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Bacteria are continually challenged with variety of synthetic chemicals/xenobiotics in their immediate surroundings, including pesticides. Chlorpyrifos is one of the most commonly used organophosphate pesticides in the world. The non-environmental strain of Escherichia coli, BL21 (DE3) displayed high tolerance to chlorpyrifos but with no/negligible degradation. The intrinsic resistance mechanisms that aid the organism in its high tolerance are probed. Efflux pumps being ubiquitous in nature and capable of conferring resistance against wide variety of xenobiotics were found to be over-expressed in the presence of CP. Also, an efflux pump inhibitor PAβN increased the susceptibility of E. coli to chlorpyrifos due to the intracellular accumulation of CP. The tripartite efflux pump EmrAB-TolC with increased expression in both transcript and protein on CP exposure, might play a major role in CP tolerance. The transcriptional regulators involved in multidrug resistance along with transporters belonging to all the major families conferring antimicrobial resistance were up-regulated. Also up-regulated were the genes involved in phopshonate metabolism and all the genes in the copper or silver export system. The common resistance mechanisms i.e, activation of efflux pumps between CP, antibacterial metals and antibiotics resistance might result in cross-resistance, ultimately increasing the prevalence of multidrug resistant strains, making infections hard to treat.

Bacteria respond to xenobiotics in their immediate environment and can either tolerate/ evade high concentrations of it, or are capable of their removal through degradation. A highly efficient and rapid chlorpyrifos (CP) degrader Pseudomonas nitroreducens AR-3 was previously reported, and the current work focused on elucidating the molecular mechanisms by which the bacterium achieves this. The bacterial response to CP included remodelling of the proteome and induction of global response pathways. Sixty-four out of the 794 significantly up-regulated proteins were specifically involved in carbohydrate metabolism, environmental and genetic information processing, biosynthesis of secondary metabolites and metabolism in diverse environments. There was an induction of (mainly) periplasmic proteins, and those involved in chemical degradation pathways. Interestingly, an unprecedented repression of stress responsive pathways and xenobiotic transporter elements were also observed, presumably due to the non-recognition of CP as a stress factor. A presumptive MBL-fold metallo-hydrolase protein showing 44% sequence similarity to an organophosphate hydrolase (ophC2) was identified from the genome. Understanding of the dynamic changes in the protein profiles shed light into the basic cellular and molecular mechanisms underlying CP degradation and enabling the organism to sustain under adverse conditions.

The indiscriminate use of pesticides leads to serious food safety and toxicity issues and threatens the environment and biodiversity. Pseudomonas nitroreducens AR-3 isolated from pesticide contaminated agricultural soil removed 97% of chlorpyrifos (CP) in just 8 h, in a mineral salt medium (MSM) containing glucose (1.0 g/L) and yeast extract (0.5 g/L) at 30 °C and 2% (v/v) inoculum when challenged with 100 mg/L CP. 3, 5, 6-trichloro 2-pyridinol (TCP), the degradation product of CP was detected only in low levels, indicating its further degradation. Organophosphate hydrolase (OPH), the enzyme considered responsible for CP degradation, had an intracellular localization. Crude OPH (1 mg/ml) removed 42% of 100 mg/L chlorpyrifos in just 2 h, indicating a rapid rate of degradation. Ultra-fast degradation of chlorpyrifos with an inducible OPH marks the potential of P. nitroreducens AR-3 for bioremediation of organophosphates. The strain AR-3 has the fastest rate of organophosphate degradation reported till date among Pseudomonads.