The effects of exopolymers on cell morphology and culturability of Leuconostoc mesenteroides during starvation

High Levels of CO2 Induce Spoilage by Leuconostoc mesenteroides by Upregulating Dextran Synthesis Genes

During nonventilated storage of carrots, CO2 gradually accumulates to high levels and causes modifications in the carrot's microbiome toward dominance of Lactobacillales and Enterobacteriales The lactic acid bacterium Leuconostoc mesenteroides secretes a slimy exudate over the surface of the carrots. The objective of this study was to characterize the slime components and the potential cause for its secretion under high CO2 levels. A proteomic analysis of the exudate revealed bacterial glucosyltransferases as the main proteins, specifically, dextransucrase. A chemical analysis of the exudate revealed high levels of dextran and several simple sugars. The exudate volume and dextran amount were significantly higher when L. mesenteroides was incubated under high CO2 levels than when incubated in an aerated environment. The treatment of carrot medium plates with commercial dextransucrase or exudate protein extract resulted in similar sugar profiles and dextran production. Transcriptome analysis demonstrated that dextran production is related to the upregulation of the L. mesenteroides dextransucrase-encoding genes dsrD and dsrT during the first 4 to 8 h of exposure to high CO2 levels compared to aerated conditions. A phylogenetic analysis of L. mesenteroides YL48 dsrD revealed a high similarity to other dsr genes harbored by different Leuconostoc species. The ecological benefit of dextran production under elevated CO2 requires further investigation. However, this study implies an overlooked role of CO2 in the physiology and fitness of L. mesenteroides in stored carrots, and perhaps in other food items, during storage under nonventilated conditions.IMPORTANCE The bacterium Leuconostoc mesenteroides is known to cause spoilage of different types of foods by secreting a slimy fluid that damages the quality and appearance of the produce. Here, we identified a potential mechanism by which high levels of CO2 affect the spoilage caused by this bacterium by upregulating dextran synthesis genes. These results have broader implications for the study of the physiology, degradation ability, and potential biotechnological applications of Leuconostoc.

Potential of wheat bran to promote indigenous microbial enhanced oil recovery

Microbial enhanced oil recovery (MEOR) is an emerging oil extraction technology that utilizes microorganisms to facilitate recovery of crude oil in depleted petroleum reservoirs. In the present study, effects of wheat bran utilization were investigated on stimulation of indigenous MEOR. Biostimulation conditions were optimized with the response surface methodology. The co-application of wheat bran with KNO₃ and NH₄H₂PO₄ significantly promoted indigenous MEOR (IMEOR) and exhibited sequential aerobic (O-), facultative (A_n-) and anaerobic (A₀-) metabolic stages. The surface tension of fermented broth decreased by approximately 35%, and the crude oil was highly emulsified. Microbial community structure varied largely among and in different IMEOR metabolic stages. Pseudomonas sp., Citrobacter sp., and uncultured Burkholderia sp. dominated the O-, A_n- and early A₀-stages. Bacillus sp., Achromobacter sp., Rhizobiales sp., Alcaligenes sp. and Clostridium sp. dominated the later A₀-stage. This study illustrated occurrences of microbial community succession driven by wheat bran stimulation and its industrial potential.

Production, properties, and industrial food application of lactic acid bacteria-derived exopolysaccharides

Exopolysaccharides (EPS)-producing lactic acid bacteria (LAB) are industrially important microorganisms in the development of functional food products and are used as starter cultures or coadjutants to develop fermented foods. There is large variability in EPS production by LAB in terms of chemical composition, quantity, molecular size, charge, presence of side chains, and rigidity of the molecules. The main body of the review will cover practical aspects concerning the structural diversity structure of EPS, and their concrete application in food industries is reported in details. To strengthen the food application and process feasibility of LAB EPS at industrial level, a future academic research should be combined with industrial input to understand the technical shortfalls that EPS can address.

Nutrient shielding in clusters of cells

Cellular nutrient consumption is influenced by both the nutrient uptake kinetics of an individual cell and the cells' spatial arrangement. Large cell clusters or colonies have inhibited growth at the cluster's center due to the shielding of nutrients by the cells closer to the surface. We develop an effective medium theory that predicts a thickness ℓ of the outer shell of cells in the cluster that receives enough nutrient to grow. The cells are treated as partially absorbing identical spherical nutrient sinks, and we identify a dimensionless parameter ν that characterizes the absorption strength of each cell. The parameter ν can vary over many orders of magnitude among different cell types, ranging from bacteria and yeast to human tissue. The thickness ℓ decreases with increasing ν, increasing cell volume fraction φ, and decreasing ambient nutrient concentration ψ(∞). The theoretical results are compared with numerical simulations and experiments. In the latter studies, colonies of budding yeast, Saccharomyces cerevisiae, are grown on glucose media and imaged under a confocal microscope. We measure the growth inside the colonies via a fluorescent protein reporter and compare the experimental and theoretical results for the thickness ℓ.

Exopolysaccharide production by a genetically engineered Enterobacter cloacae strain for microbial enhanced oil recovery

Microbial enhanced oil recovery (MEOR) is a petroleum biotechnology for manipulating function and/or structure of microbial environments existing in oil reservoirs for prolonged exploitation of the largest source of energy. In this study, an Enterobacter cloacae which is capable of producing water-insoluble biopolymers at 37°C and a thermophilic Geobacillus strain were used to construct an engineered strain for exopolysaccharide production at higher temperature. The resultant transformants, GW3-3.0, could produce exopolysaccharide up to 8.83 g l(-1) in molasses medium at 54°C. This elevated temperature was within the same temperature range as that for many oil reservoirs. The transformants had stable genetic phenotype which was genetically fingerprinted by RAPD analysis. Core flooding experiments were carried out to ensure effective controlled profile for the simulation of oil recovery. The results have demonstrated that this approach has a promising application potential in MEOR.

Biofilm formation by strains of Leuconostoc citreum and L. mesenteroides

Although biofilms produced by various Leuconostoc sp. are economically important as contaminants of sugar processing plants, very few studies are available on these systems. Twelve strains of Leuconostoc citreum and L. mesenteroides that produce a variety of extracellular glucans were compared for their capacity to produce biofilms. 16s rRNA sequence analysis was used to confirm the species identity of these strains, which included four isolates of L. mesenteroides, five isolates of L. citreum, and three glucansucrase mutants of L. citreum strain NRRL B-1355. Strains identified as L. mesenteroides produce glucans that are generally similar to commercial dextran. Nevertheless, these strains differed widely in their capacity to form biofilms, with densities ranging from 2.7 to 6.1 log cfu/cm(2). L. citreum strains and their derivatives produce a variety of glucans. These strains exhibited biofilm densities ranging from 2.5 to 5.9 log cfu/cm(2). Thus, biofilm-forming capacity varied widely on a strain-specific basis in both species. The types of polysaccharides produced did not appear to affect the ability to form biofilms.

Biofilm formation by exopolysaccharide mutants of Leuconostoc mesenteroides strain NRRL B-1355

Leuconostoc mesenteroides strain NRRL B-1355 produces the soluble exopolysaccharides alternan and dextran in planktonic cultures. Mutants of this strain are available that are deficient in the production of alternan, dextran, or both. Another mutant of NRRL B-1355, strain R1510, produces an insoluble glucan in place of alternan and dextran. To test the effect of exopolysaccharide production on biofilm formation, these strains were cultured in a biofilm reactor. All strains grew well as biofilms, with comparable cell densities, including strain NRRL B-21414, which produces neither alternan nor dextran in planktonic cultures. However, the exopolysaccharide phenotype clearly affected the appearance of the biofilms and the sloughed-off biofilm material produced by these biofilms. For all strains, soluble glucansucrases and soluble polysaccharides produced by biofilm cultures appeared to be similar to those produced by planktonic cultures. Biofilms from all strains also contained insoluble polysaccharides. Strain R1510 biofilms contained an insoluble polysaccharide similar to that produced by planktonic cultures. For most other strains, the insoluble biofilm polysaccharides resembled a mixture of alternan and dextran.

Increased internal and external bacterial load during Drosophila aging without life-span trade-off

The role of microbial load during aging of the adult fruit fly Drosophila melanogaster is incompletely understood. Here we show dramatic increases in aerobic and anaerobic bacterial load during aging, both inside the body and on the surface. Scanning electron microscopy and cell staining analyses of the surface of aged flies detected structures resembling abundant small bacteria and bacterial biofilms. Bacteria cultured from laboratory flies included aerobic species Acetobacter aceti, Acetobacter tropicalis, and Acetobacter pasteurianus and anaerobic species Lactobacillus plantarum and Lactobacillus sp. MR-2; Lactobacillus homohiochii, Lactobacillus fructivorans, and Lactobacillus brevis were identified by DNA sequencing. Reducing bacterial load and antimicrobial peptide gene expression by axenic culture or antibiotics had no effect on life span. We conclude that Drosophila can tolerate a significant bacterial load and mount a large innate immune response without a detectable trade-off with life span; furthermore, microbes do not seem to limit life span under optimized laboratory conditions.

Adoption of the transiently non-culturable state — a bacterial survival strategy?

Microbial culturability can be ephemeral. Cells are not merely either dead or alive but can adopt physiological states in which they appear to be (transiently) non-culturable under conditions in which they are known normally to be able to grow and divide. The reacquisition of culturability from such states is referred to as resuscitation. We here develop the idea that this "transient non-culturability" is a consequence of a special survival strategy, and summarise the morphological, physiological and genetic evidence underpinning such behaviour and its adaptive significance.

Interaction between water flow and spatial distribution of microbial growth in a two-dimensional flow field in saturated porous media

Bacterial growth and its interaction with water flow was investigated in a two-dimensional flow field in a saturated porous medium. A flow cell (56 x 44 x 1 cm) was filled with glass beads and operated under a continuous flow of a mineral medium containing nitrate as electron acceptor. A glucose solution was injected through an injection port, simulating a point source contamination. Visible light transmission was used to observe the distribution of the growing biomass and water flow during the experiment. At the end of the experiment (on day 31), porous medium samples were destructively collected and analyzed for abundance of total and active bacterial cells, bacterial cell volume and concentration of polysaccharides and proteins. Microbial growth was observed in two stripes along the length of the flow cell, starting at the glucose injection port, where highest biomass concentrations were obtained. The spatial distribution of biomass indicated that microbial activity was limited by transverse mixing between glucose and nitrate media, as only in the mixing zone between the media high biological activities were achieved. The ability of the biomass to change the flow pattern in the flow cell was observed, indicating that the biomass was locally reducing the hydraulic conductivity of the porous medium. This bioclogging effect became evident when the injection of the glucose solution was turned off and water flow still bypassed the area around the glucose injection port, preserving the flow pattern as it was during the injection of the glucose solution. As flow bypass was possible in this system, the average hydraulic properties of the flow cell were not affected by the produced biomass. Even in the vicinity of the injection port, the total volume of the bacterial cells remained below 0.01% of the pore space and was unlikely to be responsible for the bioclogging. However, the bacteria produced large amounts of extracellular polymeric substances (EPS), which likely caused the observed bioclogging effects.

Production of insoluble dextran using cell-bound dextransucrase of Leuconostoc mesenteroides NRRL B-523

Water-insoluble, cell-free dextran biosynthesis from Leuconostoc mesenteroides NRRL B-523 has been examined. Cell-bound dextransucrase is used to produce cell-free dextran in a sucrose-rich acetate buffer medium. A comparison between the soluble and insoluble dextrans is made for various sucrose concentrations, and 15% sucrose gave the highest amount of cell-free dextran for a given time. L. mesenteroides B-523 produces more insoluble dextran than soluble dextran. The near cell-free synthesis was validated in a batch reactor, by monitoring the cell growth which is a small (10(6)-10(7) CFU/mL) and constant value throughout the synthesis.

Survival of Campylobacter jejuni in biofilms isolated from chicken houses

Campylobacter jejuni is a thermophilic and microaerophilic enteric pathogen associated with poultry. Biofilms may be a source of C. jejuni in poultry house water systems since they can protect constituent microorganisms from environmental stress. In this study, the viability of C. jejuni in biofilms of gram-positive chicken house isolates (P1, Y1, and W1) and a Pseudomonas sp. was determined using a cultural method (modified brucella agar) and direct viable count (DVC). Two-day biofilms grown on polyvinyl chloride (PVC) coupons in R2A broth at 12 and 23 degrees C were incubated with C. jejuni for a 6-h attachment period. Media were then refreshed every 24 h for 7 days to allow biofilm growth. Two-day biofilms of P1, Y1, and Pseudomonas spp. enhanced attachment (P < 0.01) of C. jejuni (4.74, 4.62, and 4.78 log cells/cm2, respectively) compared to W1 and controls without preexisting biofilm (4.31 and 4.22 log cells/cm2, respectively). On day 7, isolates P1 and Y1 and Pseudomonas biofilms covered 5.4, 7.0, and 21.5% of the surface, respectively, compared to 4.9% by W1. Viable C. jejuni on the surface decreased (P < 0.05) with time, with the greatest reduction occurring on surfaces without a preexisting biofilm. The number of viable C. jejuni determined by DVC was greater than that determined by the cultural method, indicating that C. jejuni may form a viable but nonculturable state within the biofilm. Both DVC and the cultural method indicate that biofilms enhance (P < 0.01) the survival of C. jejuni during incubation at 12 and 23 degrees C over a 7-day period.

Assembly properties and applications of a new exopolymeric compound excreted by Pseudoalteromonas antarctica NF3

The self assembly properties and applications of an exopolymeric compound (EC) of a glycoprotein character excreted by a new gram-negative species, Pseudoalteromonas antarctica NF3, have been reviewed. This compound exhibited surface-active properties in water, with a concentration of 0.20 mg ml(-1) being the key value associated with its physicochemical properties. Unsonicated EC aqueous dispersions showed the coexistence of concentric multilamellar and small unilamellar aggregates by transmission electron microscopy (TEM). Sonication of these dispersions revealed that each lamellae of the initial multilamellar structures were made up of various subunits coiled coils. As for the ability of this exopolymeric biomaterial to coat phosphatidylcholine (PC) liposomes and to protect these vesicles against different surfactants, freeze-fracture TEM micrographs of liposome/EC aggregates revealed that the addition of the EC to liposomes led to the formation of a film (polymer adsorbed onto the bilayers) that coated very well the PC bilayers. The complete coating was already achieved at a PC:EC weight ratio of about 9:1. An increasing resistance of PC liposomes to surfactants (in particular sodium dodecyl sulfate) occurred as the proportion of EC in the system rose, although this effect was more effective at low EC proportions (PC:EC weight ratios from 9:1 to 8:2). Although a direct dependence was found between the growth of the enveloping structure and the resistance of the coated liposomes to be affected by the surfactants, the best protection occurred when this structure was a thin film of about 20-25 nm formed by nine to ten layers of about 2-3 nm.

Biomass evolution in porous media and its effects on permeability under starvation conditions

The purpose of this study was to understand bacteria profile modification and its applications in subsurface biological operations such as biobarrier formation, in situ bioremediation, and microbial-enhanced oil recovery. Biomass accumulation and evolution in porous media were investigated both experimentally and theoretically. To study both nutrient-rich and carbon-source-depleted conditions, Leuconostoc mesenteroides was chosen because of its rapid growth rate and exopolymer production rate. Porous micromodels were used to study the effects of biomass evolution on the permeability of a porous medium. Bacterial starvation was initiated by switching the feed from a nutrient solution to a buffer solution in order to examine biofilm stability under nutrient-poor conditions. Four different evolution patterns were identified during the nutrient-rich and nutrient-depleted conditions used in the micromodel experiments. In phase I, the permeability of the porous micromodel decreased as a result of biomass accumulation in pore bodies and pore throats. In phase II, starvation conditions were initiated. The depletion of nutrient in the phase II resulted in slower growth of the biofilm causing the permeability to reach a minimum as all the remaining nutrients were consumed. In phase III, permeability began to increase due to biofilm sloughing caused by shear stress. In phase IV, shear stress remained below the critical shear stress for sloughing and the biofilm remained stable for long periods of time during starvation. The critical shear stress for biofilm sloughing provided an indication of biofilm strength. Shear removal of biofilms occurred when shear stress exceeded critical shear stress. A network model was used to describe the biofilm formation phenomenon and the existence of a critical shear stress. Simulations were in qualitative agreement with the experimental results, and demonstrate the existence of a critical shear stress.