Biodegradation of Xenobiotic Compounds

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Xenobiotic compounds
Xenobiotics or xenobiotic compounds are human made chemicals that are present in the environment and pollute the environment when present in high concentrations. The word “xeno”means foreign.  A compound that is normal to one organism may be a xenobiotic to another. For example, antibiotics can be referred as xenobiotics in a human body because human body does not contain them or produce them naturally. Organs transplanted in foreign bodies of different species are termed as xenotransplantation.
The xenobiotic compounds are either not produced naturally, or are produced at much lower concentrations.Microorganism has the capability of degrading all naturally occurring compounds; this is known as the principle of microbial infallibility proposed by Alexander in 1965. Microorganisms are also able to degrade many of the xenobiotic compounds, but they are unable to degrade many others. 
Types of Xenobiotic compounds
There are two types of xenobiotic compounds. They may be biodegradable or non degradable (recalcitrant). Biodegradable xenobiotic compounds are those that get degraded by the action of microbes or other reactions while recalcitrant compounds are resistant to degradation by any reactions.
Types of Recalcitrant Xenobiotic Compounds
The recalcitrant xenobiotic compounds can be grouped into the following six types:
1)  halocarbons, 
2)  polychlorinated biphenyls,
3) synthetic polymers, 
4) alkylbenzyl sulphonate, 
5) oil mixture and
6) others. 
The xenobiotic compounds may be recalcitrant due to one or more of the following reasons:
1) they are not recognized as substrate by the existing degradative Enzymes,
2) they are highly stable, i.e., chemically and biologically inert due to the presence of substitution groups like halogens, nitro-, sulphonate, amino-, methoxy- and carbamyl groups (substitution of –H),                                                                                          
3) they are insoluble in water, or are adsorbed to external matrices like soil,
4) they are highly toxic or give rise to toxic products due to microbial activity,
5) their large molecular size prevents entry into microbial cells,
6) inability of the compounds to induce the synthesis of degrading Enzymes, and
7) lack of the permease needed for their transport into the microbial cells.
Xenobiotics and Environment
Xenobiotics pose threat to the environment. They pollute the environment as some of them are recalcitrant. Synthetic polymers such as plastics and nylon are insoluble in water. Oil is also a pollutant; many of its compounds are biodegradable and are degraded at different rates. Oil is recalcitrant mainly because of its insolubility in water and due to the toxicity of some of its compounds.
 Biodegradation of xenobiotic compounds depends on their concentration (too high concentration may be toxic), pH of the medium, temperature, availability of water and other nutrients and presence of organic compounds (these may be co-metabolites, inhibitors or preferred substrate, in place of the xenobiotic, by microorganisms). 
In general, the xenobiotic compound should be available in an acceptable concentration and toxic levels should not occur. In a treatment system, a constant supply of the compound should be available for selective maintenance of microbes capable of its degradation.
In addition, interfering organic compounds should not be present in the environment. Practical application of microbes for xenobiotic degradation is facilitated by: 
1) supply of sufficient nutrients or co metabolites, 
2) maintenance of the xenobiotic compounds at nontoxic levels and 
3) provision of microbial inoculum.