Efficient utilization of waste wheat straw through humic acid and ferric chloride co-assisted hydrothermal pretreatment for fermentation to produce bioethanol

The adsorbed ash and lignin contained in waste wheat straw (WWS) have been the essential factors restricting its high-value utilization in biorefinery. Hence, humic acid (HA) and FeCl3 as the additives of hydrothermal pretreatment were applied to simultaneously enhance the removal of lignin and eliminate the acid buffering of ash in WWS, respectively. The results showed that the xylan and lignin removal of WWS pretreated with 10 g/L HA and 20 mM FeCl3 could be efficiently increased from 61.4% to 72.9% and from 14.7% to 38.7%, respectively. The enzymatic hydrolysis efficiency and ethanol yield of WWS were increased this way from 44.4% to 82.7% and from 20.55% to 36.86%, respectively. According to the characterization of WWS, the synergistic interaction between HA and FeCl3 was beneficial to the cellulose accessibility and surface lignin area of WWS changed in positive directions, leading to the improvement of hydrolysis efficiency.

Effects of Humic Substances on the Growth of Pseudomonas plecoglossicida 2,4-D and Wheat Plants Inoculated with This Strain

Both rhizosphere bacteria and humic substances (HSs) can promote plant growth when applied individually and even greater effects of their combination have been demonstrated. We aimed to elucidate the relative importance of the stimulating effects of HSs on bacterial growth and the effects of the combination of bacteria and HSs on plants themselves. The effects of humic (HA) and fulvic acids (FA) (components of humic substances) on the growth of Pseudomonas plecoglossicida 2,4-D in vitro were studied. We also studied the effects of this bacterial strain and HSs applied individually or in combination on the growth of wheat plants. Although the 2,4-D strain showed low ability to use HSs as the sole source of nutrition, the bacterial growth rate was increased by FA and HA, when other nutrients were available. HSs increased root colonization with bacteria, the effect being greater in the case of HA. The effects on roots and shoots increased when bacteria were associated with HSs. FA+ 2,4-D was more effective in stimulating shoot growth, while HA + 2,4-D was in the case of root growth. The latter effect is likely to be beneficial under edaphic stresses.

Simulated sunlight-induced inactivation of tetracycline resistant bacteria and effects of dissolved organic matter

The transmission of antibiotic resistance in surface water has attracted much attention due to its increasing threat to human health. The role of sunlight irradiation and the effect of dissolved organic matter (DOM) on the transmission of antibiotic resistance are still unclear. In this study, photo-inactivation of antibiotic resistant bacteria (ARB) was investigated using antibiotic resistant E. coli (AR E. coli) that contained the tetracycline resistance gene (Tc-ARG) as a representative. The results showed that AR E. coli underwent significant photo-inactivation due to the membrane damage induced by direct irradiation and by the generated reactive oxygen species. Simulated sunlight irradiation specifically suppressed the expression of tetracycline resistance, which is attributed to the destruction of tetracycline-specific efflux pump. Tetracycline inhibited the photo-inactivation of AR E. coli due to its selective pressure on tetracycline resistant E. coli and competitive light absorption effect. Suwannee River fulvic acid (SRFA), a representative DOM, promoted the inactivation of AR E. coli and further inhibited the expression of tetracycline resistance gene due to the generation of its excited triplet state, singlet oxygen, and hydroxyl radical. The extracellular Tc-ARG also underwent fast photodegradation under light irradiation and in the presence of SRFA, which leads to the decrease of its transformation efficiency. This study provided insight into the sunlight-induced inactivation of ARB, which is of significance for understanding the transmission of tetracycline resistance in surface water.

Escherichia coli O157 survival in liquid culture and artificial soil microcosms with variable pH, humic acid and clay content

AIMS

This research was performed to investigate the influence of clay and humic acid on Escherichia coli O157 survival in model soils. Additionally, the influence of pH and humic acid on E. coli O157 in liquid culture was investigated.

METHODS AND RESULTS

Artificial soil microcosms were prepared with sand, kaolinite, bentonite and humic acid. Artificial soil microcosms pH was adjusted (6·0-7·0) with aluminium sulphate before E. coli O157 inoculation. After 56 days of incubation at 30°C, significant differences in E. coli O157 log CFU per gram were observed between 0 and 1000 ppm (P < 0·0001) and 0 and 5000 ppm (P < 0·0001) humic acid in 1·5% clay soils, but not in 7·5 or 15% clay soils. Significant differences (P < 0·05) in E. coli O157 log CFU per ml were observed in liquid culture influenced by humic acid concentrations after 8 h at 37°C.

CONCLUSIONS

The developed model soils support E. coli O157 populations over 28 days, and higher clay soils may aid in E. coli O157 survival.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results provide insights into physicochemical properties of soil that may influence E. coli O157 in the environment and help explain E. coli O157 survival in various soils and geographical regions.

Roles of different humin and heavy-metal resistant bacteria from composting on heavy metal removal

This study aims to assess the roles of different humin and heavy-metal resistant bacterial community from composting on heavy metal removal. The results showed that the concentration of Cu²⁺, Zn²⁺, Ni²⁺, Pb²⁺, Cr³⁺ and Cd²⁺ decreased with adding the compost-derived humin, but the removal rates were relatively low (<30% on average). The heavy metal resistant bacteria from composting have better metal binding capacities than humin, and the combined addition of humin and bacteria could further stimulate the biosorption of heavy metals with 60-80% removal of metals and improve the diversity and biomass of bacterial community. There was obviously increased synergy between the humin from maturity phase and bacteria for metal biosorption ("1 + 1 > 2"). Structural equation modeling showed that microbial biomass and humin humification are the key factors for the biosorption of heavy metals. Combining humin from maturity phase with heavy-metal resistant bacteria was suggested to control heavy metal pollution in composts.

Cd(II) Sorption on Montmorillonite-Humic acid-Bacteria Composites

Soil components (e.g., clays, bacteria and humic substances) are known to produce mineral-organic composites in natural systems. Herein, batch sorption isotherms, isothermal titration calorimetry (ITC), and Cd K-edge EXAFS spectroscopy were applied to investigate the binding characteristics of Cd on montmorillonite(Mont)-humic acid(HA)-bacteria composites. Additive sorption and non-additive Cd(II) sorption behaviour is observed for the binary Mont-bacteria and ternary Mont-HA-bacteria composite, respectively. Specifically, in the ternary composite, the coexistence of HA and bacteria inhibits Cd adsorption, suggesting a "blocking effect" between humic acid and bacterial cells. Large positive entropies (68.1~114.4 J/mol/K), and linear combination fitting of the EXAFS spectra for Cd adsorbed onto Mont-bacteria and Mont-HA-bacteria composites, demonstrate that Cd is mostly bound to bacterial surface functional groups by forming inner-sphere complexes. All our results together support the assertion that there is a degree of site masking in the ternary clay mineral-humic acid-bacteria composite. Because of this, in the ternary composite, Cd preferentially binds to the higher affinity components-i.e., the bacteria.

Influence of Dissolved Organic Matter on Tetracycline Bioavailability to an Antibiotic-Resistant Bacterium

Complexation of tetracycline with dissolved organic matter (DOM) in aqueous solution could alter the bioavailability of tetracycline to bacteria, thereby alleviating selective pressure for development of antibiotic resistance. In this study, an Escherichia coli whole-cell bioreporter construct with antibiotic resistance genes coupled to green fluorescence protein was exposed to tetracycline in the presence of DOM derived from humic acids. Complexation between tetracycline and DOM diminished tetracycline bioavailability to E. coli, as indicated by reduced expression of antibiotic resistance genes. Increasing DOM concentration resulted in decreasing bioavailability of tetracycline to the bioreporter. Freely dissolved tetracycline (not complexed with DOM) was identified as the major fraction responsible for the rate and magnitude of antibiotic resistance genes expressed. Furthermore, adsorption of DOM on bacterial cell surfaces inhibited tetracycline diffusion into the bioreporter cells. The magnitude of the inhibition was related to the amount of DOM adsorbed and tetracycline affinity for the DOM. These findings provide novel insights into the mechanisms by which the bioavailability of tetracycline antibiotics to bacteria is reduced by DOM present in water. Agricultural lands receiving livestock manures commonly have elevated levels of both DOM and antibiotics; the DOM could suppress the bioavailability of antibiotics, hence reducing selective pressure on bacteria for development of antibiotic resistance.