Xenobiotic (Gk. xenos = foreign and biotics = of or pertaining to life) compounds are man-made chemicals that are present in the environment at unnaturally high concentrations, toxic and detrimental to animal life and our natural ecosystem. The xenobiotic character means that their structures and functions are not easily understood by existing degradative enzymes, which results in their huge accumulation in the environment. Many kinds of xenobiotics occur in the waste effluents produced by the manufacture and consumption of commonly used synthetic products
from pharmaceutical, textile and food industries.
Xenobiotics, majorly released into the environment are numerous different halogenated aliphatic and aromatic compounds, polychlorinated biphenyls (PCBs), alkyl-benzyl sulphonates, nitro- aromatics, and polycyclic aromatic hydrocarbons. These toxic compounds often enter the environment as components of pesticides, fertilizers, persistent organic compounds (POPs), drugs, pharmaceuticals and personal care products (PPCPs) and herbicides and are progressively becoming more concentrated in each link of a food chain, a process known as bio-magnification. An example would be the increased use of DDT as a pesticide (a health hazard banned from US in 1972) which was recognised as a carcinogen and had implications in the reproductive failure as observed in birds. The aquatic life is profoundly more predisposed to the ill effects of environmental accumulation of these compounds in wastewater. Accidental oil spills are one of the major sources of marine pollution due to this. It has been observed that the biochemical and physiological properties of soil get altered hugely which again negatively impacts the survival and sustainability of plants and other living organisms.
Fundamental research has revealed that various microorganisms are capable of degrading a wide variety of organic pollutants in the environment. Microbial metabolism is generally regarded as the most important mechanism of xenobiotic degradation in soil, and it constitutes the basis for all bioremediation, bio-reclamation and bioaugmentation strategies. Microorganisms are generally able to degrade many of the xenobiotic compounds, but there are compounds that resist biodegradation and thereby persist in the environment called recalcitrant xenobiotic compounds.Environmental conditions that favor microbial growth and activity in soil, such as temperature, humidity and moisture, nutrient availability, pH, oxygen availability will also generally promote the metabolic degradation of xenobiotic compounds.
Bioremediation techniques aim to accelerate the naturally occurring biodegradation process by optimizing these crucial environmental conditions. Mechanisms of bioremediation include both genetically engineering microorganisms and isolating the naturally occurring xenobiotic degrading microbes. The genes responsible for degradation and xenobiotic metabolism are generally encoded by transposons, plasmids or even cluster genes grouped in the chromosomes. Research has been conducted to identify these genes responsible for the ability of microorganisms to metabolize certain xenobiotics which in turn can be used to genetically manipulate and engineer microorganisms specifically for this purpose.
A significant drawback observed is that in some cases a single microorganism may not be totally capable of performing all metabolic processes required for degradation of a xenobiotic compound and so “syntrophic bacterial consortia” are utilised where a group of bacteria work together in conjunction, resulting in dead end products from one organism being further degraded by another organism and in this way the entire product undergoes degradation. In depth research of metabolic pathways for the biodegradation of specific organic compounds as well as well as other related remediation technologies will make purposeful application of biodegradation of xenobiotics
possible, evading their transformation to hazardous and toxic compounds, unsafe for the environment.
Conclusion– With rapid increase in urbanization and population growth, the indirect increase in xenobiotic contamination in food and water is also much anticipated. Even though there are physical as well as microbiological methods for detection and environmental eradication of such compounds, xenobiotic based contamination is fast becoming a peril to our natural ecosystem. The failure of microorganisms in degrading organic pollutants will open up new arenas for focused understanding and research to come up with new technology-based innovations, to determine the efficacy of these remediation techniques. Hence it is utmost need of the hour for environmentalists and researchers to come together, address this issue and develop novel preventive methods or remedial approaches to combat this crucial problem for a safer and friendlier environment.