[ashwathi]

Man made chemicals present in the nature at high concentrations polluting the environment is known as Xenobiotic compounds. These compounds are not commonly produced by nature. Some microbes have been seen to be capable of breaking down of xenobiotics to some extent. But most of the xenobiotic compounds are non degradable in nature. Such compounds are known to be recalcitrant in nature.

The properties of xenobiotic compounds attributing to its recalcitrant properties are:

(i) Non recognizable as substrate by microbes to act upon and degrade it.
(ii) It does not contain permease which is needed for transport into microbial cell.
(iii) Large molecular nature makes it difficult to enter microbial cell.
(iv) They are highly stable and insolubility to water adds to this property.
(v) Mostly toxic in nature.
The recalcitrant xenobiotic compounds can be divided into different groups depending on their chemical composition

Halocarbons: They consist of halogen group in their structure. Mainly used in solvents, pesticides, propellants etc. They are highly volatile and escape into nature leading to destruction of ozone layer of atmosphere. The compounds present in insecticides, pesticides etc,. leach into soil where they accumulate and result in biomagnification.
Polychlorinated biphenyls (PCBs): They consist of a halogen group and benzene ring. They are mainly used in plasticisers, insulator coolants in transformers etc. They are chemically and biologically inert adding on to its recalcitrant nature.

Synthetic polymers: These are mainly used to form plastics like polyester, polyvinyl chloride etc. They are insoluble in water and of high molecular weight explaining the recalcitrant property.

Alkylbenzyl Sulphonates: They consist of a sulphonate group which resists break down by microbes. They are mostly found in detergents.

Oil mixtures: When oil spills occur covering a huge area the break down by action of microbes becomes non effective. They become recalcitrant as they are insoluble in water and some components of certain oils are toxic in higher concentrations.

The recalcitrant property of xenobiotic compound is directly linked to its complexity so that the higher the complexity the stronger recalcitrant property.

Hazards posed by xenobiotic compounds
The hazards posed by xenobiotics are huge. These compounds are highly toxic in nature and can affect survival of lower as well as higher eukaryotes. It also poses health hazards to humans like various skin problems, reproductively and even known as a trigger for causing cancer. These compounds are persistent and remain in the environment for many years leading to bioaccumulation or biomagnification. They also find a way into the food chains and the concentrations of such compounds was found to be high even in organisms that do not come in contact with xenobiotics directly.

Mechanisms involved in biodegradation of xenobiotics
Xenobiotic compounds, owing to its recalcitrant nature, is hard to break down and degrade. The complexity of its chemical composition adds to this. For breaking down such compounds the enzymes act on certain groups present in the compound. For eg: in the halocarbons the halogen group is targeted. Enzymes like oxygenases play a major role. The bonds like ester-, amide-, or ether bonds present in the compounds are first attacked leading to breaking down of compounds. In some cases the aliphatic chains and in aromatic compounds the aromatic components may be targeted. The site and mode of attack depends on the action of enzyme, its concentration and the favourable conditions. Often it is seen that the xenobiotics do not act as a source of energy to microbes and as a result they are not degraded. The presence of a suitable substrate induces its breakdown. This substrate is known as co – metabolite and the process of degradation are known as co metabolism. In another process, the xenobiotics serve as substrates and are acted upon to release energy. This is called gratuitous metabolism.

Biodegradation:
Certain microbes on continuous exposure to xenobiotics develop the ability to degrade the same as a result of mutations. Mutations resulted in modification of gene of microbes so that the active site of enzymes is modified to show increased affinity to xenobiotics. Certain mutations also resulted in developing new enzymatic pathway for xenobiotic degradation. Use of mixed population of microbes is usually recommended as it has been seen to yield faster results as the two different microbes attack different parts through different mechanisms resulting in effective break down. It also creates a condition of co metabolism. The modification of certain genes of microbes to break down xenobiotics is also recommended and is seen to produce high level of accuracy.