Antimicrobials are probably one of the most successful forms of chemotherapy in

Antimicrobials are probably one of the most successful forms of chemotherapy in the history of medicine against microbial agents such as bacteria, virus, and protozoa. The discovery of potent and safe antimicrobial agents in the 20th century was the greatest advance in healthcare, which have rapidly reduced the morbidity and mortality associated with the fatal infections. (Rice LB., 2008) The successful implementation of any therapeutic agent is jeopardized by the potential emergence of tolerance or resistance against the activity of compound from the time it is first used.

Unfortunately, the use of antibiotics has been accompanied by the development of resistant strains of bacteria that can evade the effects of these drugs. Earlier, the spread of resistance was limited, as the resistant strains are often less virulent and, consequently, less competitive than the sensitive strains from which they originated in the absence of selective pressure. Over past decades, we have conducted a global experiment in evolutionary selection pressure by applying tons of antibiotics to the planet, to treat patients and to promote growth in animals used for food production.

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It has resulted into the widespread antibiotic resistance in pathogens. Many environmental cues can lead to the temporary acquisition of resistance to a given antibiotic, including ion concentrations, temperature, and, very importantly, exposure to nonlethal doses of antimicrobials. (Fernndez et. al., 2012) Bacteria achieved their adaptive capability through mutation and a stunning genetic plasticity via horizontal gene transfer, coevolving with natural antimicrobial compounds for past billions of years.The present scenario has been described by some apocalyptic commentators as approaching a pharmageddon - a world in which bacterial pathogens have evolved to become resistant to the full range of clinically useful antibiotics, the consequence of which will be the fears about resurgence of some infectious diseases, the microbiology of invasive surgery, postoperative infection, and sepsis.

(Salmond et. al., 2008)In this study, we emphasized on ESKAPE bugs (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) " which represent paradigms of pathogenesis, transmission, and resistance among the majority of the hospital and general community infections all over the world. These pathogens can effectively escape' the bactericidal action of most of the currently available antibacterial drugs. It has resulted in increased morbidity and mortality rates with attendant fears about resurgence of some infectious diseases, the microbiology of invasive surgery, postoperative infection, and sepsis.(Salmond et. al., 2008) Our therapeutic options against these pathogens are so extremely limited that clinicians are forced to use older, previously discarded drugs, such as colistin, that are associated with significant toxicity and there is a lack of sufficient data to guide selection of dosage regimen or duration of therapy. (Boucher et. al., 2009)A dwindling pipeline of new antibiotics against the emerging multidrug - resistant bacteria has increased health burden on researchers, health care specialists, and pharmaceutical industry workers. The new forms of antibiotics being produced are the modified version of pre-existing antibiotics which targets the same cellular processes as their natural or synthetic predecessors, for which resistance will sooner or later develop among the pathogens. Therefore, there is an urgent need to screen chemically diverse compounds for the discovery of novel paradigm of antimicrobial therapy.There are various screening methods used for the identification of novel compounds with potential antimicrobial activity against resistant pathogenic bacteria including broth/agar dilution methods, disc/well diffusion methods etc. " of which broth micro-dilution method being commonly preferred. (Patel et. al., 2015).In our study, compounds obtained from the CSIR-CDRI library were screened for antimicrobial activity against bacterial strains namely Escherichia coli (ATCC 25922), Staphylococcus aureus (ATCC 29213), Klebsiella pneumoniae (BAA-1705), Acinetobacter baumannii (ATCC BAA-1605) and Pseudomonas aeruginosa (ATCC 27853) collectively referred to as ESKAPE'. Primary screening of the compounds was performed to identify their antibacterial activities and to calculate the minimum inhibitory concentration of the compound per the standard micro-dilution method recommended by CLSI.The hit compounds were selected for cytotoxicity assay to test the percentage proliferation of Vero cell lines in the presence of drug. The assay is also useful in calculating the selectivity index (SI) of the test compound which is an indirect indication of their in vivo activity. Compounds with high selectivity are further taken for time kill assay, while the compounds showing toxicity against Vero cell lines are not considered for further assays.Furthermore, one of the FDA approved drug, Ivacaftor, a quinolinone oral cystic fibrosis transmembrane conductance regulator, was reported for its antimicrobial activity against S. aureus and A. baumannii. Repurposing could highly reduce the cost of drug development as it has already gone through the clinical trials for their PK-PD, safety and tolerability. Experiments were carried out to elucidate its mechanism of action on S.aureus - via developing mutants against the drug and using wild S. aureus as a control.