Xenobiotic compounds are man-made chemicals that are present in the environment at unnaturally high concentrations. The xenobiotic compounds are either not produced naturally, or are produced at much lower concentrations than man. Micro-organism have the capability of degrading all naturally occurring compounds; this is known as the principle of microbial infallibility proposed by Alexander in 1965. Micro-organisms are also able to degrade many others. The compounds that resist  biodegradation and thereby persist in the environment are called recalcitrant. The xenobiotic compounds may be recalcitrant due to one or more of the following reasons: (i) they are not recognized as substrate by the existing degradative enzymes, (ii) 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, (iii) they are insoluble in water, or are adsorbed to external matrices like soil, (iv) they are highly toxic or give rise to toxic products due to microbial activity, (v) their large molecular size prevents entry into microbial cells, (vi) inability of the compounds to induce the synthesis of degrading enzymes, and (vii) lack of the permease needed for their transport into the microbial cells.

Types of recalcitrant xenobiotic compounds

The recalcitrant compounds can be grouped into the following 6 types: (i) halocarbons, (ii) polychlorinated biophenyls, (iii) synthetic polymers, (iv) alkylbenzyl sulphonates, (v) oil mixture and (vi) others. The structural features that make these compounds resistant to microbial degradation include the following: (i) presence of halogens in the place of hydrogen in the molecule; the carbon-halogen bond is highly stable and its cleavage requires considerable energy, (ii) substitution of H by other groups like nitro-, sulphonate, methoxy-, amino- and carbomyl groups, (iii) cyclic structures, aromatic compounds, cycloalkanes and heterocyclic compounds are more recalcitrant than linear chain or aliphatic compounds, (iv) branched linear chains resist biodegradation etc. In general, the more complex is the structure of a xenobiotic compound, the more resistant it is to biodegradation. Many other xenobiotics resist biodegradation due to their large molecular size and insolubility in water.

Hazards from xenobiotics

A number of pesticides are based on aliphatic, cyclic ring structures containing substitution of nitro-, sulphonate, methoxy-, amino- and carbomyl groups; in addition, they also contain halogens. These substitutions make them recalcitrant.

The xenobiotics present a number of potential hazards to man and the environment which are briefly listed below:

1. Toxicity: Many xenobiotics like halogenated and aromatic hydrocarbons are toxic to bacteria, lower eukaryotes and even humans. At low concentrations they may cause various skin problems and reduce reproductive potential.

2. Carcinogenicity: Certain halogenated hydrocarbons have been shown to be carcinogenic.

3. Many xenobiotics are recalcitrant and persist in the environment so that there is a buildup in their concentrations with time.

4. Many xenobiotics including DDT and PCB's are recalcitrant and lipophilic; as a consequence they show bioaccumulation or biomagnifications often by a factor of 10⁴ - 10⁶. Biomagnifications occurs mainly taken up from the two reasons: (i) these compounds are continuously taken up from the environment and accumulated in the lipid deposits of body, e.g. a 100-fold accumulation of DDT by plankton from water. (ii) such organisms are consumed by other organism in a sequential manner constituting the food chain.