Peng-Fei Xia

Characterization of the interactions between tetracycline antibiotics and microbial extracellular polymeric substances with spectroscopic approaches

The antibiotics have attracted global attentions for their impact on aquatic ecosystem. The knowledge about the fate of antibiotics encountering extracellular polymeric substances (EPS) is, however, limited. In this study, we investigated the interacting mechanisms of tetracycline (TC) to EPS extracted from aerobic activated sludge. The contributions of the main components of EPS, extracellular proteins, and polysaccharides were evaluated using bovine serum albumin and alginate sodium, respectively. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and nuclear magnetic resonance indicated that hydroxyl, carboxyl, and amino groups were the domain chemical groups involved in the interaction between TC and EPS, and the binding of TC onto EPS changed the structure of these chemical groups, thus causing shifts in their UV–visible absorption spectra. In addition, we found that extracellular proteins, rather than polysaccharides, were the major active contents involved in the interaction. Three-dimensional excitation–emission matrix fluorescence spectroscopy showed that the fluorophores in EPS were clearly quenched by TC and the static quenching process was observed, implying the complex formation of TC and EPS. Furthermore, thermodynamic analysis indicated that the binding of TC with EPS is spontaneous and dominated by electrostatic forces.

Extracellular polymeric substances protect Escherichia coli from organic solvents

The roles and functions of extracellular polymeric substances (EPS) have been intensively studied. Besides the significance of EPS in microbial multi-cellular structure formation, the interaction between EPS and external stressors was also investigated. However, seldom research studied how EPS response to organic solvents, a general type of chemicals in industry and potential pollutants in environments. In the present study, we found that EPS can response to n-butanol (n-BuOH) stress via excess secretion and varied composition, and EPS protect Escherichia coli from the n-BuOH stress. The extracellular proteins in EPS played a significant role in the protective effects via contributing to the surface hydrophobicity putatively by changing the secondary structure. We believe these findings would enable us to understand the functions of EPS in bacterial physiological activities better and benefit the EPS-related bioprocesses. This study would also provide us alternatives to utilize microorganisms with EPS excretion routine.

Investigation of fate and behavior of tetracycline in nitrifying sludge system

This study aims to investigate the fate and behavior of tetracycline (TC) in nitrifying sludge system, as well as the effects of TC dosage on sludge performance. For this purpose, two TC spiked and two control laboratory reactors were operated for two months, while the spiked reactors (designated as RI and RII) were intermittently fed with TC at the concentrations of 10 and 1 mg L−1, respectively. TC could be effectively removed via initial adsorption and subsequent biodegradation, while biodegradation was the primary mechanism in this study. Compared to RII, no significant negative effects were found on dehydrogenase activity under higher TC stress in RI. It is interesting that RI showed better nitrification performance than RII, especially higher nitrite oxidation capacity. Moreover, exposure to TC also promoted the formation of aggregation and affected the composition of nitrifying bacteria. The relative contents of nitrite-oxidizing bacteria (NOB) in RII decreased by almost 50%, from 11.4 ± 3.2% to 6.5 ± 2.5% while there was a slight change in RI, from 11.9 ± 5.8% to 11.2 ± 3.8%. Furthermore, the mean sludge diameter increased from 218.3 ± 7.8 μm to 512.4 ± 7.8 μm and 353.8 ± 11.1 μm in RI and RII, respectively. This indicated that larger aggregations were discovered in reactors with high TC stress. The aggregation might lead to a multilayer structure of sludge to protect the microorganism inside, which would explain the higher relative abundance of NOB in reactors with high TC stress. This work expands our vision about the fate and behavior of antibiotics in activated sludge system, which has far-reaching implications in activated sludge processes.

AraBAD Based Toolkit for Gene Expression and Metabolic Robustness Improvement in Synechococcus elongatus

As a novel chemical production platform, controllable and inducible modules in Synechococcus elongatus plus the ability of working in diurnal conditions are necessary. To the endeavors, inducible promoters, such as PTrc, have been refined from Escherichia coli, but the inducer isopropyl-β-D-thiogalactoside may cause several side-effects. Meanwhile, to promote the efficiency, photomixotrophic cultivation has been applied in S. elongatus with the additional organic carbon sources. In this study, we developed L-arabinose based modules consisted of both the PBAD inducible promoter and the metabolism of L-arabinose in S. elongatus, since L-arabinose is an ideal heterologous feedstock for its availability and economic and environmental benefits. As expected, we achieved homogeneous and linear expression of the exogenous reporter through the PBAD promoter, and the biomass increased in diurnal light condition via introducing L-arabinose metabolism pathway. Moreover, the combined AraBAD based toolkit containing both the PBAD inducible module and the L-arabinose metabolism module could obtain gene expression and metabolic robustness improvement in S. elongatus. With the only additive L-arabinose, the novel strategy may generate a win-win scenario for both regulation and metabolism for autotrophic bio-production platforms.

Ecological insights into low-level antibiotics interfered biofilms of Synechococcus elongatus

The interactions between antibiotics and microorganisms have attracted considerable interests. As reported, low-level concentrations of antibiotics have been detected in various environments. Although many studies reported the inducing effects of antibiotics on biofilms, few research discussed their impacts on the ecological functions of microorganisms with biogeochemical importance. In this study, we investigated the effects of low-levels of kanamycin on the biofilm formation of Synechococcus elongatus as a proof of principle study. The results indicated that the biofilms of S. elongatus would be promoted in the presence of kanamycin, and the related photosynthesis-mediated calcification, a fundamental bio-machinery contributing to the local and global carbon cycle, will probably be enhanced as well. We believe this study would offer new information to evaluate the environmental risks of antibiotics and inspire more investigations on the ecological impacts of emerging pollutants.

Influences of D-tyrosine on the stability of activated sludge flocs.

The sludge floc stability is essential for the solid/liquid separation in biological wastewater treatment. In this study, the effect of an exogenous d-tyrosine on the shear stability and surface characteristics of activated sludge flocs was investigated. Sludge flocs were found to be less stable in the addition of d-tyrosine. d-Tyrosine inhibited the production of extracellular polymeric substances (EPS) especially for the proteins. A high correlation coefficient was observed between the composition of EPS fraction and d-tyrosine content. In addition, the hydrophobicity of sludge flocs was reduced and the zeta potential was more negative with the content of d-tyrosine increased. A linear relationship between the extracellular polymeric substances and surface characteristics for sludge flocs indicated that the inhibited EPS production may be responsible for the instability of sludge upon the addition of d-tyrosine.

Influences of graphene oxide on biofilm formation of gram-negative and gram-positive bacteria

AbstractIn this study, we evaluated the influences of graphene oxide (GO) on biofilm formation. Escherichia coli MG1655 and Bacillus subtilis 168 were used as models for Gram-negative and Gram-positive bacteria. The growth profiles and viability assays indicated that GO exhibited a high antibacterial activity, of which the negative effects on bacteria growth raised with the increasing GO concentration. The antibacterial activity of GO was mainly attributed to the membrane stress and ROS-independent oxidative stress. Moreover, it was worthy to note that the biofilm formation was enhanced in the presence of GO at low dosage whereas inhibited in the high-concentration GO environment. These results could be explained by the roles of the dead cells, which were inactivated by GO. When the concentration of GO was limited, only a part of the cells would be inactivated, which may then serve as a protection barrier as well as the necessary nutrient to the remaining living cells for the formation of biofilm. In contrast, with a sufficient presence of GO, almost all cells can be inactivated completely and thus the formation of biofilm could no longer be triggered. Overall, the present work provides significant new insights on the influence of carbon nanomaterials towards biofilm formation, which has far-reaching implications in the field of biofouling and membrane bioreactor. Graphical abstractᅟ

Micelle-mediated transport disturbance providing extracellular strategy for alleviating n -butanol stress on Escherichia coli

One barrier inhibiting further progress in biofuel production is the toxicity of biofuels towards their producers. It is promising to apply gene-based intracellular techniques to engineer better strains with higher organic solvent tolerance. These methods are, however, complex. In the present study, we developed a simple, manageable, and commercial extracellular prototypal strategy to alleviate n-butanol (n-BuOH) stress on Escherichia coli via a micelle-mediated transport disturbance. When the concentration of sodium dodecyl sulfate, a typical anionic surfactant, is high enough to form micelles, n-BuOH will be trapped into/onto the micelles, and the negative charge prevents the n-BuOH from approaching the cells. Our study provides an extracellular strategy to relieve the stress from n-BuOH, and it also exhibits a new angle to advance microbial factories through extracellular routines.