Recombinant DNA techniques for bioremediation and environmentally-friendly synthesis

Substrate-dependent expression of laccase in genetically modified Escherichia coli: design and construction of an inducible phenol-degrading system

Phenolic compounds that are produced by variety of industrial and urban activities pose dangers to live organisms and the environment. Here, an inducible phenol-degrading system was designed and constructed in Escherichia coli as the host. CapR as a transcription activator in Pseudomonas species shows sensitivity towards most common phenolic pollutants. Upon presence of inducible pollutants and conformational changes of CapR, an inducible promoter will trigger the expression of a bacterial laccase gene, which had been isolated previously from a local Bacillus species. Laccase as a multicopper oxidase has the ability to oxidize wide variety of mono and polyphenols. The sensitivity of the inducible system was verified in the presence of phenol with the concentration range of 1 nM-10 mM. A linear correlation was observed between laccase expression and phenol concentration up to 1 mM. Laccase was expressed even in the lowest concentration of phenol (1 nM) after 2 hr of exposure. 2,2-Azinobis (3-ethylbenzthiazoline-6-sulfonate) (ABTS) as a mediator of laccase oxidative reactions could induce laccase expression through conformational changes of CapR. Recognition of ABTS by CapR not only results in expression of the remediating enzyme but also extends its substrate range to nonphenolic compounds.

Metabolic and molecular methods to evaluate the organoclay effects on a bacterial community

The aim of this work was to evaluate the influence exerted by two different commercial organoclays (DELLITE 43B and DELLITE 67G) on a model microbial consortium using microbial metabolic characterization with BIOLOG system and denaturing gradient gel electrophoresis (DGGE) molecular approach. The information obtained from the molecular analyses, in their complex, account for the differences in species composition induced on the reference consortium by the contact with the organoclays under study. DELLITE 43B resulted to produce a marked selective effect, stimulating the quantitative increase especially of Pseudomonas pseudoalcaligenes. A weaker effect was found for DELLITE 67G. On the other hand, Biolog analyses indicated a depressing action exerted by DELLITE 43B on the metabolic activity of the model microbial consortium as a whole. The presence of P. pseudoalcaligenes and B. borstelensis in the bacterial community after the treatments confirmed that a positive change in the microbial structure consortium occurred.

Comparison of 2-chlorobenzoic acid biodegradation in a membrane bioreactor by B. cepacia and B. cepacia bearing the bacterial hemoglobin gene

Degradation of 2-chlorobenzoic acid (2-CBA), a model recalcitrant chlorinated organic compound, by pure cultures of Burkholderia cepacia strain DNT with (transformed B. cepacia) and without (untransformed B. cepacia) the bacterial hemoglobin (Vitreoscilla hemoglobin, VHb) gene, vgb, was investigated in parallel membrane bioreactors (MBRs). This was done aseptically to prevent contamination during the operation of the MBRs. The objective was to determine whether the degradation of 2-CBA by cometabolism, using acetate as a primary carbon source, under hypoxic conditions might be enhanced for vgb-bearing cells in MBRs. The 2-CBA removal efficiency of transformed B. cepacia (97-99%) was slightly higher than that of untransformed B. cepacia (95-99%) at all stages. The average amount of chloride released from 2-CBA by transformed cells was also higher than for untransformed cells, 92-96% compared to 64-84% of the maximum theoretical amount, the exact value depending on the operating conditions. These results indicate that 2-CBA degradation/transformation is not accompanied by the stoichiometric release of chloride for the untransformed strain. The difference between percentages of 2-CBA removal and chloride release by untransformed cells was attributed to persistence, under hypoxic conditions, of the 2-CBA chlorine atom in 2-CBA metabolites. Growth of transformed cells was also significantly enhanced under hypoxic conditions compared to untransformed cells. For varying media compositions, the transformed cells reached higher cell densities (3.2-5.4 g/L) relative to untransformed cells (2.8-4.7 g/L) at food to microorganism ratios ranging from 0.44-0.59 to 0.38-0.49 g COD/g biomass-d The observed yields thus ranged from 0.16-0.20 and 0.15-0.18 g TSS/g COD for untransformed and transformed cells, respectively. The value of the yield depended on medium composition. The MBR system using vgb-containing B. cepacia maintained a high biomass concentration without oxygen limitations and provided cell-free effluent. Hence, it may be useful for treating high volumes of water contaminated with low levels of recalcitrant organics.

2-Chlorobenzoate biodegradation by recombinant Burkholderia cepacia under hypoxic conditions in a membrane bioreactor

The feasibility of applying bacterial hemoglobin technology to degrade 2-chlorobenzoate (2-CBA) through co-metabolism under hypoxic conditions in a membrane bioreactor (MBR) process has been studied in the laboratory. 2-chlorobenzoate removal and chloride release rates in the MBR system varied from 99 to 78% and 98 to 73%, respectively, depending on the operation conditions. Chemical oxygen demand (COD) removal efficiencies were more than 90% at food-to-microorganism ratios ranging from 0.32 to 0.62 g/g/d, and the observed yield was 0.13 to 0.20 g biomass/g COD. The bacterial cell-floc size-distribution analysis showed that there is a significant change in bacterial floc size due to high shear stress during operation of the MBR. To characterize growth kinetics of Burkholderia cepacia strain dinitrotoluene, a mathematical model that describes co-metabolic oxidation of 2-CBA in an MBR has been developed.

Stress-induced evolution and the biosafety of genetically modified microorganisms released into the environment

This article is focused on the problems of reduction of the risk associated with the deliberate release of genetically modified microorganisms (GMMs) into the environment. Special attention is given to overview the most probable physiological and genetic processes which could be induced in the released GMMs by adverse environmental conditions, namely: (i) activation of quorum sensing and the functions associated with it, (ii) entering into a state of general resistance, (iii) activation of adaptive mutagenesis, adaptive amplifications and transpositions and (iv) stimulation of inter-species gene transfer. To reduce the risks associated with GMMs, the inactivation of their key genes responsible for stress-stimulated increase of viability and evolvability is proposed.

Development of a gene cloning and inactivation system for halorespiring Desulfitobacterium dehalogenans

Efficient host-vector systems have been developed for the versatile, strictly anaerobic, halo- and fumarate-respiring gram-positive bacterium Desulfitobacterium dehalogenans. An electroporation-based transformation procedure resulting in approximately 10(3) to 10(4) transformants per microg of the cloning vector pIL253 was developed and validated. The broad-host-range vector pG+host9 was shown to replicate at a permissive temperature of 30 degrees C, whereas the replicon was not functional at 40 degrees C. The D. dehalogenans frdCAB operon, predicted to encode a fumarate reductase, was cloned, characterized, and targeted for insertional inactivation by pG+host9 carrying a 0.6-kb internal frdA fragment. Single-crossover integration at the frdA locus occurred at a frequency of 3.3 x 10(-4) per cell and resulted in partially impaired fumarate reductase activity. The gene cloning and inactivation systems described here provide a solid basis for the further elucidation of the halorespiratory network in D. dehalogenans and allow for its further exploitation as a dedicated degrader.

A gene containment strategy based on a restriction-modification system

Engineering barriers to the spread of specific genes are of great interest both to increase the predictability of recombinant microorganisms used for environmental applications and to study the role of gene transfer in the adaptation of microbial communities to changing environments. We report here a new gene containment circuit based on a toxin-antidote pair that targets the cell DNA, i.e. the type II EcoRI restriction-modification system. The set-up involved linkage of the ecoRIR lethal gene encoding the EcoRI endonuclease (toxin) to the contained character in a plasmid and chromosomal insertion of the ecoRIM gene encoding the cognate EcoRI methylase (antidote) that protects the target DNA from restriction. Transfer of the contained character to a recipient cell lacking the antidote caused EcoRI-mediated chromosomal breaks, leading to cell death, thereby preventing gene spread. Using transformation and conjugation as mechanisms of DNA transfer and different environmentally relevant bacteria as recipients, we have shown that the potentially universal EcoRI-based containment system decreases gene transfer frequencies by more than four orders of magnitude. Analyses of the survivors escaping killing revealed a number of possible inactivation mechanisms.

Field applications of genetically engineered microorganisms for bioremediation processes

Genetically engineered microorganisms (GEMs) have shown potential for bioremediation applications in soil, groundwater, and activated sludge environments, exhibiting enhanced degradative capabilities encompassing a wide range of chemical contaminants. However, the vast majority of studies pertaining to genetically engineered microbial bioremediation are supported by laboratory-based experimental data. In general, relatively few examples of GEM applications in environmental ecosystems exist. Unfortunately, the only manner in which to fully address the competence of GEMs in bioremediation efforts is through long-term field release studies. It is therefore essential that field studies be performed to acquire the requisite information for determining the overall effectiveness and risks associated with GEM introduction into natural ecosystems.