Shorter Lag Phase Research Articles

Presence of galactose in precultures induces lacS and leads to short lag phase in lactose-grown Lactococcus lactis cultures

Lactose conversion by lactic acid bacteria is of high industrial relevance and consistent starter culture quality is of outmost importance. We observed that Lactococcus lactis using the high-affinity lactose-phosphotransferase system excreted galactose towards the end of the lactose consumption phase. The excreted galactose was re-consumed after lactose depletion. The lacS gene, known to encode a lactose permease with affinity for galactose, a putative galactose–lactose antiporter, was upregulated under the conditions studied. When transferring cells from anaerobic to respiration-permissive conditions, lactose-assimilating strains exhibited a long and non-reproducible lag phase. Through systematic preculture experiments, the presence of galactose in the precultures was correlated to short and reproducible lag phases in respiration-permissive main cultivations. For starter culture production, the presence of galactose during propagation of dairy strains can provide a physiological marker for short culture lag phase in lactose-grown cultures.

Effects of nanobubble water on the growth of Lactobacillus acidophilus 1028 and its lactic acid production.

Nanobubble water (NBW) has been applied in various fields due to the unique properties of nanobubbles (NBs) including long-term stability, negative zeta potential and generation of free radicals. In this study, the performance of four kinds of NBW from different gases (air, N2, H2, and CO2) in addition to deionized water (DW) were investigated and compared in terms of the growth of the probiotic Lactobacillus acidophilus 1028. The NB density, size distribution, zeta potential, pH and dissolved oxygen (DO) of the NBW were firstly investigated. Results indicate that N2-NBW had the highest absolute value of zeta potential and NB density (−25.3 ± 5.43 mV and 5.73 ± 1.0 × 107 particles per mL, respectively), while the lowest was detected in CO2-NBW (−6.96 ± 2.36 mV and 3.39 ± 1.73 × 107 particles per mL, respectively). With the exception of CO2-NBW, all the other types of NBW showed promotion effects on the growth of the strain at the lag and logarithmic phases. Among them, N2-NBW demonstrated the best performance, achieving the highest increase ratio of 51.1% after 6 h cultivation. The kinetic models (Logistic and Gompertz) indicate that the culture with N2-NBW had the shortest lag phase and the maximum specific growth rate when compared to the H2-NBW and DW groups under the same cultivation conditions. Preliminary analysis on the mechanisms suggested that these effects were related to the properties (zeta potential and density) of the NBs, which might affect the transport of substances. This study suggests that NBW has the potential for promoting the production efficiency of probiotics via fermentation.

Adaptive Evolution of Escherichia coli K-12 MG1655 Grown on Ethanol and Glycerol

The FEt and FGl derivatives of the E. coli K-12 MG1655 and YA1461 strains with improved ethanol and glycerol utilization, respectively, under aerobic conditions were obtained by laboratory adaptive evolution. These fitness strains have a shorter lag-phase and higher growth rate on glucose, xylose, and their mixture as compared with the parental strains. DNA sequencing and cross-transduction analysis showed that the change in the growth rate of the rifampicin-resistant fitness mutants FGl and YA1461 RifRII are caused by mutations in the rpoB gene encoding the RNA polymerase β-subunit. These mutations affect conservative amino acid segments, which leads to the amino acid substitutions of RpoBD483G and RpoBH526N, respectively.

Development of Freeze-Thaw Tolerant Lactobacillus rhamnosus GG by Adaptive Laboratory Evolution.

The industrial application of microorganisms as starters or probiotics requires their preservation to assure viability and metabolic activity. Freezing is routinely used for this purpose, but the cold damage caused by ice crystal formation may result in severe decrease in microbial activity. In this study, adaptive laboratory evolution (ALE) technique was applied to a lactic acid bacterium to select tolerant strains against freezing and thawing stresses. Lactobacillus rhamnosus GG was subjected to freeze-thaw-growth (FTG) for 150 cycles with four replicates. After 150 cycles, FTG-evolved mutants showed improved fitness (survival rates), faster growth rate, and shortened lag phase than those of the ancestor. Genome sequencing analysis of two evolved mutants showed genetic variants at distant loci in six genes and one intergenic space. Loss-of-function mutations were thought to alter the structure of the microbial cell membrane (one insertion in cls), peptidoglycan (two missense mutations in dacA and murQ), and capsular polysaccharides (one missense mutation in wze), resulting in an increase in cellular fluidity. Consequently, L. rhamnosus GG was successfully evolved into stress-tolerant mutants using FTG-ALE in a concerted mode at distal loci of DNA. This study reports for the first time the functioning of dacA and murQ in freeze-thaw sensitivity of cells and demonstrates that simple treatment of ALE designed appropriately can lead to an intelligent genetic changes at multiple target genes in the host microbial cell.

Transcriptome profiling and identification of the functional genes involved in berry development and ripening in Vitis vinifera

The length of berry lag phase determines the overall time needed for grape berries to get mature, but the functional gene networks in this phase have not been well documented. In order to reveal the origin of the somatic variation and regulation mechanism of grape berry development and ripening, an early ripening mutant of Vitis vinifera with a shorter lag phase was used for transcriptome profiling. The RNA-seq results revealed that 2021 and 2470 genes were up- and down-regulated, respectively, in the early ripening mutant compared to the wild type. The GO and KEGG enrichment analysis indicated that the up-regulated genes belonged to several pathways and metabolisms, among which the most significant constituents were for biosynthesis of secondary metabolites and flavonoid biosynthesis. The down-regulated genes were involved in biosynthesis of secondary metabolites, plant hormone signal transduction, and photosynthesis. Many transcription factors including WRKYs, AP2-EREBPs, and MYBs were also differentially expressed, suggesting their regulatory roles in berry development and ripening. The transcriptomic comparisons suggested that the prominent up-regulation of an Arabidopsis SnRK3.23, CIPK23 or PKS17 homolog could have driven the early ripening phenotype in the mutant by activating the downstream VvABF2 transcription factor in the ABA signaling. At the same time, ethylene and auxin were also involved in this process. As a result, the major ripening related genes, e.g., MYBA1, MYBA2, VvUFGT, GRIP22, and STS were activated in the mutant. The results are of importance for future studies on manipulation of grape berry ripening time.

Nitrogen removal through “Candidatus Brocadia sinica” treating high-salinity and low-temperature wastewater with glycine addition: Enhanced performance and kinetics

Freshwater-derived anaerobic ammonia oxidation (anammox) bacteria (“Candidatus Brocadia sinica”) were investigated to remove nitrogen from high-salinity and low-temperature wastewater with glycine addition. The reactor was operated at 15 ± 0.5 °C with influent pH of 7.5 ± 0.1. When glycine were 0.2, 0.4, and 0.6 mM, respectively, nitrite removal rate (NRR) increased by 27.7%, 47.3%, and 70.4% accordingly. Optimal ammonia removal rate (0.32 kg/(m3·d)) and NRR (0.45 kg/(m3·d)) were achieved at 0.8 mM glycine. Effect resulting from glycine on nitrite reductase was higher than hydrazine synthase. Moreover, ΔNO2−-N/ΔNH4+-N increased with glycine addition while ΔNO3−-N/ΔNH4+-N first increased and then decreased. The remodified Logistic model and modified Boltzmann model were appropriate to describe nitrogen removal with glycine addition. Kinetic parameter λ achieved through the remodified Logistic model revealed that “Candidatus Brocadia sinica” had a shorter lag phase than that of marine anammox bacteria.

Evaluation of the effect of various main elements on the PrPSc detection by real-time quaking-induced conversion assay.

In order to definitively diagnosis sporadic Creutzfeldt-Jakob disease (sCJD), brain tissue is currently required. Therefore, there is a great need for tests that can detect sCJD in body fluids or other types of tissues. Different variables, including the amount of recombinant celluar prion protein (rPrPC), salt, cleaning surfactants and thioflavin T (ThT), in human cerebrospinal fluid (CSF) were evaluated. The reagent concentrations of 1X PBS, 170 mM NaCl, 1 mM EDTA, 0.01 mM ThT and 0.001% SDS, and the amounts of 10 µg rPrPC and 10 µl CSF were considered to be optimal for the real-time quaking-induced conversion (RT-QuIC) assay. Using these conditions, the RT-QuIC assay for prion protein (PrPSc) detection was observed to be sensitive to 10−8 diluted brain homogenates of hamsters infected with the 263K scrapie strain. Furthermore, CSF samples from 70 probable sCJD cases and 48 non-CJD cases were preliminarily screened. A substantial proportion of sCJD samples (57.14%) tested positive by RT-QuIC, with a short lag phase (<50 h post-reaction) and high peak ThT values (>25,000 relative fluorescence units). By contrast, only a small number of non-CJD samples displayed weakly positive results, and these were detected at a later stage (>50 h post-reaction) and had much lower ThT values. In conclusion, the RT-QuIC assay in CSF samples reported in the present study may provide a useful pre-mortem tool for the diagnosis of sCJD, particularly in China where postmortem examination is rarely conducted.

Characteristics of high frequency interruption for vacuum DC breakers

Post arc current can reflect the residual plasma in the electrode gap and the zero zone characteristics, so it has a great influence on the interruption of the breakers. In this paper, a permanent magnet-repulsive force hybrid actuator is designed, and the post arc current measurement system is established. Based on the forced current zero method, a series of high frequency DC interruption experiments are carried out in the synthetic test circuit with vacuum interrupters. The influence of the commutation frequency and recovery voltage on the vacuum DC interruption is investigated. The results show that a short lag phase about 80 ns exists between the post arc current and recovery voltage. High commutation frequency leads to the rise of the post arc current, which is disadvantageous for the vacuum DC interruption. The post arc current also rises with the increase in the recovery voltage. The interruption is not just affected by the di/dt and du/dt. In this study, failing interruption at the first current zero happens when the transient recovery voltage is near 2800 V. The current chopping occurs easily at high commutation frequency. And the chopping current value is proportional to the commutation frequency, which is up to 120 A at 4.0 kHz.

Characterization of Canine Adipose-Derived Mesenchymal Stromal/Stem Cells in Serum-Free Medium.

In this article, we report on the development of a defined serum-free medium capable of supporting the culture expansion of mesenchymal stromal/stem cells (MSCs) from canine adipose tissue (canine Ad-MSCs). The potential benefits of serum-free media can only be utilized if cells cultured in serum-free media maintain the same functional characteristics as cells cultured in serum-containing media. Therefore, we analyze the characteristics of canine Ad-MSCs cultured in this serum-free medium or in serum-containing medium through evaluation of growth kinetics, clonogenic capacity, senescence, and differentiation capacity. Both, serum-containing medium and our serum-free medium, supported efficient growth and colony formation of canine Ad-MSCs. In addition, canine Ad-MSCs cultured in both media demonstrated similar viability after freeze/thaw, similar cell surface marker expression, and were capable of trilineage differentiation. While canine Ad-MSCs cultured in both media were generally similar, under the conditions of our study, canine Ad-MSCs cultured in serum-free medium demonstrated a shorter lag phase and higher colony-forming capacity, accelerated population doubling, maintained multipotentiality at higher passage numbers, and underwent senescence at higher passage numbers compared to canine Ad-MSCs cultured in conventional serum-containing medium. These results suggest that canine Ad-MSCs cultured in serum-free medium retain the basic characteristics associated with canine Ad-MSCs cultured in serum-containing medium, although some differences in growth kinetics were observed.

Effect of food microstructure on growth dynamics of Listeria monocytogenes in fish-based model systems

Traditionally, predictive growth models for food pathogens are developed based on experiments in broth media, resulting in models which do not incorporate the influence of food microstructure. The use of model systems with various microstructures is a promising concept to get more insight into the influence of food microstructure on microbial dynamics. By means of minimal variation of compositional and physicochemical factors, these model systems can be used to study the isolated effect of certain microstructural aspects on microbial growth, survival and inactivation.In this study, the isolated effect on microbial growth dynamics of Listeria monocytogenes of two food microstructural aspects and one aspect influenced by food microstructure were investigated, i.e., the nature of the food matrix, the presence of fat droplets, and microorganism growth morphology, respectively. To this extent, fish-based model systems with various microstructures were used, i.e., a liquid, a second more viscous liquid system containing xanthan gum, an emulsion, an aqueous gel, and a gelled emulsion. Growth experiments were conducted at 4 and 10 °C, both using homogeneous and surface inoculation (only for the gelled systems).Results regarding the influence of the growth morphology indicated that the lag phase of planktonic cells in the liquid system was similar to the lag phase of submerged colonies in the xanthan system. The lag phase of submerged colonies in each gelled system was considerably longer than the lag phase of surface colonies on these respective systems. The maximum specific growth rate of planktonic cells in the liquid system was significantly lower than for submerged colonies in the xanthan system at 10 °C, while no significant differences were observed at 4 °C. The maximum cell density was higher for submerged colonies than for surface colonies. The nature of the food matrix only exerted an influence on the maximum specific growth rate, which was significantly higher in the viscous systems than in the gelled systems. The presence of a small amount of fat droplets improved the growth of L. monocytogenes at 4 °C, resulting in a shorter lag phase and a higher maximum specific growth rate. The obtained results could be useful in the determination of a set of suitable microstructural parameters for future predictive models that incorporate the influence of food microstructure on microbial dynamics.

Potential prebiotic effects of rice wine on Lactobacillus and Streptococcus

Rice wine has been associated with multiple health benefits due to various functional compounds it contains such as short‐chain fatty acids, organic acids, oligosaccharides and peptides. Lactobacillus and Streptococcus are considered as beneficial probiotics with protective effects on gastro‐intestinal health. In this study, rice wine was subjected to the simulated gastrointestinal digestion consisting of mouth, stomach and small intestinal phases. The final digesta was used to determine its potential prebiotic effects on probiotics, including Lactobacillus helveticus MB2‐1, Lactobacillus plantarum 70810, Lactobacillus plantarum D1501, Streptococcus thermophilus M5, Lactobacillus bulgaricus 1‐4, Lactobacillus plantarum 17‐1, Lactobacillus casei subsp. rahamnosus LS‐8, Lactobacillus plantarum B1‐6. Chemical analysis revealed that rice wine contained a considerate amount of citric acid, succinic acid, malic acid, lactic acid, acetic acid, propanoic acid and formic acid. The rice wine digesta dose‐dependently promoted the growth of probiotics. For example, the lag phase of Lactobacillus helveticus MB2‐1 was shortened from 10 h to 7 h, and μmax was increased by 16% due to the treatment of rice wine digesta. Similarly, the growth of Streptococcus thermophilus M5 and Lactobacillus bulgaricus 1‐4 was increased by addition of rice wine digesta, which was evidenced by shortened lag phase and increased maximum growth rate and maximum OD600. This is the first report on the prebiotic effects of rice wine digesta, which provided a basis to use rice wine and related products as prebiotics.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Heavy metal tolerance in marine strains of Yarrowia lipolytica.

Heavy metal tolerance of two marine strains of Yarrowia lipolytica was tested on solid yeast extract peptone dextrose agar plates. Based on minimum inhibitory concentration esteems, it is inferred that the two strains of Y. lipolytica were tolerant to heavy metals such as Pb(II), Cr(III), Zn(II), Cu(II), As(V), and Ni(II) ions. The impact of various heavy metal concentrations on the growth kinetics of Y. lipolytica was likewise assessed. With increased heavy metal concentration, the specific growth rate was reduced with delayed doubling time. Furthermore, biofilm development of both yeasts on the glass surfaces and in microtitre plates was assessed in presence of different heavy metals. In microtitre plates, a short lag phase of biofilm formation was noticed without the addition of heavy metals in yeast nitrogen base liquid media. A lag phase was extended over increasing metal concentrations of media. Heavy metals like Cr(VI), Cd(II), and As(V) are contrastingly influenced on biofilms' formation of microtitre plates. Other heavy metals did not much influence on biofilms development. Thus, biofilm formation is a strategy of Y. lipolytica under stress of heavy metals has significance in bioremediation process for recovery of heavy metals from contaminated environment.

A comparison of the performance of natural hybrids Saccharomyces cerevisiae × Saccharomyces kudriavzevii at low temperatures reveals the crucial role of their S. kudriavzevii genomic contribution

Fermentation performance at low temperature is a common approach to obtain wines with better aroma, and is critical in industrial applications. Natural hybrids S. cerevisiae × S. kudriavzevii, isolated from fermentations in cold-climate European countries, have provided an understanding of the mechanisms of adaptation to grow at low temperature. In this work, we studied the performance of 23 S. cerevisiae × S. kudriavzevii hybrids at low temperature (8, 12 and 24 °C) to characterize their phenotypes. Kinetic parameters and spot tests revealed a different ability to grow at low temperature. Interestingly, the genome content of the S. kudriavzevii in hybrids was moderately correlated with a shorter lag phase, and the genetic origin of hybrids influenced their performance at low temperature (8 °C).The parental expression of cold marker genes (NSR1, GUT2 and GPD1) showed that the relative expression of the S. kudriavzevii alleles was higher than the expression of the S. cerevisiae alleles in hybrids with a better growth at low-temperatures. These results suggest that the genomic contribution of S. kudriavzevii to hybrids is important for improving the fitness of these strains at low temperature.

Hydrodynamic Conditions Influence Bacterial Growth and Phenol Biodegradation of Strains with Different Morphology and Motility

Microorganisms are frequently exposed to flowing fluid, thus to investigate bacterial characteristics under different hydrodynamic conditions is of great importance in microbial ecology. This study characterized bacterial growth and phenol biodegradation of three strains, i.e., Microbacterium oxydans (rod-shaped, non-motile), Alcaligenes faecalis (rod-shaped, motile), and Staphylococcus haemolyticus (spherical, non-motile) in shake-flask cultures at various rotating speeds. For all the strains, a higher rotating speed always resulted in a shorter lag phase, indicating that the strains showed a superior adaptability under higher hydrodynamic conditions. The maximum specific growth rate of M. oxydans, A. faecalis, and S. haemolyticus increased rapidly with the increase of energy dissipation rate till the highest value of 0.386, 0.240, and 0.323 1/h and then decreased as the rotating speed further increased. The phenol biodegradation rate was also dependent on rotating speed, and the trends were consistent with the growth rate variations. A predictive model similar to Haldane model was proposed and was fitted well (R2 > 0.913) with bacterial growth under different hydrodynamic conditions. According to the predictive model, the optimum hydrodynamic conditions for the growth of M. oxydans, A. faecalis, and S. haemolyticus were 3.099, 2.197, and 2.289 m2/s3, respectively. The results suggested that non-motile and rod-shaped bacteria were more dependent on hydrodynamic conditions than motile and spherical ones, which could be attributed to the discrepancies in bacterial morphology and motility. The results provide a better understanding on bacterial responses to various hydrodynamic conditions and could be further applied in the bioremediation of contaminated water.

The Effect of Carbohydrates and Bacteriocins on the Growth Kinetics and Resistance of Listeria monocytogenes.

The aim of this study was to determine if different carbohydrates influence the growth of Listeria monocytogenes in the presence of carnocyclin A or leucocin A. Carnobacterium maltaromaticum ATCC PTA-5313 and Leuconostoc gelidum UAL187 were used to produce carnocyclin A and leucocin A, respectively. Growth curves were modeled for five strains of L. monocytogenes grown in basal medium supplemented with glucose, sucrose, fructose, mannose, or cellobiose, in the presence of carnocyclin A or leucocin A. The growth of L. monocytogenes to leucocin A or carnocyclin A was influenced by carbohydrate and/or strain. Carnocyclin A inhibited the growth of L. monocytogenes more than leucocin A. Growth in media containing glucose, mannose, and fructose increased the sensitivity of some strains of L. monocytogenes to bacteriocins, while growth in cellobiose and sucrose increased the resistance of L. monocytogenes to bacteriocins, as evidenced by a shorter lag phase. Strains of L. monocytogenes developed resistance to both leucocin A and carnocyclin A, but the time to develop resistance was longer when strains are treated with carnocyclin A. Carbohydrate influences the development of resistance of L. monocytogenes to the bacteriocins, but the ability of strains to develop resistance to leucocin A or carnocyclin A differs. Results of this study indicate that carbohydrates influence the ability of L. monocytogenes to grow in the presence of bacteriocins.

Protective Role of Intracellular Melatonin Against Oxidative Stress and UV Radiation in Saccharomyces cerevisiae

Melatonin (Mel) is considered a potent natural antioxidant molecule given its free-radical scavenging ability. Its origin is traced back to the origin of aerobic life as early defense against oxidative stress and radiation. More complex signaling functions have been attributed to Mel as a result of evolution in different biological kingdoms, which comprise gene expression modulation, enzyme activity, and mitochondrial homeostasis regulation processes, among others. Since Mel production has been recently reported in wine yeast, we tested the protective effect of Mel on Saccharomyces cerevisiae against oxidative stress and UV light. As the optimal conditions for S. cerevisiae to synthesize Mel are still unknown, we developed an intracellular Mel-charging method to test its effect against stresses. To assess Mel’s ability to protect S. cerevisiae from both stresses, we ran growth tests in liquid media and viability assays by colony count after Mel treatment, followed by stress. We also analyzed gene expression by qPCR on a selection of genes involved in stress protection in response to Mel treatment under oxidative stress and UV radiation. The viability in the Mel-treated cells after H2O2 stress was up to 35% greater than for the untreated controls, while stress amelioration reached 40% for UVC light (254 nm). Mel-treated cells showed a significant shortened lag phase compared to the control cells under the stress and normal growth conditions. The gene expression analysis showed that Mel significantly modulated gene expression in the unstressed cells in the exponential growth phase, and also during various stress treatments.

Altered plasma clot properties increase the risk of recurrent deep vein thrombosis: a cohort study

AbstractIt has been demonstrated that fibrin clots generated from plasma samples obtained from patients with prior thromboembolic events are denser and less susceptible to lysis. Such a prothrombotic fibrin clot phenotype has been suggested as a new risk factor for venous thromboembolism, but its prognostic value is unclear. To assess whether abnormal clot properties can predict recurrent deep vein thrombosis (DVT), we studied 320 consecutive patients aged 18 to 70 years following the first-ever DVT. Plasma clot properties were evaluated after 3 months of anticoagulant treatment since the index event. A mean duration of anticoagulation was 10 months (range, 4-20). Recurrent DVT was observed in 77 patients (25%; 6.6%/year) during a median follow-up of 44 months. Recurrences of DVT were associated with faster formation (−9% lag phase) of denser fibrin networks (−12% fibrin clot permeability [Ks]) and 4% higher maximum absorbance of plasma clots that displayed impaired fibrinolytic degradation (+25% prolonged clot lysis time [CLT]) and a 5% slower rate of increase in D-dimer levels during clot degradation (D-Drate; all P < .05). Proximal DVT alone, higher C-reactive protein, D-dimer, peak thrombin, lower Ks, shorter lag phase, decreased D-Drate, and prolonged CLT were independent predictors of recurrences (all P < .05). Individuals characterized by low Ks (≤7.3 × 10−9 cm2) and prolonged CLT (>96 min) were at the highest risk of recurrent DVT (odds ratio, 15.8; 95% confidence interval, 7.5-33.5). Kaplan-Meier curves showed that reduced Ks and prolonged CLT predicted recurrent DVT. We demonstrate that unfavorably altered clot properties may predict recurrent DVT after anticoagulation withdrawal.

Listeria monocytogenes Growth Kinetics in Milkshakes Made from Naturally and Artificially Contaminated Ice Cream.

This study assessed the growth of Listeria monocytogenes in milkshakes made using the process-contaminated ice cream associated with a listeriosis outbreak in comparison to milkshakes made with artificially contaminated ice cream. For all temperatures, growth kinetics including growth rates, lag phases, maximum populations, and population increases were determined for the naturally and artificially derived contaminants at 5, 10, 15, and 25°C storage for 144 h. The artificially inoculated L. monocytogenes presented lower growth rates and shorter lag phases than the naturally contaminated populations at all temperatures except for 5°C, where the reverse was observed. At 25°C, lag phases of the naturally and artificially contaminated L. monocytogenes were 11.6 and 7.8 h, respectively. The highest increase in population was observed for the artificially inoculated pathogen at 15°C after 96 h (6.16 log CFU/mL) of storage. Growth models for both contamination states in milkshakes were determined. In addition, this study evaluated the antimicrobial effectiveness of flavoring agents, including strawberry, chocolate and mint, on the growth of the pathogen in milkshakes during 10°C storage. All flavor additions resulted in decreased growth rates of L. monocytogenes for both contamination states. The addition of chocolate and mint flavoring also resulted in significantly longer lag phases for both contamination states. This study provides insight into the differences in growth between naturally and artificially contaminated L. monocytogenes in a food product.

Biohydrogen Production by Antarctic Psychrotolerant Klebsiella sp. ABZ11.

Lower temperature biohydrogen production has always been attractive, due to the lower energy requirements. However, the slow metabolic rate of psychrotolerant biohydrogen-producing bacteria is a common problem that affects their biohydrogen yield. This study reports on the improved substrate synthesis and biohydrogen productivity by the psychrotolerant Klebsiella sp. strain ABZ11, isolated from Antarctic seawater sample. The isolate was screened for biohydrogen production at 30°C, under facultative anaerobic condition. The isolate is able to ferment glucose, fructose and sucrose with biohydrogen production rate and yield of 0.8 mol/l/h and 3.8 mol/g, respectively at 10 g/l glucose concentration. It also showed 74% carbohydrate uptake and 95% oxygen uptake ability, and a wide growth temperature range with optimum at 37°C. Klebsiella sp. ABZ11 has a short biohydrogen production lag phase, fast substrate uptake and is able to tolerate the presence of oxygen in the culture medium. Thus, the isolate has a potential to be used for lower temperature biohydrogen production process.

Evolutionary Adaptation of Kluyveromyces marxianus NIRE-K3 for Enhanced Xylose Utilization

The evolutionary adaptation was approached on the thermotolerant yeast Kluyveromyces marxianus NIRE-K3 at 45oC on xylose as a sole source of carbon for enhancement of xylose uptake. After 60 cycles, evolved strain K. marxianus NIRE-K3.1 showed comparatively 3.75-fold and 3.0-fold higher specific growth and xylose uptake rates, respectively than that of native strain. Moreover, the short lag phase was also observed on adapted strain. During batch fermentation with xylose concentration of 30 g l-1, K. marxianus NIRE-K3.1 could utilize about 96 % of xylose in 72 h and produced 4.67 and 15.7 g l-1 of ethanol and xylitol, respectively, which were 9.72-fold and 4.63-fold higher than that of native strain. Similarly, specific sugar consumption rate, xylitol and ethanol yields were 5.07-fold, 1.15-fold and 2.44-fold higher as compared to the native strain, respectively. The results obtained after evolutionary adaptation of K. marxianus NIRE-K3 show the significant improvement in the xylose utilization, ethanol and xylitol yields, and productivities. By understanding the results obtained, the significance of evolutionary adaptation has been rationalized, since the adapted culture could be more stable and could enhance the productivity.