Keywords

1 Introduction

The term ‘bioremediation’ means transformation of a chemical compound from highly complicated form (organic) to simple (inorganic) form through biological means. If we say a compound is biodegradable, it means that it can be converted into various inorganic forms or can be mineralized i.e., possible to convert into carbon dioxide and water.

Xenobiotics are the chemical compounds, which are manufactured by human and their degradation is not possible. These are foreign compounds, when collected in the atmosphere pose dangerous effects on the atmosphere [1,2,3]. Examples of xenobiotics are pesticides, fuels, solvents, alkanes, polycyclic hydrocarbons (PAH’s), antibiotics, synthetic azo dyes, pollutants, polyaromatic, chlorinated and nitro aromatic compounds [4]. Moreover, certain compounds are recalcitrant i.e. the compound are resistant to being broken down through chemical process because of the presence of halogen, nitro or sulphonyl groups.

The major examples of xenobiotics are DDT and halogenated aromatic compounds because they pose hazardous impact on the atmosphere. Although, many such compounds are highly beneficial to the humans but their long term presence in the environment cause deleterious effects.

Recalcitrant toxic pesticides such as BHC, PCBs, and DDT are not easily biodegraded and their concentration goes on increasing in the soil and water with time. For example, there is a continuous increase in the concentration of DDT at successive trophic levels in food chain. Model compounds of DDT such as diphenymethane and dichlorodiphenylmethane are biodegrade by various microbes. Pseudomonas putida converts DDT to several compounds under anaerobic conditions. In aerobic conditions, ring cleavage yield p-chlorophenylacetic acid that in turn served as a substrate for an Arthrobactor species. A number of reviews have been available on the bioremediation of xenobiotics [4,5,6,7].

2 Basis of Bioremediation

Microbes play the most important role in the process of biodegradation. Certain abiotic mechanisms and photo-oxidation also play an important role in the degradation of certain organic chemicals but such transformations are generally incomplete because these processes cannot convert the compound into inorganic form (Fig. 1).

Fig. 1
figure 1

Classification of different xenobiotic compounds

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Before the industrialization, biosphere of earth remained constant because of more or less balanced biosynthesis and biodegradation reactions. During the course of evolution, a vast variety of chemical compounds are biosynthesized in nature and microbes were exposed to these compounds. For this millions of year’s exposure, they have developed the capacity and mechanism to attack these compounds. Out of several chemical compounds synthesized by chemists, many have structural features and bonding similar to that of natural compounds, so can be biodegraded.

The xenobiotic compounds having recalcitrant characteristics have following features

  1. (i)

    These compounds are not recognized by the microorganisms as substrate. So, can’t be degraded by them.

  2. (ii)

    For their transportation into the microbial cell, they do not have permease enzyme.

  3. (iii)

    These are complex molecules. So, very difficult to enter inside the microbial cell.

  4. (iv)

    These compounds are highly stable in the nature. In addition to this they are insoluble in water.

  5. (v)

    Xenobiotic compounds are very poisonous in nature.

On the basis of chemical composition, the recalcitrant xenobiotic compounds can be divided into the following types (Fig. 2).

Fig. 2
figure 2

Types of recalcitrant xenobiotic compounds

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2.1 Halocarbons

These compounds are mainly used in the preparation of pesticides, insecticides etc. These are volatile, when free into the environment cause damage to the ozone layer and when gathered in soil, leading to biomagnifications.

2.2 Polychlorinated Biphenyls (PCBs)

These compounds are inert in nature and are used in the manufacturing of plasticisers, insulator coolants in transformers etc.

2.3 Synthetic Polymers

These are high molecular weight compounds, which are insoluble in water and used in the manufacturing of plastics like polymer, polyvinylchloride.

2.4 Alkylbenzyl Sulphonates

These compounds are not degraded by microorganisms because of the presence of sulphonate group. They are used in the manufacturing of detergents.

2.5 Oil Mixtures

Due to accidental oil spilling in the ocean, oil spread over the water surface and its breakdown by microorganisms results ineffective. Oil is insoluble in water. So, it becomes recalcitrant.

From this discussion, it has been clear that the recalcitrant characteristic of a xenobiotic compound is directly connected with its complexity and it is also noted that recalcitrant property is enhanced as their complexity increased.

3 Hazards from Xenobiotic Compounds

The impacts of xenobiotic compounds are very hazardous in nature. Their dangerous effect is not only on lower eukaryotes but higher eukaryotes are also affected. They are very toxic to human beings also. Their exposure may cause certain skin diseases and prolong exposure may cause cancer also. Because they posses recalcitrant property, they are not degraded in the nature and may remain persist for long time in the environment may result in the form of bioaccumulation or biomagnifications. In bioaccumulation or biomagnifications, they enter into the food chain and their concentration will increase with the increase in the trophic level of that ecosystem.

4 Removal of Xenobiotics

Unluckily, xenobiotics are used widely among the modern society. To survive the better life, many industries—manufactured different chemical compounds but majority of them are poisonous and toxic for the living beings. e.g. the use of plastics, pesticides, paints, textiles and pharmaceuticals can not be eradicated from our daily needs but now the time has come that we should aware to overcome this huge problem and protect our future from the curse of xenobiotics. One of the most efficient tools to get rid of this problem is the use of microorganisms that may degrade these toxic chemicals from the atmosphere. Because microorganisms are easily grow and multiply in laboratory. So, they can be a useful device for the degradation of such chemical compounds. The enzymes present in these microorganisms are able to degrade these chemical compounds. The enzymes present in these microbes are able to degrade these chemicals into harmless end products. Different types of microorganisms are used to degrade different type of waste but unfortunately there are some artificially human made chemical compounds, which are not degraded by the microorganisms because they are not able to break certain chemical bonds present in the chemical compounds.

One of the recent branches of the science i.e. genetic engineering makes it possible to construct such type of microorganisms, that able to degrade these xenobiotic compounds. Among such microorganisms Pseudomonas genus are mostly employed for their degradation. Such bacteria have extra chromosomal DNA i.e. TOL plasmid, that have the capacity to degrade toluene and xylene.

There is one more approach to degrade these xenobiotic compounds by using the combined efforts of human and microbial activity. In this technique the recalcitrant chemicals are first break into smaller parts and then subjected to microbial degradation. An example is the accidental oil spilling, which is very dangerous for the life of aquatic organisms. By the combining efforts of human and microbes, aquatic organisms may survive but if the area of oil spill is very vast, then the rate of oil degradation by microbes becomes very slow due to the toxicity of oil. But now, recently a chemical is used for solving this problem.

A chemical name SOT II (solid oil treatment) is an inorganic solid absorbent, which is chemically inert. Whenever a oil spill occurs, this chemical is spread over the oil spill area. Since it is a strong absorbent, it will compact in small particles with oil and sink at the bottom of the sea. Due to the property of strong absorbent SOT II particles can’t leave the oil spill. As the superficial layer of oil has removed from the water, the living organism survives as they come in direct contact with oxygen and sunlight. The particles of SOT II with oil, which sinks at the bottom of sea will become the food of for oil degrading organisms i.e. bacteria, algae, fungi and protozoa. This process takes place rapidly at the bottom of the sea and within a week these oil degrading organisms degrade the oil with the production of end products i.e. water and CO2.

This technique shows 99% success in the removal of oil from the sea and its degradation and elimination from the ecosystem.

As we all know, xenobiotics are the toxins and detoxification is a successful remedy to neutralize these toxins. In detoxification process, the negative impact of these toxins can be minimized by planning the detox diet. The main objective of detox diet is to eradicate these toxins because this diet contains 40% solid and 60% liquids. Solid food contains certain phytochemicals e.g. indole, flavanoids etc. e.g. pineapple contains bromelain. Papaya contains papain, which is a colon cleaner and improves immune system. Consumption of raw food such as sprouts, dried fruits, nuts, milk etc. are also the examples of detoxification because these foods contains antioxidants that enhance the immune system of our body.

5 Microbial Degradation of Xenobiotics

To overcome the needs of increasing population, the use of pesticide in modern agriculture is increasing day by day but the results are very dangerous because these pesticides persist in the ecosystem and don’t undergo biological transformation (Table 1).

Table 1 Duration of persistence of insecticides and herbicides in soil
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In the degradation of xenobiotics compounds, the role of microorganisms is very important because these microorganisms degrade the xenobiotics into the simpler or non hazardous compounds. Among these microbes, bacteria have the noble detoxifying abilities. Examples of aerobic and anaerobic xenobiotics degradative bacteria are Pseudomonas, Gordonia, Bacillus, Moraxella, Micrococcus, Escherichia, Sphingobium, Pandoraea, Rhodococcus, and anaerobic xenobiotics degradative bacteria are Pelatomaculum, Desulphovibrio, Methanospirillum, Methanosaeta desulfotomaculum, Syntrophobacter, Syntrophus [6, 9]. Among them, Pseudomonas species and Bacillus species have been the most widely studied. Many other bacterial species which assist in degradation of recalcitrant xenobiotic compounds are listed in Table 2.

Table 2 List of xenobiotic compounds degraded by microorganisms
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In the present scenario, the problem of xenobiotics remediation can be sorted out by the use of microorganisms that play a key role in the degradation of these xenobiotic compounds. An example of xenobiotics degradation is pentachlorophenol (PCP) that is a broad spectrum biocide, which has been used as fungicide, insecticide, herbicide, algicide, disinfectant and antifouling agent.

Bioreactors containing alginate immobilized along with Polyurethane foam immobilized PCP degrading Flavobacterium (ATCC39723) cells have been used to remove PCP from contaminated water. Absorption of PCP by Polyurethane immobilized matrix plays a role in reducing the toxicity of PCP (Fig. 3).

Fig. 3
figure 3

Outline of aerobic and anaerobic degradation of pentachlorophenol

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6 Conclusion

Xenobiotic compounds present a big problem in the environment but microorganisms play an important role in their bioremediation. From this paper, it has been clear that oil spilling is the world’s greatest problem but with the help of genetically modified microorganisms, these oil spills can be cleaned up within few days. So, from this review, it may be concluded that microbial degradation is an important tool for the remediation of different xenobiotic compounds.