**6. Conclusions**

Bacteria, which are present ubiquitously in nature, with a simple unicellular structure and ability to grow faster, have made their niches very dynamic, metabolically as well as genotypically plastic, and hence hot spots for the evolution of new trait(s). During the process of adaptation to strenuous conditions, microbes might acquire new genes and functions to enhance survivability. In addition, they might mutate already existing and/or acquired genes. The former approach is mediated through MGEs, which are the major players responsible for genome plasticity. Microbes evolve genes for novel pathways encoding xenobiotic degradation by the above mechanisms, which are further assembled as transcription unit(s)/operon(s), so as to finetune their regulation. Thus, only the required enzymes of the degradative pathway or segmen<sup>t</sup> of the pathway are activated by the respective carbon source. Further, the mutations in the acquired gene(s), shu ffling/rearrangemen<sup>t</sup> of genes and compartmentalization of metabolic steps gran<sup>t</sup> an additional advantage to the organism to adapt and utilize the compound

more efficiently, as observed in the case of Carbaryl metabolism by *Pseudomonas* sp. C5pp. Apart from plasmids and transposons, the genes for aromatic degradation have also been found to be present as a part of GEIs/ICEs in the genome of various organisms including *Pseudomonas*. Integration of the degradation property into the host genome may impart additional stability to the phenotype, as generally the plasmids are lost in the absence of selection pressure. Therefore, by employing various strategies, microbes display an ability to adapt to challenging environments.

**Author Contributions:** All three authors contributed equally towards conceptualization, writing original draft, review and editing. P.S.P. acquired funds through various governmen<sup>t</sup> agencies to support the research activities.

**Funding:** PP acknowledges research grants from BRNS, DBT and DST, Govt. of India.

**Acknowledgments:** Thanks to Sravanti Kasarlawar for suggestions and discussion. BAS thanks DST for project Junior Research Fellowship and HM thanks CSIR, Govt. of India for Junior Research Fellowship.

**Conflicts of Interest:** The authors declare no conflict of interest.
