Exopolysaccharide Biosynthesis Research Articles

Interlink between ExoD (Alr2882), exopolysaccharide synthesis and metal tolerance in Nostoc sp. strain PCC 7120: Insight into its role, paralogs and evolution

Exopolysaccharides (EPS) produced by bacterial species are an important component of bacteria's survival strategy. Synthesis of EPS, principal component of extracellular polymeric substance, occurs through multiple pathways involving multitude of genes. While stress-induced concomitant increase in exoD transcript levels and EPS content have been shown earlier, experimental evidence for direct correlation is lacking. In the present study, role of ExoD in Nostoc sp. strain PCC 7120 was evaluated by generating a recombinant Nostoc strain AnexoD+, wherein the ExoD (Alr2882) protein was constitutively overexpressed. AnexoD+ exhibited higher EPS production, propensity for formation of biofilms and tolerance to Cd stress compared to vector control AnpAM cells. Both Alr2882 and its paralog All1787 exhibited 5 transmembrane domains, with only All1787 predicted to interact with several proteins in polysaccharide synthesis. Phylogenetic analysis of orthologs of these proteins across cyanobacteria indicated that the two paralogs Alr2882 and All1787 and their corresponding orthologs arose divergently during evolution, and could have distinct roles to perform in the biosynthesis of EPS. This study has thrown open the possibility of engineering overproduction of EPS and inducing biofilm formation through genetic manipulation of EPS biosynthesis genes in cyanobacteria, thus building a cost-effective green platform for large scale production of EPS.

Small Molecule Inhibition of an Exopolysaccharide Modification Enzyme is a Viable Strategy To Block Pseudomonas aeruginosa Pel Biofilm Formation.

Biosynthesis of the Pel exopolysaccharide in Pseudomonas aeruginosa requires all seven genes of the pelABCDEFG operon. The periplasmic modification enzyme PelA contains a C-terminal deacetylase domain that is necessary for Pel-dependent biofilm formation. Herein, we show that extracellular Pel is not produced by a P. aeruginosa PelA deacetylase mutant. This positions PelA deacetylase activity as an attractive target to prevent Pel-dependent biofilm formation. Using a high-throughput screen (n = 69,360), we identified 56 compounds that potentially inhibit PelA esterase activity, the first enzymatic step in the deacetylase reaction. A secondary biofilm inhibition assay identified methyl 2-(2-pyridinylmethylene) hydrazinecarbodithioate (SK-017154-O) as a specific Pel-dependent biofilm inhibitor. Structure-activity relationship studies identified the thiocarbazate as a necessary functional group and that the pyridyl ring could be replaced with a phenyl substituent (compound 1). Both SK-017154-O and compound 1 inhibit Pel-dependent biofilm formation in Bacillus cereus ATCC 10987, which has a predicted extracellular PelA deacetylase in its pel operon. Michaelis-Menten kinetics determined SK-017154-O to be a noncompetitive inhibitor of PelA, while compound 1 did not directly inhibit PelA esterase activity. Cytotoxicity assays using human lung fibroblast cells showed that compound 1 is less cytotoxic than SK-017154-O. This work provides proof of concept that biofilm exopolysaccharide modification enzymes are important for biofilm formation and can serve as useful antibiofilm targets. IMPORTANCE Present in more than 500 diverse Gram-negative and 900 Gram-positive organisms, the Pel polysaccharide is one of the most phylogenetically widespread biofilm matrix determinants found to date. Partial de-N-acetylation of this α-1,4 linked N-acetylgalactosamine polymer by the carbohydrate modification enzyme PelA is required for Pel-dependent biofilm formation in Pseudomonas aeruginosa and Bacillus cereus. Given this and our observation that extracellular Pel is not produced by a P. aeruginosa PelA deactylase mutant, we developed an enzyme-based high-throughput screen and identified methyl 2-(2-pyridinylmethylene) hydrazinecarbodithioate (SK-017154-O) and its phenyl derivative as specific Pel-dependent biofilm inhibitors. Michaelis-Menten kinetics revealed SK-017154-O is a noncompetitive inhibitor and that its noncytotoxic, phenyl derivative does not directly inhibit P. aeruginosa PelA esterase activity. We provide proof of concept that exopolysaccharide modification enzymes can be targeted with small molecule inhibitors to block Pel-dependent biofilm development in both Gram-negative and Gram-positive bacteria.

Inulin increases the EPS biosynthesis of Lactobacillus delbrueckii ssp. bulgaricus LDB-C1.

Its eps gene cluster, the antioxidant activity and monosaccharide composition of exopolysaccharides, the expression levels of related genes at different fermentations were analyzed for clarifying the exopolysaccharide biosynthesis mechanism of Lactobacillus delbrueckii subsp. bulgaricus LDB-C1. The comparison analysis of eps gene clusters indicated that the gene clusters present diversity and strain specificity. The crude exopolysaccharides from LDB-C1 exhibited a good antioxidant activity. Compared with glucose, fructose, galactose, and fructooligosaccharide, inulin significantly improved the exopolysaccharide biosynthesis. The structures of EPSs were significantly different under different carbohydrate fermentation conditions. Inulin obviously increased the expressions of most EPS biosynthesis related genes at fermentation 4h. Inulin accelerated the beginning of the exopolysaccharide production in LDB-C1, and the enzymes promoted by inulin was beneficial for the accumulation of exopolysaccharide at the whole fermentation process.

EXOPOLYSACCHARIDES OF LACTIC ACID BACTERIA: THE MANIPULATION OF ENVIRONMENTAL STRESSES TO MODULATE BIOSYNTHESIS FOR INDUSTRIAL APPLICATIONS

Exopolysaccharides are biopolymers produced by microorganisms in response to adverse environmental conditions. Exopolysaccharides of lactic acid bacteria are applied in many different fields such as food, pharmaceuticals, and medicine. This review discusses the biological functions of the exopolysaccharide derived from lactic acid bacteria and their response to exopolysaccharide synthesis under environmental stresses. Environmental stress can alter the expression of eps genes, resulting in changes in yield and monosaccharide composition of obtained exopolysaccharides. Therefore, the evaluation aims to clarify the feasibility of using environmental stress to modify exopolysaccharide biosynthesis in lactic acid bacteria for various applications.

Molecular characterization of Paenibacillus antarcticus IPAC21, a bioemulsifier producer isolated from Antarctic soil.

Paenibacillus antarcticus IPAC21, an endospore-forming and bioemulsifier-producing strain, was isolated from King George Island, Antarctica. As psychrotolerant/psychrophilic bacteria can be considered promising sources for novel products such as bioactive compounds and other industrially relevant substances/compounds, the IPAC21 genome was sequenced using Illumina Hi-seq, and a search for genes related to the production of bioemulsifiers and other metabolic pathways was performed. The IPAC21 strain has a genome of 5,505,124 bp and a G + C content of 40.5%. Genes related to the biosynthesis of exopolysaccharides, such as the gene that encodes the extracellular enzyme levansucrase responsible for the synthesis of levan, the 2,3-butanediol pathway, PTS sugar transporters, cold-shock proteins, and chaperones were found in its genome. IPAC21 cell-free supernatants obtained after cell growth in trypticase soy broth at different temperatures were evaluated for bioemulsifier production by the emulsification index (EI) using hexadecane, kerosene and diesel. EI values higher than 50% were obtained using the three oil derivatives when IPAC21 was grown at 28°C. The bioemulsifier produced by P. antarcticus IPAC21 was stable at different NaCl concentrations, low temperatures and pH values, suggesting its potential use in lower and moderate temperature processes in the petroleum industry.

Broad-host-range lytic Erwinia phage Key with exopolysaccharide degrading activity

In this study, the genome of the lytic broad-host-range phage Key infecting Erwinia amylovora, Erwinia horticola, and Pantoea agglomerans strains was characterized. Key phage has a 115,651 bp long double-stranded DNA genome with the G + C ratio of 39.03%, encoding 182 proteins and 27 tRNA genes. The majority (69%) of predicted coding sequences (CDSs) encode proteins with unknown functions. The protein products of 57 annotated genes were found to have probable functions in nucleotide metabolism, DNA replication, recombination, repair, and packaging, virion morphogenesis, phage-host interaction and lysis. Furthermore, the product of gene 141 shared amino acid sequence similarity and conserved domain architecture with the exopolysaccharide (EPS) degrading proteins of Erwinia and Pantoea infecting phages as well as bacterial EPS biosynthesis proteins. Due to the genome synteny and similarity to the proteins of T5-related phages, phage Key, together with its closest relative, Pantoea phage AAS21, was suggested to represent a novel genus within the Demerecviridae family, for which we tentatively propose the name “Keyvirus”.

Biopolymers Produced by Lactic Acid Bacteria: Characterization and Food Application.

Plants, animals, bacteria, and food waste are subjects of intensive research, as they are biological sources for the production of biopolymers. The topic links to global challenges related to the extended life cycle of products, and circular economy objectives. A severe and well-known threat to the environment, the non-biodegradability of plastics obliges different stakeholders to find legislative and technical solutions for producing valuable polymers which are biodegradable and also exhibit better characteristics for packaging products. Microorganisms are recognized nowadays as exciting sources for the production of biopolymers with applications in the food industry, package production, and several other fields. Ubiquitous organisms, lactic acid bacteria (LAB) are well studied for the production of exopolysaccharides (EPS), but much less as producers of polylactic acid (PLA) and polyhydroxyalkanoates (PHAs). Based on their good biodegradability feature, as well as the possibility to be obtained from cheap biomass, PLA and PHAs polymers currently receive increased attention from both research and industry. The present review aims to provide an overview of LAB strains' characteristics that render them candidates for the biosynthesis of EPS, PLA, and PHAs, respectively. Further, the biopolymers' features are described in correlation with their application in different food industry fields and for food packaging. Having in view that the production costs of the polymers constitute their major drawback, alternative solutions of biosynthesis in economic terms are discussed.

Exopolysaccharides from Lactic Acid Bacteria: A Review on Functions, Biosynthesis and Applications in Food Industry

Lactic acid bacteria are the substantial source for producing polysaccharides used in technological applications as thickeners and viscosifiers in the food industry. A broad variety of lactic acid bacteria species secrete structurally diverse exopolysaccharides that contribute to their surface attachment, protection against abiotic or biotic stress factors and nutrient uptake. The exopolysaccharides are produced naturally during fermentation process by living lactic acid bacteria cells and accepted as postbiotic for these metabolites having various physiological health-promoting effects. Exopolysaccharide producer lactic acid bacteria encode a great number of enzymes and regulatory proteins involved exopolysaccharide biosynthesis process. This process is a complex and occurs through presence of multiple genes. However, it is crucial the understanding of structure, composition, function, chemical, and physical properties of exopolysaccharides which vary from one type of bacteria to another via chemical analysis methods. In this review, the use of lactic acid bacteria exopolysaccharides, their structures, genetic modules and biosynthesis, and the use of exopolysaccharides derived from lactic acid bacteria in the food industry are described, discussed and focused on recent developments.

Draft Genome Sequence of Lactococcus lactis Strain 12-16-PSH, Isolated from Prostokvasha from the Arkhangelsk Region of Russia.

A draft genome sequence of Lactococcus lactis strain 12-16-PSH, which was isolated from prostokvasha, is reported. The genome assembly of strain 12-16-PSH contained 63 contigs, with a total length of 2,468,647 bp. A total of 2,421 protein-coding genes were predicted, among which 6 encoded bacteriocins while 15 encoded glycosyl transferases, presumably involved in exopolysaccharide biosynthesis.

Exopolysaccharide Biosynthesis in Rhizobium leguminosarum bv. trifolii Requires a Complementary Function of Two Homologous Glycosyltransferases PssG and PssI.

The Pss-I region of Rhizobium leguminosarum bv. trifolii TA1 comprises more than 20 genes coding for glycosyltransferases, modifying enzymes, and polymerization/export proteins, altogether determining the biosynthesis of symbiotically relevant exopolysaccharides. In this study, the role of homologous PssG and PssI glycosyltransferases in exopolysaccharide subunit synthesis were analyzed. It was shown that the glycosyltransferase-encoding genes of the Pss-I region were part of a single large transcriptional unit with potential downstream promoters activated in specific conditions. The ΔpssG and ΔpssI mutants produced significantly lower amounts of the exopolysaccharide, while the double deletion mutant ΔpssIΔpssG produced no exopolysaccharide. Complementation of double mutation with individual genes restored exopolysaccharide synthesis, but only to the level similar to that observed for the single ΔpssI or ΔpssG mutants, indicating that PssG and PssI serve complementary functions in the process. PssG and PssI interacted with each other in vivo and in vitro. Moreover, PssI displayed an expanded in vivo interaction network comprising other GTs involved in subunit assembly and polymerization/export proteins. PssG and PssI proteins were shown to interact with the inner membrane through amphipathic helices at their C-termini, and PssG also required other proteins involved in exopolysaccharide synthesis to localize in the membrane protein fraction.

Glycoside hydrolase processing of the Pel polysaccharide alters biofilm biomechanics and Pseudomonas aeruginosa virulence

Pel exopolysaccharide biosynthetic loci are phylogenetically widespread biofilm matrix determinants in bacteria. In Pseudomonas aeruginosa, Pel is crucial for cell-to-cell interactions and reducing susceptibility to antibiotic and mucolytic treatments. While genes encoding glycoside hydrolases have long been linked to biofilm exopolysaccharide biosynthesis, their physiological role in biofilm development is unclear. Here we demonstrate that the glycoside hydrolase activity of P. aeruginosa PelA decreases adherent biofilm biomass and is responsible for generating the low molecular weight secreted form of the Pel exopolysaccharide. We show that the generation of secreted Pel contributes to the biomechanical properties of the biofilm and decreases the virulence of P. aeruginosa in Caenorhabditis elegans and Drosophila melanogaster. Our results reveal that glycoside hydrolases found in exopolysaccharide biosynthetic systems can help shape the soft matter attributes of a biofilm and propose that secreted matrix components be referred to as matrix associated to better reflect their influence.

Genomic Insights and Functional Analysis Reveal Plant Growth Promotion Traits of Paenibacillus mucilaginosus G78.

Paenibacillus mucilaginosus has widely been reported as a plant growth-promoting rhizobacteria (PGPR). However, the important genomic insights into plant growth promotion in this species remain undescribed. In this study, the genome of P. mucilaginosus G78 was sequenced using Illumina NovaSeq PE150. It contains 8,576,872 bp with a GC content of 58.5%, and was taxonomically characterized. Additionally, a total of 7337 genes with 143 tRNAs, 41 rRNAs, and 5 ncRNAs were identified. This strain can prohibit the growth of the plant pathogen, but also has the capability to form biofilm, solubilize phosphate, and produce IAA. Twenty-six gene clusters encoding secondary metabolites were identified, and the genotypic characterization indirectly proved its resistant ability to ampicillin, bacitracin, polymyxin and chloramphenicol. The putative exopolysaccharide biosynthesis and biofilm formation gene clusters were explored. According to the genetic features, the potential monosaccharides of its exopolysaccharides for P. mucilaginosus G78 may include glucose, mannose, galactose, fucose, that can probably be acetylated and pyruvated. Conservation of the pelADEFG compared with other 40 Paenibacillus species suggests that Pel may be specific biofilm matrix component in P. mucilaginosus. Several genes relevant to plant growth-promoting traits, i.e., IAA production and phosphate solubilization are well conserved compared with other 40 other Paenibacillus strains. The current study can benefit for understanding the plant growth-promoting traits of P. mucilaginosus as well as its potential application in agriculture as PGPR.

Шляхи підвищення ефективності технологій синтезу мікробних екзополісахаридів. Частина 1. Встановлення оптимальних умов біосинтезу

Conducting experiments in which only one factor is varied is the easiest way to establish the optimal conditions of producer cultivation. Usually, in such studies, a consistent determination of the nature and concentration of the carbon and nitrogen source, pH, temperature, duration of cultivation, as well as the method of inoculum preparation is carried out. Such single-factor studies remain an effective tool for the initial determination of the most critical factors which cause the greatest impact on the biosynthesis process and require additional optimization. Further optimization of producer cultivation conditions using multivariate studies involves the use of various methods of mathematical planning of experiments, one of which is the Plackett-Burman design, which allows determining the level of influence on the process of a large number of variables while conducting a smaller number of experiments. For the combined analysis of factors which have the greatest influence on the process of exopolysaccharides biosynthesis, other more complex mathematical models are usually used, in particular, the Box-Wilson method, the central composite design, or the Box-Behnken design. A significant advantage of the Box-Behnken design is that it does not contain combinations for which all factors are at the highest or lowest levels at the same time. The analyzed literature data of recent years regarding ways to increase the efficiency of microbial exopolysaccharides technologies proved the need to use an integrated approach to solve this problem. So, at the first stage, it is necessary to establish the optimal composition of the nutrient medium. At the same time, the most modern approach is to conduct multifactorial studies using complex mathematical models, which take into account the interrelationship of various studied factors and make it pos sible to more accurately establish the optimal conditions of the cultivation process, which ensure increasing the concentration of microbial exopolysaccharides by 2—10 times compared to that before optimization.

Novel Fermented Ice Cream Formulations with Improved Antiradical and Anti-Inflammatory Features

Autochthonous strains belonging to Lacticaseibacillus paracasei and Lacticaseibacillus casei were screened based on pro-technological (kinetics of growth and acidification, exopolysaccharides biosynthesis), functional (cell viability during processing and storage, in vitro protein digestibility, and in vitro gastrointestinal batch digestion), and sensory properties to ferment milk intended to be included in the ice-cream formulation. The main discrimination among the strains resulted from the sensory evaluation by the panelists, who assigned the highest scores to the ice cream produced with milk fermented with L. casei F14. Antiradical and anti-inflammatory features of such fermented ice cream (with and without the addition of hydroxytyrosol) were shown in Caco-2 cell culture. The pretreatment of Caco-2 cells with fermented ice creams counteracted the accumulation of pro-inflammatory interleukin mediators IL-8 and IL-6 and reactive oxygen species (ROS) induced by detrimental stimuli, and preserved the Caco-2 cell monolayer integrity. The fermentation with L. casei F14 per se played a key role, whereas the inclusion of hydroxytyrosol only contributed to further enhancing the antiradical activity of ice cream. No protective effect on Caco-2 cells was brought about by the addition of dietary fiber.

An RpoN-dependent PEP-CTERM gene is involved in floc formation of an Aquincola tertiaricarbonis strain

BackgroundThe floc is a characteristic of microbial aggregate growth, displaying cloudy suspensions in water. Floc formation has been demonstrated in a series of bacteria and the floc-forming bacteria play a crucial role in activated sludge (AS) process widely used for municipal sewage and industrial wastewater treatment over a century. It has been demonstrated that some exopolysaccharide biosynthesis genes and the sigma factor (sigma54 or rpoN) were required for floc forming in some bacteria. However, the mechanism underlying the floc formation stills need to be elucidated.ResultsIn this study, we demonstrate that a TPR (Tetratricopeptide repeats) protein-encoding gene prsT is required for floc formation of Aquincola tertiaricarbonis RN12 and an upstream PEP-CTERM gene (designated pepA), regulated by RpoN1, is involved in its floc formation but not swarming motility and biofilm formation. Overexpression of PepA could rescue the floc-forming phenotype of the rpoN1 mutant by decreasing the released soluble exopolysaccharides and increasing the bound polymers.ConclusionOur results indicate that the wide-spread PEP-CTERM proteins play an important role in the self-flocculation of bacterial cells and may be a component of extracellular polymeric substances required for floc-formation.

Rhizobial migration toward roots mediated by FadL-ExoFQP modulation of extracellular long-chain AHLs

Migration from rhizosphere to rhizoplane is a key selecting process in root microbiome assembly, but not fully understood. Rhizobiales members are overrepresented in the core root microbiome of terrestrial plants, and here we report a genome-wide transposon-sequencing of rhizoplane fitness genes of beneficial Sinorhizobium fredii on wild soybean, cultivated soybean, rice, and maize. There were few genes involved in broad-host-range rhizoplane colonization. The fadL mutant lacking a fatty acid transporter exhibited high colonization rates, while mutations in exoFQP (encoding membrane proteins directing exopolysaccharide polymerization and secretion), but not those in exo genes essential for exopolysaccharide biosynthesis, led to severely impaired colonization rates. This variation was not explainable by their rhizosphere and rhizoplane survivability, and associated biofilm and exopolysaccharide production, but consistent with their migration ability toward rhizoplane, and associated surface motility and the mixture of quorum-sensing AHLs (N-acylated-L-homoserine lactones). Genetics and physiology evidences suggested that FadL mediated long-chain AHL uptake while ExoF mediated the secretion of short-chain AHLs which negatively affected long-chain AHL biosynthesis. The fadL and exoF mutants had elevated and depleted extracellular long-chain AHLs, respectively. A synthetic mixture of long-chain AHLs mimicking that of the fadL mutant can improve rhizobial surface motility. When this AHL mixture was spotted into rhizosphere, the migration toward roots and rhizoplane colonization of S. fredii were enhanced in a diffusible way. This work adds novel parts managing extracellular AHLs, which modulate bacterial migration toward rhizoplane. The FadL-ExoFQP system is conserved in Alphaproteobacteria and may shape the “home life” of diverse keystone rhizobacteria.

Genetic potential for exopolysaccharide synthesis in activated sludge bacteria uncovered by genome-resolved metagenomics

A good floc formation of activated sludge (AS) is crucial for solid-liquid separation and production of clean effluent during wastewater treatment. Floc formation is partly controlled by self-produced extracellular polymeric substances (EPS) such as exopolysaccharides, proteins, and nucleic acids. Little is known about the composition, structure, and function of EPS in AS and which bacteria produce them. To address this knowledge gap for the exopolysaccharides, we took advantage of 1083 high-quality metagenome-assembled genomes (MAGs) obtained from 23 Danish wastewater treatment plants. We investigated the genomic potential for exopolysaccharide biosynthesis in bacterial species typical in AS systems based on genome mining and gene synteny analyses. Putative gene clusters associated with the biosynthesis of alginate, cellulose, curdlan, diutan, hyaluronic acids, Pel, poly-β-1,6-N-acetyl-d-glucosamine (PNAG), Psl, S88 capsular polysaccharide, salecan, succinoglycan, and xanthan were identified and linked to individual MAGs, providing a comprehensive overview of the genome-resolved potential for these exopolysaccharides in AS bacteria. The approach and results provide a starting point for a more comprehensive understanding of EPS composition in wastewater treatment systems, which may facilitate a more refined regulation of the activated sludge process for improved stability.

Genomic potential for exopolysaccharide production and differential polysaccharide degradation in closely related Alteromonas sp. PRIM-21 and Alteromonas fortis 1T.

Members of the genus Alteromonasare widely distributed in diverse marine environments and are often associated with marine organisms. Their ability to produce exopolysaccharides (EPS) and depolymerize sulfated algal polysaccharides has provided industrial importance to some species. Here, we describe the draft genome of an algae-associatedstrain namely, Alteromonassp. PRIM-21 isolated from the southwest coast of India to understand the EPS biosynthetic pathways as well as polysaccharide depolymerization system in comparison to the closely related strainAlteromonas fortis1T that shares 99.8% 16S rRNA gene sequence similarity. Whole-genome shotgun sequencing of Alteromonas sp. PRIM-21 yielded 50 contigs with a total length of 4,638,422bp having 43.86% GC content. The resultant genome shared 95.9% OrthoANI value withA. fortis1T, and contained 4125 predicted protein-coding genes, 71 tRNA and 10 rRNA genes. Genes involved in Wzx/Wzy-, ABC transporter- and synthase-dependent pathways for EPS production and secretion were common in both Alteromonas sp. PRIM-21 andA. fortis1T. However, the distribution of carbohydrate-active enzymes (CAZymes) was heterogeneous. The strain PRIM-21 harbored polysaccharide lyases for the degradation of alginate, ulvan, arabinogalactan and chondroitin. This was further validated from the culture-based assays using seven different polysaccharides. The depolymerizing ability of the bacteria may be useful in deriving nutrients from the biopolymers produced in the algal host while the EPS biosynthesis may provide additional advantages for life in the stressful marine environment. The results also highlight the genetic heterogeneity in terms of polysaccharide utilization among the closely relatedAlteromonasstrains.

Effects of galactosyltransferase on EPS biosynthesis and freeze-drying resistance of Lactobacillus acidophilus NCFM

Galactosyltransferase (GalT) is an important enzyme in synthesizing exopolysaccharide (EPS), the major polymer of biofilms protecting cells from severe conditions. However, the contribution to, and regulatory mechanism of GalT, in stressor resistance are still unclear. Herein, we successfully overexpressed GalT in Lactobacillus acidophilus NCFM by genetic engineering. The GalT activity and freeze-drying survival rate of the recombinant strain were significantly enhanced. The EPS yield also increased by 17.8%, indicating a positive relationship between freeze-drying resistance and EPS. RNA-Seq revealed that GalT could regulate the flux of the membrane transport system, pivotal sugar-related metabolic pathways, and promote quorum sensing to facilitate EPS biosynthesis, which enhanced freeze-drying resistance. The findings concretely prove that the mechanism of GalT regulating EPS biosynthesis plays an important role in protecting lactic acid bacteria from freeze-drying stress.

Metagenomics indicates abundance of biofilm related genes and horizontal transfer of multidrug resistant genes among bacterial communities in nano zinc oxide polluted soil

The unsafe and reckless disposal of metal oxide nanoparticles like ZnO (nZnO) into the soil could seriously impact bacterial behavioural responses and functions. Under such stress, biofilm formation is considered to be a robust mechanism for bacterial survival in soil. We examined the response of bacterial metagenomes in soils exposed to varying levels of Zn (50, 200, 500 and 1000 mg kg−1) as nano Zn oxide (nZnO) in terms of biofilm genesis and regulation and their co-occurrences with multidrug resistance genes (MDRGs) and mobile genetic elements (MGEs). The size-specific effects of nZnO were verified using its bulk counterpart (bZnO). Both nZnO and bZnO facilitated profusion of biofilm related genes (BGs) especially at higher Zn levels (500 and 1000 mg kg−1 Zn), though maximum abundance was registered at a comparatively lower level under nZnO. In general, nZnO favoured an enhancement of genes involved in exopolysaccharide biosynthesis and attachment, while bZnO favoured genes related to capsule formation, chemotaxis and biofilm dispersion. Co-occurrence network analysis revealed significant positive correlations between abundances of BGs, MDRGs and MGEs, indicating an enhanced probability for horizontal gene transfer of MDRGs in nZnO polluted soils.