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Xenobiotic Compounds and their Biodegradation
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.

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.
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Elimination of xenobiotics

Unfortunately, xenobiotics are part of human life in the modern society. Different industries develop and produce various compounds that should improve human’s life, but most of them are highly toxic and consequences to the biomes around the globe are usually terrible. Xenobiotics associated with production of plastics, pesticides, paints, textile and pharmaceuticals can’t be eliminated from our lives, but we should put more effort in elimination of toxic materials before our planet turn into one big bag of waste.

Microorganisms have always been the easiest and most efficient tool used for elimination of abundant or harmful chemicals from our environment. They can be found all around the planet, they are easily grown in laboratory conditions, scientists are familiar with their affinities and characteristics, they possess needed enzymes and they enjoy “eating” chemical waste. They can be used for different types of organic waste, but some artificially created compounds are simply un-digestible: microorganisms are not metabolically “equipped” to break down chemical bonds in compounds of interest. Luckily, another scientific branch, genetic engineering, allowed scientist to modify microorganisms and turn them into organisms able to degrade xenobiotics. Genetically modified Pseudomonas species are mostly used in the process of bioremediation. Pseudomonas harboring TOL plasmid, for example, degrades toluene, m-xylene and p-xylene. Insertion of bioluminescence gene allows researchers to monitor Pseudomonas during bioremediation.

Another way to help nature get rid of all those waste materials could be achieved by combining human and microbial activity. Chemicals that can’t be degraded by microorganisms directly, can be chemically altered to their smaller or easier-to-break parts and then degraded using already known microorganisms. Combination of human and microbial activity could be a life savior in the case of accidental oil spills that have devastating consequences on the marine organisms. Large oil spill can’t be degraded solely by microorganisms due to large quantities of spilled oil and because oil contains some toxic ingredients. Newly invented chemical could solve this problem.

SOT 11 (SOT stands for solid oil treatment) is an inorganic solid absorbent that consists of nontoxic mineral granules, which are chemically inert. This chemical is developed by Oil Treatment International (OTI), a Swiss based company, over a period of 20 years. How this mineral mixture works? Since it acts as strong absorbent, once it is deposited over the spill, oil will compact in small particles that will (because of the gravity) sink to the bottom of the sea. Due to firm bond between oil and SOT 11, oil can’t leave this complex and enter the water or water surface again. Elimination of the superficial oil film allows animals to come in contact with oxygen and establish normal breathing again. Also, removal of the spill will allow sunlight to reach underwater photosynthetic organisms (without light, they can’t produce food), and prevent animals that spend their life both on the land and in the water to end up covered in oil. Birds can’t fly if their feather is glued in oil, and it has toxic effect when animal swallow it. What is happening with oil-absorbent particles once they precipitate on the bottom of the ocean? Closer examination showed that living creatures on the marine floor don't change their normal life activities during this process. Once on the bottom, shredded in small pieces, oil-SOT 11 will become food for the oil-digesting organisms such as fungi, algae, protozoa and bacteria. Digestion includes several species that are active in different stages because chemical degradation of oil happens in cascades. Low level of oxygen on the bottom is not a problem, because oil degradation can happen both under aerobic and anaerobic conditions. Whole process happens really quickly and end products are water and carbon dioxide. Since this method is still new and needs to be assessed for both safety and efficacy, regulatory body requested regular checks of the underwater processes that are happening after the SOT 11 is added. Sampling of the sand from the sea bottom showed that almost all oil-SOT 11 particles vanished (digested by the sea microorganisms) within a week. This result is amazing and when compared to absorbent non-treated area, it showed 99% more success in oil removal and its final degradation & elimination from the ecosystem.

Oil spills are dangerous and unfortunately, often ecological disasters. Although there isn’t a way to prevent them from happening, at least we can help ecosystem recuperate after oil is spilled.
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Xenobiotics are toxins and there are various ways to neutralize the effect of these toxins.
Detoxification is a process that decreases the negative impact of xenobiotics or toxins, on the body.
Practicing detoxification cleanses the body from within and reduces the side effects of xenobiotics. The best practice is to plan detox diet. The first action of such practices is on the accumulated toxins within body. The good detox plan is that which contains 60 per cent liquids and 40 percent solids. Solid food means the one that contains phyto chemicals such as indoles flavonoids, nutrients and enzymes. Example are Bromalain which is found in Pineapple. Papain which is in papaya which act as colon cleanser and helps in improving the digestive system.
Apart from above mentioned things, the detox diet may involve periods of fasting, consuming only fluids and eliminating heavy foods. Sometimes syrups are also used. Example Maple syrup provides energy that can be absorbed easily in the body while it also cleanses the toxins found within the body. This way it obliterates cravings for junk food, alcohol and other toxins.
Eating raw food is another way of detoxification. It is natural way of eating and it avoids processed food and preserved foods intake. Examples of raw food are sprouts, nuts, dried fruits, raw milk, carrots, and various fruits. This food contains antioxidants and other essential substances which strengthens immune system. Another method is to eat only one type of fruit. Examples are Apple, Grapefruit, Bananas and Melons. Other such fruits can also be considered. The fruit supply organic water and helps in cleansing. Fruit consumption can be easily digested and this helps body to repair the internal system. These are various detoxification methods and should be followed as per doctor’s recommendations only.
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Most notorious persistent organic pollutants (POPs) are called as dirty dozens: these are
1. DDT
2. Toxaphane
3. PCB's
4. Dioxins & Furans
5. Hexachloro Benzene
6. Chlordane
7. Dieldrin
8. Endrin
9. Aldrin
10. Mirex
[+] 1 user Likes ajithnandanam's post
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Xenobiotic compounds are man-made chemicals that are present in the atmosphere at unusually high concentrations. Microorganisms are able to digest most of the naturally occurring xenobiotic compounds and this property is called as microbial infallibility. Those xenobiotic compounds that resist digestion from even microbes are called recalcitrant.

General Features of Xenobiotic Degradation:

Since xenobiotics consist of a wide variety of compounds, their degradation occurs via a large number of metabolic pathways.

Degradation of alkanes and aromatic hydrocarbons generally occurs as follows:
1. An oxygenase first introduces a hydroxyl group to make the compound reactive
2. The hydroxyl group is then oxidized to a carboxyl group
3. The ring structure is opened up in case of cyclic compounds
4. The linear molecule is degraded by beta oxidation to yield acetyl-CoA, which is then utilized in the usual manner to carbon dioxide.

Similarly, an alicyclic hydrocarbon e.g. cyclohexane is oxidized as follows:
1. First an oxygenase adds a –OH group in the ring
2. Then another oxygenase forms an ester in the form of a lacone
3. The lactone is then hydrolyzed to open the ring structure to give a linear molecule

In both these oxidations, mono-oxygenases are involved, which add oxygen to a single position in the molecule. In contrast, oxidation of benzene ring may involve a di-oxygenase, which adds oxygen at two positions in the molecule in a single step.

Both mono- and di-oxygenases are of a variety of types: some react best with short chain alkanes while others act on cyclic alkanes. But these enzymes are not very specific and each enzyme oxidizes a limited range of compounds. Thus xenobiotics are degraded by a wide variety of microorganisms, each of which degrades a small range of compounds. Frequently, oxidation of xenobiotic compounds involves cytochrome P450 or rubredoxin. In addition, the halogens and/or other substituent groups are either modified or removed usually as one of the initial reactions or sometimes it is achieved later in the process.
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Chemicals that are foreign to the biosphere are known as xenobiotic compounds. Release of chemical substances due to rapid industrial progress has now become a serious problem causing environmental pollution. Pollutants resembling structural features of xenobiotics mostly include organic sulfonic acids, halogenated aliphatic and polycyclic aromatic hydrocarbons, s-triazines, nitroaromatic compounds, azo compounds and synthetic polymers. Over the years huge quantity of hazardous waste sites is being generated throughout the world due to accumulation of xenobiotic compounds in soil and water. Polycyclic aromatics, nitroaromatic compounds (NACs), and other hydrocarbons (PAHs) constituting crude oil, are among the diverse group of xenobiotic chemicals responsible for immense environmental pollution. The conventional physico-chemical strategies for remediation of xenobiotics to clean up contaminated sites are not quite costly and adequate. Therefore, research focused on biodegradation and elimination of these hazardous compounds are gaining importance. The process of biodegradation wherein Xenobiotic contaminated sites are remediated by means of bacteriological actions, utilizes the capability of microorganisms in reducing the toxicity and concentration of a large number of Xenobiotic pollutants. This is an ecofriendly and economical and efficient treatment technique that is developing quite rapidly in the field of environmental restoration. Bacteria and fungi can utilize the xenobiotic compounds as substrates, by mineralizing or converting them into less toxic products. Microorganisms gain access to xenobiotics pollutants depending on the availability of these compounds in air, water, soil various environmental compartments. Transformation and degradation of xenobiotic residues are mostly carried out by microorganisms. Bioremediation is a major mechanism in aquatic and terrestrial environments, and provides the very foundation of the modern wastewater treatment plants. Biodegradation performance in the natural habitat is affected and controlled by the physicochemical properties of the environment. Micropore entrapment as well as soil accumulation are the major causes xenobiotic persistence. Both aerobic and anaerobic bacteria, and fungi are effectively involved in the biodegradation of xenobiotics. These microbial transformations are sometimes fortuitous, a phenomenon quite common in microbiology. Xenobiotic compounds are often utilized as a source of energy, nitrogen, carbon, or sulfur by various microorganisms. But few xenobiotics are there that are resistant to microbial attack. Therefore, research should focus on understanding mechanism of the interaction between microorganisms and xenobiotic compounds in the environment that must include biochemical as well as genetic engineering areas. Such effective strategies then can play a pivotal role for successful environmental clean up by a diverse group of microorganisms for efficient remediation of toxic xenobiotic compounds.
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1. Can any one tell what is the Status of recalcitrant/xenobiotic compounds in water bodies in India?
2. How to measure recalcitrant compounds?

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