MicroRNAs bacterial pathogens and viruses (4). (5). The importance

MicroRNAs(miRNAs) are small noncoding endogenous RNA that consistsapproximately 19–25 nucleotides in length (Kaeuferle et al. 2014). They play asignificant role by post-transcriptionally regulating the gene expression inplants and animals (Bartel, 2004; Chekulaeva and Filipowicz, 2009).Furthermore, miRNA can influence target mRNA by binding to protein-codingregions, thereby modulating the stability or translation of mRNA(1) miRNAsP1  affect a varietyof key biological processes such as cell differentiation, proliferation andapoptosis, development, immunity, metabolism and stem cell maintenance (2). Therefore,function of miRNAs has provided potential pathways in various areas, such astherapeutic intervention of miRNA against pathogens has led to conduct studiesrelated to investigate miRNA based therapy (MicroRNA therapeutics: Discoveringnovel targets and developing specific therapy). P2 As well differentapproaches have led to identify the effective use of miRNA profiling fordiverse applications in disease diagnosis (MicroRNA profiling: approaches andconsiderations).

P3  It is noted thataberrant miRNA levels can cause diseases such as cancer, age-related diseases,neurological disorders and immune function disorders. Consequently, appropriateregulation of miRNA is essential for disease prevention (3). Moreover, miRNAshave pivotal roles in eukaryotic defense mechanism of the host againstextracellular, invasive bacterial pathogens and viruses (4). (5).

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Theimportance of miRNA in bacterial infections was initially discovered with theaid of miR-393, regulates auxin signaling P4 against a extracellularpathogen Pseudomonas  in Arabidopsis thaliana plant.  According to this discovery of miRNAs hostdefence function in plants, latterly miRNAs were implicated in bacterialinfections in mammalians (6,7). However, immune regulationsof miRNAs and characterization of miRNAs has not been studied extensively inteleost fish when compared with mammals. In  8 teleost species from five orders, 1,250miRNAs were identified (Table 1).

However, some specific miRNA genes are availableonly in teleost fish but not in higher vertebrates, such as miRNA-734 and miRNA-7132 P5 (Yang et al.,2015; Yan et al., 2012; Andreassen et al., 2013). Therefore, studies haveconducted on identifying the miRNAs that are differentially expressed betweencontrols and challenged groups in teleost fish.

According to those reports,  altered expression levels of miRNAs wasobserved in viral as well as bacterial infections in several teleost fish. Moreover,miRNAs contribute as regulators of Toll-like P6 receptors (tl) andfollowing stimulation of nuclear factor (NF-kB ) signaling in bacterialinfections. It is noted miRNAs responding to infection depends on the targetgene(s) they regulate and many of the targets are genes from the host’s immuneresponse gene networks (8)global Aquaculture fishindustry often suffers with severe infections caused by viruses, bacteria andparasites. Aeromonashydrophila is also such bacterium; gram-negative, motile, rod-shaped whichcan cause motile aeromonas septicaemia (MAS) disease in fish that leads to highmortility (9, Harikrishnan and Balasundarama, 2005). Therefore,studying the immunity and host defence mechanisms of host by proper regulationof miRNA will support the control and prevention of infectious aeromonassepticaemia (MAS) disease in aquaculture industry.  Zebra fishis suitable animal model for drug screening and many applications because ofeasy handling, fecundity and due to low cost screening.The current studyhas been carried out using a control and challenged groups to screen miRNAs in zebrafish (Daniorerio) gut after an infectionof Aeromonas hydrophila and statethose identified miRNAs directly regulate the anti-bacterial activity of zebrafish.