Bacterial lifestyle shapes pangenomes

Pangenomes vary across bacteria. Some species have fluid pangenomes, with a high proportion of genes varying between individual genomes. Other species have less fluid pangenomes, with different genomes tending to contain the same genes. Two main hypotheses have been suggested to explain this variation: differences in species' bacterial lifestyle and effective population size. However, previous studies have not been able to test between these hypotheses because the different features of lifestyle and effective population size are highly correlated with each other, and phylogenetically conserved, making it hard to disentangle their relative importance. We used phylogeny-based analyses, across 126 bacterial species, to tease apart the causal role of different factors. We found that pangenome fluidity was lower in i) host-associated compared with free-living species and ii) host-associated species that are obligately dependent on a host, live inside cells, and are more pathogenic and less motile. In contrast, we found no support for the competing hypothesis that larger effective population sizes lead to more fluid pangenomes. Effective population size appears to correlate with pangenome variation because it is also driven by bacterial lifestyle, rather than because of a causal relationship.

Long-term evolution of antibiotic tolerance in Pseudomonas aeruginosa lung infections

Pathogenic bacteria respond to antibiotic pressure with the evolution of resistance but survival can also depend on their ability to tolerate antibiotic treatment, known as tolerance. While a variety of resistance mechanisms and underlying genetics are well characterized in vitro and in vivo, an understanding of the evolution of tolerance, and how it interacts with resistance in situ is lacking. We assayed for tolerance and resistance in isolates of Pseudomonas aeruginosa from chronic cystic fibrosis lung infections spanning up to 40 years of evolution, with 3 clinically relevant antibiotics: meropenem, ciprofloxacin, and tobramycin. We present evidence that tolerance is under positive selection in the lung and that it can act as an evolutionary stepping stone to resistance. However, by examining evolutionary patterns across multiple patients in different clone types, a key result is that the potential for an association between the evolution of resistance and tolerance is not inevitable, and difficult to predict.

SOCfinder: a genomic tool for identifying social genes in bacteria

Bacteria cooperate by working collaboratively to defend their colonies, share nutrients, and resist antibiotics. Nevertheless, our understanding of these remarkable behaviours primarily comes from studying a few well-characterized species. Consequently, there is a significant gap in our understanding of microbial social traits, particularly in natural environments. To address this gap, we can use bioinformatic tools to identify genes that control cooperative or otherwise social traits. Existing tools address this challenge through two approaches. One approach is to identify genes that encode extracellular proteins, which can provide benefits to neighbouring cells. An alternative approach is to predict gene function using annotation tools. However, these tools have several limitations. Not all extracellular proteins are cooperative, and not all cooperative behaviours are controlled by extracellular proteins. Furthermore, existing functional annotation methods frequently miss known cooperative genes. We introduce SOCfinder as a new tool to find bacterial genes that control cooperative or otherwise social traits. SOCfinder combines information from several methods, considering if a gene is likely to [1] code for an extracellular protein [2], have a cooperative functional annotation, or [3] be part of the biosynthesis of a cooperative secondary metabolite. We use data on two extensively-studied species (P. aeruginosa and B. subtilis) to show that SOCfinder is better at finding known cooperative genes than existing tools. We also use theory from population genetics to identify a signature of kin selection in SOCfinder cooperative genes, which is lacking in genes identified by existing tools. SOCfinder opens up a number of exciting directions for future research, and is available to download from https://github.com/lauriebelch/SOCfinder.

Signatures of kin selection in a natural population of the bacteria Bacillus subtilis

Laboratory experiments have suggested that bacteria perform a range of cooperative behaviors, which are favored because they are directed toward relatives (kin selection). However, there is a lack of evidence for cooperation and kin selection in natural bacterial populations. Molecular population genetics offers a promising method to study natural populations because the theory predicts that kin selection will lead to relaxed selection, which will result in increased polymorphism and divergence at cooperative genes. Examining a natural population of Bacillus subtilis, we found consistent evidence that putatively cooperative traits have higher polymorphism and greater divergence than putatively private traits expressed at the same rate. In addition, we were able to eliminate alternative explanations for these patterns and found more deleterious mutations in genes controlling putatively cooperative traits. Overall, our results suggest that cooperation is favored by kin selection, with an average relatedness of r = .79 between interacting individuals.

Gene transferability and sociality do not correlate with gene connectivity

The connectivity of a gene, defined as the number of interactions a gene's product has with other genes' products, is a key characteristic of a gene. In prokaryotes, the complexity hypothesis predicts that genes which undergo more frequent horizontal transfer will be less connected than genes which are only very rarely transferred. We tested the role of horizontal gene transfer, and other potentially important factors, by examining the connectivity of chromosomal and plasmid genes, across 134 diverse prokaryotic species. We found that (i) genes on plasmids were less connected than genes on chromosomes; (ii) connectivity of plasmid genes was not correlated with plasmid mobility; and (iii) the sociality of genes (cooperative or private) was not correlated with gene connectivity.

The role of staphylococcal bacteriocins in nasal competition

Bacteriocins are antimicrobial toxins produced by bacteria to defend and invade territories by killing unrelated strains and species. Understanding if bacteriocins shape natural populations is important for understanding the evolution of antimicrobial resistance and identifying novel antimicrobials for clinical use. Staphylococcus aureus is an opportunistic pathogen that asymptomatically colonises the nasal cavity of 1 in 3 healthy adults. S. aureus is known to produce many different bacteriocins, however we are yet to understand the extent to which they mediate the establishment of nasal populations. Here, we test the importance of bacteriocins in driving colonisation success, by screening S. aureus antimicrobial inhibition against otherS. aureus strains and three commensal species that commonly co-inhabit the nasal cavity. We use a longitudinally sampled collection of 173S. aureusnasal isolates from 14 participants over 90-months to track within-individual population changes over time. We found that 8% of allS. aureus isolates produced bacteriocins active against other species, but that between-strain bacteriocin inhibition inS. aureus was very rare, observed by only 0.5% of isolates. Therefore, while there is no evidence that intraspecific competition drives colonisation success in the nasal cavity, interspecific competition is more likely to influence strain and species dominance.

The evolution of cheating in viruses

The success of many viruses depends upon cooperative interactions between viral genomes. However, whenever cooperation occurs, there is the potential for 'cheats' to exploit that cooperation. We suggest that: (1) the biology of viruses makes viral cooperation particularly susceptible to cheating; (2) cheats are common across a wide range of viruses, including viral entities that are already well studied, such as defective interfering genomes, and satellite viruses. Consequently, the evolutionary theory of cheating could help us understand and manipulate viral dynamics, while viruses also offer new opportunities to study the evolution of cheating.

Changes in toxin production of environmental Pseudomonas aeruginosa isolates exposed to sub-inhibitory concentrations of three common antibiotics

Pseudomonas aeruginosa is an environmental pathogen that can cause severe infections in immunocompromised patients. P. aeruginosa infections are typically treated with multiple antibiotics including tobramycin, ciprofloxacin, and meropenem. However, antibiotics do not always entirely clear the bacteria from the infection site, where they may remain virulent. This is because the effective antibiotic concentration and diffusion in vitro may differ from the in vivo environment in patients. Therefore, it is important to understand the effect of non-lethal sub-inhibitory antibiotic concentrations on bacterial phenotype. Here, we investigate if sub-inhibitory antimicrobial concentrations cause alterations in bacterial virulence factor production using pyocyanin as a model toxin. We tested this using the aforementioned antibiotics on 10 environmental P. aeruginosa strains. Using on-the-spot electrochemical screening, we were able to directly quantify changes in production of pyocyanin in a measurement time of 17 seconds. Upon selecting 3 representative strains to further test the effects of sub-minimum inhibitory concentration (MICs), we found that pyocyanin production changed significantly when the bacteria were exposed to 10-fold MIC of the 3 antibiotics tested, and this was strain specific. A series of biologically relevant measured pyocyanin concentrations were also used to assess the effects of increased virulence on a culture of epithelial cells. We found a decreased viability of the epithelial cells when incubated with biologically relevant pyocyanin concentrations. This suggests that the antibiotic-induced virulence also is a value worth being enclosed in regular testing of pathogens.

A comparative approach to understanding selection on extracellular secretion

Many bacterial genes encode proteins that are secreted extracellularly. These proteins can be considered cooperative because all surrounding cells can benefit from their production. Therefore, it has been hypothesized that these cooperative genes would more frequently lie on mobile elements, such as plasmids, which can transfer to other cells. This could stabilise cooperation, leading to the prediction that plasmids should carry proportionally more cooperative genes than the less mobile chromosome. However, it is unknown whether this prediction holds across the bacterial tree of life. To address this, we analysed the gene content of the chromosome and plasmid(s) of 1620 genomes comprising 51 diverse bacterial species. We find that across species analysed, plasmids do not carry proportionally more cooperative genes than the chromosome. Contrary to prediction, the role of mobile elements in promoting cooperative behaviour is highly variable across bacterial species.

Privatisation rescues function following loss of cooperation

A single cheating mutant can lead to the invasion and eventual eradication of cooperation from a population. Consequently, cheat invasion is often considered equal to extinction in empirical and theoretical studies of cooperator-cheat dynamics. But does cheat invasion necessarily equate extinction in nature? By following the social dynamics of iron metabolism in Pseudomonas aeruginosa during cystic fibrosis lung infection, we observed that individuals evolved to replace cooperation with a ‘private’ behaviour. Phenotypic assays showed that cooperative iron acquisition frequently was upregulated early in infection, which, however, increased the risk of cheat invasion. With whole-genome sequencing we showed that if, and only if, cooperative iron acquisition is lost from the population, a private system was upregulated. The benefit of upregulation depended on iron availability. These findings highlight the importance of social dynamics of natural populations and emphasizes the potential impact of past social interaction on the evolution of private traits.

Pleiotropy, cooperation, and the social evolution of genetic architecture

Pleiotropy has been suggested as a novel mechanism for stabilising cooperation in bacteria and other microbes. The hypothesis is that linking cooperation with a trait that provides a personal (private) benefit can outweigh the cost of cooperation in situations when cooperation would not be favoured by mechanisms such as kin selection. We analysed the theoretical plausibility of this hypothesis, with analytical models and individual-based simulations. We found that (1) pleiotropy does not stabilise cooperation, unless the cooperative and private traits are linked via a genetic architecture that cannot evolve (mutational constraint); (2) if the genetic architecture is constrained in this way, then pleiotropy favours any type of trait and not especially cooperation; (3) if the genetic architecture can evolve, then pleiotropy does not favour cooperation; and (4) there are several alternative explanations for why traits may be linked, and causality can even be predicted in the opposite direction, with cooperation favouring pleiotropy. Our results suggest that pleiotropy could only explain cooperation under restrictive conditions and instead show how social evolution can shape the genetic architecture.

The structural and the photoelectrochemical properties of ZnO–ZnS/ITO 1D hetero-junctions prepared by tandem electrodeposition and surface sulfidation: on the material processing limits

ZnO nanorods were electrodeposited on ITO and immersed in a Na₂S solution for a variable time. As a function of this experimental parameter, different ZnS surface growth mechanisms take place, leading to specific microstructures.

Diversity, Prevalence, and Longitudinal Occurrence of Type II Toxin-Antitoxin Systems of Pseudomonas aeruginosa Infecting Cystic Fibrosis Lungs

Type II toxin-antitoxin (TA) systems are most commonly composed of two genes encoding a stable toxin, which harms the cell, and an unstable antitoxin that can inactivate it. TA systems were initially characterized as selfish elements, but have recently gained attention for regulating general stress responses responsible for pathogen virulence, formation of drug-tolerant persister cells and biofilms—all implicated in causing recalcitrant chronic infections. We use a bioinformatics approach to explore the distribution and evolution of type II TA loci of the opportunistic pathogen, Pseudomonas aeruginosa, across longitudinally sampled isolates from cystic fibrosis lungs. We identify their location in the genome, mutations, and gain/loss during infection to elucidate their function(s) in stabilizing selfish elements and pathogenesis. We found (1) 26 distinct TA systems, where all isolates harbor four in their core genome and a variable number of the remaining 22 on genomic islands; (2) limited mutations in core genome TA loci, suggesting they are not under negative selection; (3) no evidence for horizontal transmission of elements with TA systems between clone types within patients, despite their ability to mobilize; (4) no gain and limited loss of TA-bearing genomic islands, and of those elements partially lost, the remnant regions carry the TA systems supporting their role in genomic stabilization; (5) no significant correlation between frequency of TA systems and strain ability to establish as chronic infection, but those with a particular TA, are more successful in establishing a chronic infection.

Sociomics: Using Omic Approaches to Understand Social Evolution

All of life is social, from genes cooperating to form organisms, to animals cooperating to form societies. Omic approaches offer exceptional opportunities to solve major outstanding problems in the study of how sociality evolves. First, omics can be used to clarify the extent and form of sociality in natural populations. This is especially useful in species where it is difficult to study social traits in natural populations, such as bacteria and other microbes. Second, omics can be used to examine the consequences of sociality for genome evolution and gene expression. This is especially useful in cases where there is clear variation in the level of sociality, such as the social insects. Major tasks for the future are to apply these approaches to a wider range of non-model organisms, and to move from exploratory analyses to the testing of evolutionary theory.

Bacteriocin-mediated competition in cystic fibrosis lung infections

Bacteriocins are toxins produced by bacteria to kill competitors of the same species. Theory and laboratory experiments suggest that bacteriocin production and immunity play a key role in the competitive dynamics of bacterial strains. The extent to which this is the case in natural populations,especially human pathogens, remains to be tested. We examined the role of bacteriocins in competition using Pseudomonas aeruginosa strains infecting lungs of humans with cystic fibrosis (CF). We assessed the ability of different strains to kill each other using phenotypic assays, and sequenced their genomes to determine what bacteriocins (pyocins) they carry. We found that(i) isolates from later infection stages inhibited earlier infecting strains less,but were more inhibited by pyocins produced by earlier infecting strains and carried fewer pyocin types; (ii) this difference between early and late infections appears to be caused by a difference in pyocin diversity between competing genotypes and not by loss of pyocin genes within a lineage overtime; (iii) pyocin inhibition does not explain why certain strains outcompete others within lung infections; (iv) strains frequently carry the pyocin-killing gene, but not the immunity gene, suggesting resistance occurs via other unknown mechanisms. Our results show that, in contrast to patterns observed in experimental studies, pyocin production does not appear to have a major influence on strain competition during CF lung infections.

Photoelectrochemical properties of nanocrystalline ZnS discrete versus continuous coating of ZnO nanorods prepared by electrodeposition

The photoelectrochemical properties of nanocrystalline ZnS discrete versus continuous coating of ZnO nanorods prepared by electrodeposition were investigated.

Isolation and characterization of plant growth promoting traits of a rhizobacteria: Pantoea agglomerans lma2

The use of microbial technology in agriculture is expanding quickly with the identification of new bacterial strains which are more effective in promoting the growth of plants. The rhizobacteria that promote the growth of plants can have a positive effect on the productivity of crops especially when subjected to salt stress. A nitrogen-fixing bacterium was isolated from the wheat rhizosphere of an arid region. The strain was identified on the basis of tests API20E and 16S rRNA sequencing, as Pantoea agglomerans lma2. This strain degraded several carbon sources: sugars (fructose, ribose, dextrin, salicin...), lipids (lecithin, tributyrin and tween 80), proteins (gelatin, casein), grew on KCN and could grow from pH 4 to 8 and had an optimum at pH 7. The growth temperature showed a maximum at 30 degrees C and the bacteria could tolerate from 4 to 41 degrees C and the growth rate was higher when the NaCl concentration was between 100 and 300 mM. The performance of activities enhancing the growth of plants of P. agglomerans lma2 was significantly better in the presence of salt. Rates of Indole Acetic Acid (IAA), siderophores production and solubilization of phosphate increased between 100 and 400 mM NaCl compared to the control without salt. The maximum values were saved to 300 mM for the production of siderophores (18.32%) and solubilization of phosphate (1061.49 microg mL(-1)) and 100 mM for the production of IAA (161 microg mL(-1)). A significant correlation existed between these three activities. These results showed that P. agglomerans lma2 with its Plant Growth Promoting Rhizobacteria (PGPR) and halophilic properties could constitute a good fertilizer in arid and saline zone.

Toward an evolutionary definition of cheating

The term "cheating" is used in the evolutionary and ecological literature to describe a wide range of exploitative or deceitful traits. Although many find this a useful short hand, others have suggested that it implies cognitive intent in a misleading way, and is used inconsistently. We provide a formal justification of the use of the term "cheat" from the perspective of an individual as a maximizing agent. We provide a definition for cheating that can be applied widely, and show that cheats can be broadly classified on the basis of four distinctions: (i) whether cooperation is an option; (ii) whether deception is involved; (iii) whether members of the same or different species are cheated; and (iv) whether the cheat is facultative or obligate. Our formal definition and classification provide a framework that allow us to resolve and clarify a number of issues, regarding the detection and evolutionary consequences of cheating, as well as illuminating common principles and similarities in the underlying selection pressures.

Pseudomonas aeruginosa adaptation to lungs of cystic fibrosis patients leads to lowered resistance to phage and protist enemies

Pathogenic life styles can lead to highly specialized interactions with host species, potentially resulting in fitness trade-offs in other ecological contexts. Here we studied how adaptation of the environmentally transmitted bacterial pathogen, Pseudomonas aeruginosa, to cystic fibrosis (CF) patients affects its survival in the presence of natural phage (14/1, ΦKZ, PNM and PT7) and protist (Tetrahymena thermophila and Acanthamoebae polyphaga) enemies. We found that most of the bacteria isolated from relatively recently intermittently colonised patients (1–25 months), were innately phage-resistant and highly toxic for protists. In contrast, bacteria isolated from long time chronically infected patients (2–23 years), were less efficient in both resisting phages and killing protists. Moreover, chronic isolates showed reduced killing of wax moth larvae (Galleria mellonella) probably due to weaker in vitro growth and protease expression. These results suggest that P. aeruginosa long-term adaptation to CF-lungs could trade off with its survival in aquatic environmental reservoirs in the presence of microbial enemies, while lowered virulence could reduce pathogen opportunities to infect insect vectors; factors that are both likely to result in poorer environmental transmission. From an applied perspective, phage therapy could be useful against chronic P. aeruginosa lung infections that are often characterized by multidrug resistance: chronic isolates were least resistant to phages and their poor growth will likely slow down the emergence of beneficial resistance mutations.

Cytotoxic, genotoxic and antigenotoxic potencies of oligorutins

Rutin has been enzymatically oligomerized by laccase from Trametes versicolor. Five fractions of oligomers were obtained from the monomers having high solubility in water, which can reach 351-times that of rutin. Cytotoxicity of rutin and oligorutin fractions was evaluated towards K562 cells. Oligorutin fractions showed a lower antiproliferative effect compared with its monomer. The genotoxic potential of rutin and oligorutin fractions was assessed, at the limit of the solubility of each molecule, using the comet test. None of the tested concentrations of either rutin or oligorutin fractions has showed a genotoxic effect. Similarly, the antigenotoxic effect of these flavonoids was tested using the same assay. The obtained results showed a higher ability of oligorutin fractions to reduce the genotoxicity induced by hydrogen peroxide compared with monomeric rutin.

Effect of Azotobacter vinelandii and compatible solutes on germination wheat seeds and root concentrations of sodium and potassium under salt stress

The effect of plant growth-promoting Rhizobacteria (PGPR) and exogenous application of compatible solutes on seed germination and root concentrations of sodium and potassium of two wheat varieties (Triticum durum L.) were evaluated under saline stress. In this experiment, Azotobacter vinelandii strain DSM85, glycine betaine and proline were used. Inoculated seeds for each variety were placed on Whatman paper in 9 cm Petri dishes containing 15 mL of distilled water or NaCl solutions at various concentrations (control, 100, 200, 300 mM) supplemented with or without glycine betaine (GB) or proline at 5 mM. The results indicated that addition of proline (5 mM) stimulated the production of indol acetic acid and the growth of A. vinelandii at 200 and 300 mM NaCl, respectively. The germination rate index and the germination final percentage decreased significantly (p < 0.05) with increasing salinity level. The germination was significantly diminished at 300 mM with significant variation among varieties and Waha variety had higher germination percentage than Bousselam variety. Inoculation of seeds by A. vinelandii and exogenous application of proline had significantly positive effect on the germination at this concentration of NaCl. The rate of accumulation of Na+ in roots was important at 100 mM and increased at 200 mM. The concentration of K+ decreased when salinity increased. The effect of inoculation or inoculation with proline decreased the accumulation of Na' and reduced the loss of K+ under salt stress. From the present study we can conclude that the use of A. vinelandii strain DSM85 and external application of low concentrations of proline on seeds might be considered as a strategy for the protection of plants under saline stress.

Parental consanguinity and family history of coronary artery disease strongly predict early stenosis

BACKGROUND

Coronary artery disease (CAD) is a multifactorial disease with acquired and inherited components.

AIM

We investigated the roles of family history and consanguinity on CAD risk and age at diagnosis in 4284 patients. The compounded impact of diabetes, hyperlipidemia, hypertension, smoking, and BMI, which are known CAD risk factors, on CAD risk and age at diagnosis was also explored.

METHODS

CAD was determined by cardiac catheterization. Logistic regression and stratification were performed to determine the impact of family history and consanguinity on risk and onset of CAD, controlling for diabetes, hyperlipidemia, hypertension, smoking, and BMI.

RESULTS

Family history of CAD and gender significantly increased the risk for young age at diagnosis of CAD (p<0.001). Consanguinity did not promote risk of CAD (p=0.38), but did affect age of disease diagnosis (p<0.001). The mean age at disease diagnosis was lowest, 54.8 years, when both family history of CAD and consanguinity were considered as unique risk factors for CAD, compared to 62.8 years for the no-risk-factor patient category (p<0.001).

CONCLUSIONS

Family history of CAD and smoking are strongly associated with young age at diagnosis. Furthermore, parental consanguinity in the presence of family history lowers the age of disease diagnosis significantly for CAD, emphasizing the role of strong genetic and cultural CAD modifiers. These findings highlight the increased role of genetic determinants of CAD in some population subgroups, and suggest that populations and family structure influence genetic heterogeneity between patients with CAD.

Phase behavior and rheological properties of enzymatically synthesized trehalose decanoate aqueous solutions

Surface tension properties of an enzymatically synthesized equimolar mixture of trehalose mono- and didecanoate in aqueous solutions have been determined. At 20 degrees C a critical micellar concentration (CMC) of 50 micromol/l and a minimal surface tension of 28 mN/m have been obtained. Above the CMC, it has been shown that up to a concentration of 42 wt%, and in a 20-60 degrees C temperature range the sugar ester aqueous solutions do not form any crystalline structure, nor present any phase transition, and the trehalose decanoate molecules form an isotropic worm-like micellar phase. The rheological properties indicate however a more complicated picture in the same concentration and temperature ranges. In steady shear, the viscosity of the trehalose decanoate solutions do not exhibit any shear rate dependence from 1 to 100 s(-1) for concentrations up to 42 wt%. Below 0.8 wt%, the viscosity remains constant and close to that of water; then, between 0.8 and 23 wt%, the viscosity shows a quadratic increase with surfactant concentration. For higher concentrations, up to 42 wt%, no further significant increase in viscosity is observed. In oscillatory shear experiments, the solutions exhibit viscoelastic properties. The observed rheological behavior as a function of concentration and temperature may be due to a progressive evolution of the trehalose decanoate molecular associations: as the concentration increases, the system evolves towards an entangled and/or partially branched or cross-linked micellar network, and eventually a multiconnected network of cross-linked micelles.

Effect of acyl donor chain length and substitutions pattern on the enzymatic acylation of flavonoids

Rutin and esculin were enzymatically acylated with different aliphatic acids as acyl donors (fatty acids, dicarboxylic acids and omega-substituted fatty acids) by an immobilized lipase from Candida antarctica. The effect of the water content and the acyl donors pattern on the flavonoid initial acylation rate and conversion yield were investigated. The obtained results indicated that the water content of the medium has a strong effect on the performance of these reactions. The best conversion yields were reached when the water content was kept lower than 200 ppm. At low water content of the medium, these syntheses are influenced by carbon chain length and substitution pattern of the acyl donors. Higher conversion yields of esculin and rutin (>70%) were obtained with aliphatic acids having high carbon chain length (>12). Moreover, it has been found that the amine and thiol groups on omega-substituted fatty acid chain were unfavourable to these reactions. The 1H NMR and 13C NMR analyses of some synthesized esters (esculin and rutin palmitate) show that only monoesters were produced and that the esterification takes place on the primary OH of glucose moiety of the esculin and on the secondary 4"'-OH of the rhamnose residue of rutin.

Uptake of heavy metals from synthetic aqueous solutions using modified PEI-silica gels

The decontamination of synthetic Pb(II), Zn(II), Cd(II), Ni(II) solutions was investigated, using silica gels chemically modified with poly(ethyleneimine) (PEI) as sorbents. Two families of sorbents, i.e. silica/PEI and crosslinked silica/PEI, were prepared and characterized. Then the removal of metal ions from synthetic aqueous solutions was studied by static tests. They revealed that the sorption capacities depend on the pH, the initial concentration and to some extent on the nature of the metal. The recovery of the metal cations from the saturated sorbents was possible with diluted acid, only for the crosslinked supports. In this case, the regeneration and reuse without sorption decrease, was demonstrated. The presence of other cations (as Na(+), Ca(2+)) and metals does not affect the sorption capacities.

Study of the effects of temperature, pH and yeast extract on growth and exopolysaccharides production by Propionibacterium acidi-propionici on milk microfiltrate using a response surface methodology

AIMS

To study the effects of temperature, pH and yeast extract (YE) concentration on growth and exopolysaccharide (EPS) production by Propionibacterium acidi-propionici DSM 4900 cultivated on milk microfiltrate.

METHODS AND RESULTS

A multifactorial approach using a Response Surface Methodology (RSM) was followed. The results indicated that both growth, and EPS and organic acids production, were influenced by pH, temperature and YE concentration. Biomass and organic acids production occurred in all the tested domains, whereas EPS production was only possible in a narrow pH range (5.3-6.5). The results clearly showed that the optimal conditions for EPS production were different to those for optimal growth. The effect of YE on EPS production was not only due to an increase in growth but also to a direct effect on the production of EPS. The temperature played a major role. A decrease of temperature induced a slowing down of both growth and organic acids production, making the essential factors of the medium and the precursors of EPS biosynthesis more available and hence, leading to an increase in EPS production.

CONCLUSION

The effects of pH, temperature and YE were determined, allowing the definition of favourable, though non-optimal, conditions for EPS production: 23 degrees C, pH 6 and 3 g l(-1) YE concentration.

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of a multifactorial approach for investigating the effect of fermentation conditions on EPS production has been demonstrated.

Exopolysaccharide production by Propionibacterium acidi-propionici on milk microfiltrate

AIMS

The aims of this work were to evaluate growth and exopolysaccharide (EPS) production properties of Propionibacterium acidi-propionici DSM 4900 on milk permeate.

METHODS AND RESULTS

Anaerobic growth on milk permeate was only possible if supplemented with yeast extract (YE). Fermentation capacities of the strain were significantly improved by further increasing the supplemented YE. At 5 g l(-1) YE, consumption of 45 g l(-1) lactose to produce 9 g l(-1) biomass, 34 g l(-1) organic acids and 0.65 g l(-1) EPS was observed. From a kinetic point of view, EPS production occurred during the bacteria growth phase. At the excreted polysaccharide level, the medium showed shear-thinning behaviour with a relatively high apparent viscosity of up to 30 mPa.s (milli.Pascal.second) at a shear rate of 17 s(-1).

CONCLUSION

EPS production by P. acidi-propionici DSM 4900 on milk permeate showed promising rheological behaviour of the milk-derived medium obtained, even at a low production level.

SIGNIFICANCE AND IMPACT OF THE STUDY

A kinetic study on EPS production by a food-grade bacterium that could be used in situ in alimentation was carried out.