbiota. There are currently many bifidobacterial strains with claimed probiotic attributes. The mechanism through which these strains reside within their host and exert benefits to the host is far from fully understood. We have shown in the case of Bifidobacterium breve UCC2003 that a cell surface exopolysaccharide (EPS) plays a role in in vivo persistence. Biosynthesis of two possible EPSs is controlled by a bidirectional gene cluster which guides alternate EPS synthesis by means of a reorienting promoter. The presence of EPS impacts on host immune response: the wild type, EPS-positive B. breve UCC2003 efficiently evades the adaptive B-cell host response, while its isogenic, EPS-deficient equivalent elicits a strong adaptive immune response. Functionally, EPS positive strains were more resilient to presence of acid and bile and were responsible for reduced colonization levels of Citrobacter rodentium, a gut pathogen. In conclusion, we have found that EPS is important in host interactions and pathogen protection, the latter indicative of a probiotic ability for the EPS of B. breve UCC2003.

Insights into the role of extracellular polysaccharides in Burkholderia adaptation to different environments

The genus Burkholderia comprises more than 60 species able to adapt to a wide range of environments such as soil and water, and also colonize and infect plants and animals. They have large genomes with multiple replicons and high gene number, allowing these bacteria to thrive in very different niches. Among the properties of bacteria from the genus Burkholderia is the ability to produce several types of exopolysaccharides (EPSs). The most common one, cepacian, is produced by the majority of the strains examined irrespective of whether or not they belong to the Burkholderia cepacia complex (Bcc). Cepacian biosynthesis proceeds by a Wzy-dependent mechanism, and some of the B. cepacia exopolysaccharide (Bce) proteins have been functionally characterized. In vitro studies showed that cepacian protects bacterial cells challenged with external stresses. Regarding virulence, bacterial cells with the ability to produce EPS are more virulent in several animal models of infection than their isogenic non-producing mutants. Although the production of EPS within the lungs of cystic fibrosis (CF) patients has not been demonstrated, the in vitro assessment of the mucoid phenotype in serial Bcc isolates from CF patients colonized for several years showed that mucoid to non-mucoid transitions are relatively frequent. This morphotype variation can be induced under laboratory conditions by exposing cells to stress such as high antibiotic concentration. Clonal isolates where mucoid to non-mucoid transition had occurred showed that during lung infection, genomic rearrangements, and mutations had taken place. Other phenotypic changes include variations in motility, chemotaxis, biofilm formation, bacterial survival rate under nutrient starvation and virulence. In this review, we summarize major findings related to EPS biosynthesis by Burkholderia and the implications in broader regulatory mechanisms important for cell adaptation to the different niches colonized by these bacteria.

Pathogenicity, virulence factors, and strategies to fight against Burkholderia cepacia complex pathogens and related species

The Burkholderia cepacia complex (Bcc) is a group of 17 closely related species of the beta-proteobacteria subdivision that emerged in the 1980s as important human pathogens, especially to patients suffering from cystic fibrosis. Since then, a remarkable progress has been achieved on the taxonomy and molecular identification of these bacteria. Although some progress have been achieved on the knowledge of the pathogenesis traits and virulence factors used by these bacteria, further work envisaging the identification of potential targets for the scientifically based design of new therapeutic strategies is urgently needed, due to the very difficult eradication of these bacteria with available therapies. An overview of these aspects of Bcc pathogenesis and opportunities for the design of future therapies is presented and discussed in this work.

Distribution of cepacian biosynthesis genes among environmental and clinical Burkholderia strains and role of cepacian exopolysaccharide in resistance to stress conditions

The genus Burkholderia includes strains pathogenic to animals and plants, bioremediators, or plant growth promoters. Genome sequence analyses of representative Burkholderia cepacia complex (Bcc) and non-Bcc strains for the presence of the bce-I gene cluster, directing the biosynthesis of the exopolysaccharide (EPS) cepacian, further extended this previously described cluster by another 9 genes. The genes in the bce-II cluster were named bceM to bceU and encode products putatively involved in nucleotide sugar precursor biosynthesis and repeat unit assembly, modification, and translocation across the cytoplasmic membrane. Disruption of the B. cepacia IST408 bceQ and bceR genes, encoding a putative repeat unit flippase and a glycosyltransferase, respectively, resulted in the abolishment of cepacian biosynthesis. A mutation in the bceS gene, encoding a putative acyltransferase, did not affect EPS production yield significantly but decreased its acetylation content by approximately 20%. Quantitative real-time reverse transcription-PCR experiments confirmed the induction of genes in the bce-I and bce-II clusters in a Burkholderia multivorans EPS producer clinical isolate in comparison to the level for its isogenic EPS-defective strain. Fourier Transform infrared spectroscopy analysis confirmed that the exopolysaccharide produced by 10 Burkholderia isolates tested was cepacian. The ability of Burkholderia strains to withstand desiccation and metal ion stress was higher when bacteria were incubated in the presence of 2.5 g/liter of cepacian, suggesting that this EPS plays a role in the survival of these bacteria by contributing to their ability to thrive in different environments.

The hfq gene is required for stress resistance and full virulence of Burkholderia cepacia to the nematode Caenorhabditis elegans

The Burkholderia cepacia complex (Bcc) emerged as problematic opportunistic pathogens to cystic fibrosis (CF) patients. Although several virulence factors have been identified in Bcc, the knowledge of their relative contribution to Bcc pathogenicity remains scarce. In this work, we describe the identification and characterization of a B. cepacia IST408 mutant containing a disruption in the hfq gene. In other bacteria, Hfq is a global regulator of metabolism, acting as an RNA chaperone involved in the riboregulation of target mRNAs by small regulatory non-coding RNAs (sRNAs). The B. cepacia Hfq protein was overproduced as a histidine-tagged derivative, and we show evidence that the protein forms hexamers and binds sRNAs. When provided in trans, the B. cepacia IST408 hfq gene complemented the Escherichia coli hfq mutant strain GS081. Our results also show that the B. cepacia hfq mutant is more susceptible to stress conditions mimicking those faced by Bcc bacteria when infecting the CF host. In addition, the B. cepacia hfq mutant and two hfq mutants derived from B. dolosa and B. ambifaria clinical isolates also exhibited a reduced ability to colonize and kill the nematode Caenorhabditis elegans, used as an infection model. These data, together with the conservation of Hfq orthologues among Bcc, strongly suggest that Hfq plays a major role in the survival of Bcc under stress conditions, contributing to the success of Bcc as CF pathogens.

Plant host and sugar alcohol induced exopolysaccharide biosynthesis in the Burkholderia cepacia complex

The species that presently constitute the Burkholderia cepacia complex (Bcc) have multiple roles; they include soil and water saprophytes, bioremediators, and plant, animal and human pathogens. Since the first description of pathogenicity in the Bcc was based on sour skin rot of onion bulbs, this study returned to this plant host to investigate the onion-associated phenotype of the Bcc. Many Bcc isolates, which were previously considered to be non-mucoid, produced copious amounts of exopolysaccharide (EPS) when onion tissue was provided as the sole nutrient. EPS production was not species-specific, was observed in isolates from both clinical and environmental sources, and did not correlate with the ability to cause maceration of onion tissue. Chemical analysis suggested that the onion components responsible for EPS induction were primarily the carbohydrates sucrose, fructose and fructans. Additional sugars were investigated, and all alcohol sugars tested were able to induce EPS production, in particular mannitol and glucitol. To investigate the molecular basis for EPS biosynthesis, we focused on the highly conserved bce gene cluster thought to be involved in cepacian biosynthesis. We demonstrated induction of the bce gene cluster by mannitol, and found a clear correlation between the inability of representatives of the Burkholderia cenocepacia ET12 lineage to produce EPS and the presence of an 11 bp deletion within the bceB gene, which encodes a glycosyltransferase. Insertional inactivation of bceB in Burkholderia ambifaria AMMD results in loss of EPS production on sugar alcohol media. These novel and surprising insights into EPS biosynthesis highlight the metabolic potential of the Bcc and show that a potential virulence factor may not be detected by routine laboratory culture. Our results also highlight a potential hazard in the use of inhaled mannitol as an osmolyte to improve mucociliary clearance in individuals with cystic fibrosis.