XENOBIOTICS

XENOBIOTICS

• The term Xenobiotic comes from the Greek for xeno (foreign) and biotics (of or pertaining to life).
• Xenobiotic compounds are man­ made chemicals that are present in the environment at higher concentration. These are either not produced naturally or are produced at much lower amount than that by man.
• Microorganism have the capability of degrading all naturally occurring compounds; this is known as “The principle of Microbial Infallibility” proposed by Alexander in 1965.

Recalcitrant

• The Xenobiotic compounds that resist biodegradation and persist in the environment for long period of time, so they are called as Recalcitrant.
• The Xenobiotic compounds may be recalcitrant due to one or more of the following reasons

ü  Not recognized as substrate by degradative microorganisms and enzymes.

ü  Highly stable in nature

ü  Insoluble in water

ü  Unusual bonds or bond sequences (as in tertiary and quaternary compounds)

ü  Unusual substitution (as with chlorine and other halogens)

ü  Highly toxic or release toxic products due to microbial activity.

ü  Large molecular weight which prevents entry to microbial cells (as in polyethylene and other plastics).

Types of Recalcitrant Xenobiotic Compounds

• Halocarbons – Halogens, Insecticides, Herbicides, Freons in condenser of Cooling system and Propellants in spray cans of Cosmetics & Paints.
• Poly Chlorinated Biphenyls (PCBs) – Plasticizers and Insulator coolants.
• Synthetic polymers - Polyethylene, Polystyrene, Polyvinyl chloride and Nylons.
• Alkylbenzyl Sulphonates – Surface ­active detergents which are found to be superior compare to soaps.
• Oil mixtures – Oil spills
• Other Xenobiotic compounds - More number of pesticides consists of aliphatic, cyclic ring structures containing replacement of nitro­, sulphonate, methoxy­, amino and carbomyl groups in addition to halogen group. These modifications make them recalcitrant.

Hazards from Xenobiotic compounds

• Toxicity
• Carcinogenicity
• Many xenobiotics are recalcitrant and persist in the environment so that there is a build-up in their concentration with time.
• Many xenobiotics including DDT and PCB’s are recalcitrant and lipophilic; as a consequence they show bioaccumulation or bio-magnification.

Practical application of Microorganisms for Xenobiotic degradation

• Practical application of microbes for xenobiotic degradation is facilitated by

ü  Supply of sufficient nutrients or co-metabolites.

ü  Maintenance of the xenobiotic compounds to non-toxic levels.

• Provision of microbial population or inoculum