Recent advances in constructing artificial microbial consortia for the production of medium-chain-length polyhydroxyalkanoates.

Polyhydroxyalkanoates (PHAs) are a class of high-molecular-weight polyesters made from hydroxy fatty acid monomers. PHAs produced by microorganisms have diverse structures, variable physical properties, and good biodegradability. They exhibit similar physical properties to petroleum-based plastics but are much more environmentally friendly. Medium-chain-length polyhydroxyalkanoates (mcl-PHAs), in particular, have attracted much interest because of their low crystallinity, low glass transition temperature, low tensile strength, high elongation at break, and customizable structure. Nevertheless, high production costs have hindered their practical application. The use of genetically modified organisms can reduce production costs by expanding the scope of substrate utilization, improving the conversion efficiency of substrate to product, and increasing the yield of mcl-PHAs. The yield of mcl-PHAs produced by a pure culture of an engineered microorganism was not high enough because of the limitations of the metabolic capacity of a single microorganism. The construction of artificial microbial consortia and the optimization of microbial co-cultivation have been studied. This type of approach avoids the addition of precursor substances and helps synthesize mcl-PHAs more efficiently. In this paper, we reviewed the design and construction principles and optimized control strategies for artificial microbial consortia that produce mcl-PHAs. We described the metabolic advantages of co-cultivating artificial microbial consortia using low-value substrates and discussed future perspectives on the production of mcl-PHAs using artificial microbial consortia.

Optimal biostimulation strategy for phenol degradation with indigenous rhizobium Ralstonia taiwanensis.

This study provides a first attempt from a perspective of Gaden's classification of fermentation and phase-plane to put forward phenol degradation using various augmented nutrient media for biostimulation. It aimed to identify the most promising nutrient source(s) to attenuate synergistic interactions with phenol for optimal phenol degradation. Therefore, the growth association of phenol degradation using various nutrient media in place of combined toxic interactions was established via Gaden's classification scheme of fermentation and phase-plane analysis. In cultures grown on medium bearing dual carbon sources (glycerol and phenol) or phenol alone, phenol was found to be firstly biodegraded for microbial growth (i.e., growth-associated degradation). In contrast, when yeast extract or acetate was supplemented, a diauxic growth behavior was observed as the augmented nutrient was primarily utilized while phenol degradation was repressed. Moreover, using glycerol as the nutrient source, phenol degradation seemed to be enhanced simultaneously during the consumption of glycerol for cellular growth after ca. 2h response lag in growth. Although gluconic acid could enhance cell growth as well as phenol degradation, the phenol degradation performance was still not as good as that of glycerol. Thus, biostimulation with glycerol appeared to show the most favorable metabolic characteristics against phenol toxicity on Ralstonia taiwanensis, leading to better degradation efficiency of the toxic pollutant. Phase-plane trajectories also clearly confirmed that glycerol was the optimal biostimulating nutrient source for phenol degradation.

Surface active properties of bacterial strains isolated from petroleum hydrocarbon-bioremediated soil.

Two bacterial strains identified as Ralstonia picketti (BP 20) and Alcaligenes piechaudii (CZOR L-1B) were isolated from petroleum hydrocarbon-contaminated soil following bioremediation treatment. The surface active properties, e.g. surface tension, emulsification and foamability of their culture filtrates were evaluated. Bacterial cell-surface hydrophobicity (BAH) as measured by analyzing cell affinity towards aliphatic and aromatic compounds was also determinated. The bacteria grew in liquid cultures containing 1% (v/v) of crude oil as carbon and energy source at 30 degrees C under aerobic conditions. The surface tensions were reduced to 61 mN/m and 55 mN/m by Ralstonia picketti and Alcaligenes piechaudii, respectively. The emulsification index (EI24) was almost 100% for all tested compounds except diesel oil. The stability of the emulsions was deteminated at 4 degrees C, 45 degrees C and 65 degrees C. The emulsions were stable at 4 degrees C. Ralstonia picketti was better foam inducer (FV = 50 ml) compared to Alcaligenes piechaudii (FV = 10 ml). The BAH measurements revealed higher adhesion of Alcaligenes piechaudii cells towards different hydrocarbons compared to Ralstonia picketti cells. The strains were found to have a surface hydrophobicity in the following order: aliphatic hydrocarbons, BTEX, and PAHs. The ability to adhere to bulk hydrocarbon is mostly a characteristic of hydrocarbon-degrading bacteria. The strains were found to be better emulsifiers than surface tension reducers. They produce water-soluble extracellular bioemulsifiers. Both bacterial isolates have good properties to use them, mainly in the petroleum industry, e.g. in enhanced oil recovery and in bioremediation processes-primarily due to their emulsification property, i.e. emulsion forming and stabilizing capacity.

Phenol degradation and toxicity assessment upon biostimulation to an indigenous Rhizobium Ralstonia taiwanensis.

This study provides a first attempt from a toxicological perspective to put forward, in general terms and explanations, combined toxic interactions and biostimulation strategy upon nutrient medium to Ralstonia taiwanensis for bioremediation. Dose-response analysis clearly revealed that most of the supplemented nutrients tested (except for gluconic acid) synergistically interact with chronic toxicity to phenol, especially at low doses. Acute toxicity based upon adaptation lag is a more appropriate indicator for comparative analysis of toxicity due to similar toxic ranking at almost all effective concentrations. In addition, comparison upon acute and chronic toxicity for various nutrient media also suggests in parallel that acute toxicity is more significant than chronic toxicity possibly as the result of a more sensitive response of adaptation lag to growth in different media. Feasibility of adding extra nutrient substrates (e.g., phenol, gluconic acid, yeast extract, pyruvic acid, acetic acid, and glycerol) to stimulate proliferation of phenol degraders for better phenol degradation performance was also assessed. The results show that using acetic acid as the augmented nutrient source might be the most feasible biostimulation strategy for phenol degradation.

Construction and evaluation of nagR-nagAa::lux fusion strains in biosensing for salicylic acid derivatives.

The NagR protein is a response regulatory protein found in the bacterium Ralstonia sp. U2 that is involved in sensing for salicylic acid and the subsequent induction of the operon just upstream of its gene. The genes encoded for in this operon are involved in the degradation of salicylic acid. Escherichia coli strain RFM443 carrying a fusion of the Photorhabdus luminescens luxCDABE operon with the nagR gene and upstream region of the nagAa gene was constructed and characterized with respect to its optimum temperature, its response time and kinetics, and its ability to detect numerous benzoic acid derivatives. Although capable of detecting 0.5 mM salicylic acid at any temperature between 28 and 40 degrees C, this E. coli strain, labeled DNT5, showed its greatest relative activity at 30 degrees C, i.e., the temperature at which the largest induction was seen. Furthermore, experiments done with numerous benzoic acid derivatives found the NagR protein to be responsive to only a few of the compounds tested, including salicylic acid and 3-methyl salicylic acid, and acetyl salicylic acid was the strongest inducer. The lower limits of detection for these compounds with E. coli strain DNT5 were also established, with the native inducer, salicylic acid, giving the most sensitive response and detectable down to a concentration of about 2 microM. A second lux fusion plasmid was also constructed and transformed into an NahR background, Pseudomonas putida KCTC1768. Within this strain, NAGK-1768, the supplemental activity of the NahR protein on the nagAa promoter, was shown to extend both the range of chemicals detected and the sensitivity.