Pseudomonas putida: An Environment Friendly Bacterium 129

8.3 Degradation of Pesticides

Advances in biodegradation of toxic pesticides using microorganisms are being made with prior

knowledge of microbial metabolism and genetic techniques. Liquid cultures are used to test

microbes for the optimization of temperature, pH, organic nutrients, substrate bioavailability and

concentration that play an important role in bioremediation in soil. But at the same time, this is

not enough to overcome the problem of pesticide pollution. Using genetic techniques, genes from

microbes with desired phenotypes should be transferred to industrially useful strains. As a result,

critical enzymes for bioremediation can be economically and successfully produced. Non-native

microbial bioremediators are used in bioaugmentation. It can degrade hydrocarbons, pesticides,

explosives and other substances (Monga et al. 2021). The number of soil microorganisms, their

enzymatic capabilities and the pace of pesticide degradation in response to changes in soil conditions

play a major role in the elimination of pesticides from the environment (for example, pH). Pesticide

degrading genes are usually found on plasmids. Carbofuran, atrazine, 2,4D and parathion degrading

genes have been identified (Gong et al. 2016, Neumann et al. 2004, Dejonghe et al. 2000, Rani and

Lalithakumari 1994). In addition, a deeper understanding of microbial genomes and proteomics can

facilitate the manipulation of microorganisms for the purpose of more effective bioremediation of

contaminants (Kondakova and Cronan 2019, Mandalakis et al. 2013).

8.3.1 Organophosphates Pesticides

Organophosphate Pesticides (OP) are insecticides and herbicides, and have contaminated

groundwater. They are strong cholinesterase inhibitors and thus toxic to humans (Serdar et al.

1989, DeFrank 1991). OPs are neurotoxins that inhibit acetylcholine esterase, causing the CNS

function to be disrupted, paralysis of the muscle and death (Carvalho et al. 2003). As a result,

they are more toxic to vertebrates (Malhat and Nasr 2011). To detoxify OP, chemical treatment,

photodecomposition, incineration and other methods can be used, but they are not environmentally

friendly as they produce organophosphate triesterases, which could be toxic. On the other hand,

microbial detoxification is cheap and environment friendly. Bacteria such as Flavobacterium sp.

(Yoshida and Sathunathan 1973), Arthobacter sp. (Mallick et al. 1999) and P. diminuta are the

prominent Ops degraders (Serdar et al. 1989). Similarly, the soil microorganism P. putida was also

found to mineralize OP, parathion (DeFrank 1991). Parathion is a well-known example of OP, which

is degraded into diethyl thiophosphate (DETP) and para-nitrophenol (PNP). DETP is non-toxic, and

it can be metabolized by microbial cultures (Shelton and Somich 1988). This reaction is catalyzed

by organophosphate (encoded by the opd gene). As PNP is carcinogenic, it should be metabolized.

Bacillus, Arthrobacter (Bhushan et al. 2000), and Pseudomonas perform this function (Bang 1997).

Pseudomonas sp. ENV2030 has operons that encode PNP transformation to beta-ketoadipate

via hydroquinone (Zylstra et al. 2000). To reduce the concentration of parathion, opd and PNP

degrading operons are introduced into P. putida KT2442. The organophosphorus hydrolase (OPH)

enzyme, which is encoded by the oph gene, the mpd gene and/or the opd gene in different strains,

aids in the detoxification of OPs (Bigley and Raushel 2013). Chlorpyrifos (O, O-diethyl O-(3,5,6­

trichloro-2-pyridyl phosphorothioate) is a pesticide used in both the home and agriculture. The oph

gene oversees the chlorpyrifos degradation. Chemical mutations by ethylmethane sulphonate (EMS)

have been used to improve the yield of bacterial enzymes in order to improve bioremediation by

bacterial strains. AchM15 and AchMS1 mutants degraded 63 and 82.03% chlorpyrifos, respectively,

compared to 35% in the wild type (El-sayed et al. 2019).

8.3.2 Benzene, Toluene and p-xylene (BTX)

BTX indicates the presence of benzene, toluene and p-xylene. These water-soluble petroleum

components have poisoned the most of the bodies of potable water. A single microbe can mineralize