Directed Evolution of Bacteria for Bioremediation

The development of modern science contributed consistently to the introduction of new technologies which allow use the full potential of different branches of modern science. In this respect, the progress of in research of bacteria seems to be particularly significant because it opened the ways to the development of bioremediation. The latter becomes particularly important in the modern world, when the problem of the environment pollution and the negative impact of human activities on the environment and biodiversity is extremely negative. In such a context, the directed evolution of bacteria for bioremediation can be very helpful in the solution of the problem of the environmental protection because bioremediation can be used effectively as a tool to return the natural environment altered by contaminants to its original condition. At this point, it is important to underline that the modern science allows directing the evolution of bacteria for bioremediation that means that this process can be directed by humans and, therefore, they can minimize their negative impact on their environment.

On analyzing the essence of bioremediation and its potential and actual application, it should be pointed out that specialists (Lovley, 2003) define bioremediation as a process that uses microorganisms, fungi, green plants, or their enzymes to return the natural environment, which has been changed in the result of contamination, to its original, pure condition. On analyzing the use of bioremediation, it is primarily necessary to underline that this process emerges due the use of microorganisms, bacteria, which can accelerate bioremediation and contribute to the restoration of the natural environment.

In this respect, it should be said that enzymatic bioremediation is potentially a rapid method of removing environmental pollutants, such as pesticides residues, for instance. The practical application of bioremediation can involve the treatment of residues resulting from any activities leading to the contamination, for instance, agricultural production and processing industries, such as the treatment of irrigation waters, surface-contaminated fruit and vegetables, and spent deep liquors (Diaz, 2008). At the same time, it is obvious that the potential of the application of bioremediation is huge. In such a context, the understanding of the directed evolution of bacteria for bioremediation becomes particularly important and researches are needed to improve the existing approaches to such an evolution.

Basically, it should be said that the directed evolution of bacteria for bioremediation is also needed because many chemicals contain structural elements of substituents that do not occur in nature (Heider and Rabus, 2008). Therefore, nature cannot recover itself from the contamination and the assistance of scientists is needed. Specialists (Diaz, 2008) presume that because of the novelty of these compounds, microorganisms have not evolved appropriate metabolic pathways for them. Moreover, for some xenobiotics no derivative routes have been described, others are transformed incompletely or inefficiently. In such a situation, an efficient degradation is needed, which involves several factors, such as bioavailability of the substrates. Therefore, bioremediation can be an effective instrument which can accelerate the degradation of contaminated elements.

In fact, one of the major reasons for the prolonged persistence of hydrophobic organic compounds in the environment is their solubiliztion-limited bioavailability (Parales, 2008). A possible way to enhance their bioavailability and, therefore, their biodegradation is the application of biosurfactants, molecules that consist of both hydrophilic and hydrophobic part (Lovley, 2003). Reports on the efficacy of surfactants on bioremediation have been mixed. On the one hand, the natural roles of biosurfactants have been claimed to increase the surface area of hydrophobic, water-insoluble growth substrates, increasing their bioavailability or desorbing them from the surfaces and regulating attachment and detachment of microorganisms to and from surfaces (Lovley, 2003). In such a way, the net effect of a surfactant on biodegradation depends on the benefits that result from enhanced solubility of target compounds versus the reduction of their adhesion of bacteria to those compounds. On the other hand, some researches (Meyer and Panke, 2008) showed that the addition of surfactants reduced bacterial adhesion to the surface of non-aqueous phase liquids and, concomitantly, growth on anthracene. Obviously, the contrasting effects of surfactant application is a result of the poorly understood complexity of interaction between soil, pollutant, surfactant, and microorganisms in different environments. The recent observations (Parales, 2008) that single surfactants can have contrasting effects on the degradation of organic pollutants can explain why the application of surfactants have yielded inconclusive results.

In such a situation, it is necessary to take into consideration properties of bacteria important for bioremediation. In this respect, it should be said that various environmental contaminants are highly hydrophobic. They are toxic for microorganisms because they accumulate in and disrupt cell membranes, inactivate the cell and thereby abolish the desired biodegradative activity, even in microorganisms capable of biodegradation (Heider and Rabus, 2008). Several bacteria resistant to solvents have been isolated and possible organisms of organic solvent tolerance, such as alterations in the composition of the cytoplasmic and outer membranes, as well as cell surface have been reported (Heider and Rabus, 2008). The cis to trans isomerization of fatty acids is one of the adaptive mechanisms. Because of the higher rigidity of trans fatty acids, the membrane is less susceptible to the structural disturbances caused by the organic solvent. The gene encoding the enzyme responsible for the cis to trans isomerization of fatty acids has now been cloned and characterized (Heider and Rabus, 2008).

An increase biosynthesis of phospholipids has also been observed in solvent-tolerant microorganisms. Studies comparing the solvent-tolerant wild type with solvent-tolerant mutants (Heider and Rabus, 2008) have shown that low cell-surface hydrophobicity serves as a defense mechanism that prevents the accumulation of organic solvent molecules in the membrane. In addition to these adaptive changes, active mechanisms, such as presence of solvent efflux pump systems, contribute to organic solvent tolerance (Parales, 2008). The elimination of higher solvent concentration through its effective degradation was found not to be responsible for solvent tolerance (Heider and Rabus, 2008). Nevertheless, some of the isolated solvent-tolerant bacteria are also capable of mineralization of, for example, toluene, and the catabolic potential can be engineered to include substrates previously not mineralized by the given organism (Parales, 2008).

Furthermore, a new approach to equip bacteria adapted to a certain environment with a new catabolic potential was used to construct a recombinant Deinococcus radiodurans capable of oxidizing toluene and chlorobenzene in highly irradiating environments (Parales, 2008). In the result of the research, it became obvious that either the organisms themselves or the enzymes and information acquired thereof can help in optimizing future bioremediation efforts.

Thus, taking into account all above mentioned, it is possible to conclude that the directed evolution of bacteria for bioremediation can be very effective since such an evolution of bacteria can enlarge opportunities for the practical application of bioremediation. Nevertheless, the further researches are needed to understand the full potential and effects of directed evolution of bacteria for bioremediation.

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