Exploring the antibacterial potentials of South-East Asian natural products against multidrug resistant bacteria

• Main Author: Ali, Asghar
• Format: Thesis
• Published: 2021
• Subjects: QD Chemistry; QR Microbiology
• Online Access: http://eprints.sunway.edu.my/2389/
• Institution: Sunway University

The emergence and spread of antimicrobial resistance have been of serious concern on human health and the management of bacterial infectious diseases. Effective treatment of these diseases requires the development of novel therapeutics, preferably free of side effects. In this regard, natural products are frequently conceived to be potential alternative sources for novel antibacterial compounds. In the present study, we have evaluated the antibacterial activity of the crude extracts of selected natural products (NPs) of Malaysia, i.e., Curcuma longa, Gracilaria, edulis, Laurus nobilis, Murraya koenigii and Nephelium lappaceum against six bacteria namely, Bacillus subtilis, methicillin-resistant Staphylococcus aureus (MRSA), Streptococcus pyogenes, Pseudomonas aeruginosa, Klebsiella pneumoniae and Salmonella enterica, which have gained traction on account of their growing multidrug resistance (MDR). Among a series of NPs solvent extracts such as, chloroform, ethyl acetate, acetone, ethanol, methanol and water, the sequential fractions of ethyl acetate and acetone initially have revealed significant activity towards all the tested strains. Such activities were affirmed, further, through the direct crude extract fractions of these solvents. Chromatographic analyses of these sequential and direct crude extract fractions of ethyl acetate and acetone, via HPLC, LC-MS and GC-MS, have generated a library of potential bioactive compounds for each of these NPs, Downstream virtual screening and pharmacological prediction, towards proteins involved in the MDR, such as, RsmA, MepR, SpY_2034, RamR, AadB and LecB, respectively, of the afore-mentioned bacteria, have eventually unveiled novel potential antibacterial compounds, which can be extracted and/or utilised in future for medicinal uses. Notable among them is Eplerenone, an antihypertensive agent, which has been shown to have nice binding affinities and pharmacological properties against the tested bacterial proteins, thereby enabling to propose the same as an antibacterial agent. Simultaneously, a set of three known antibacterial pure compounds, namely, Quercetin (QT), Ar-turmerone (AT) and Eugenol (EG), unanimously found to be present across the studied NPs were purchased and tested against the six tested MDR bacteria (STMB). The results revealed remarkable activity of only AT and EG towards the STMB. Hence, both the compounds were chosen for their effect on the gene expression level of the STMB. A total of three genes were chosen for each bacterial strain. Briefly, gyrA was used as a reference gene while two other reported virulent genes for each strain were selected. Of these, dnaK was the common and remaining one gene was specific for all the tested strains. These were, for instance, mneP, sepA, acrA, mexA, acrZ and acrZ, respectively, for B. subtilis, MRSA, S. pyogenes, P. aeruginosa, K. pneumoniae and S. enterica. Upon treating the STMB with AT and EG, the expression levels, of the specific genes of interest for each bacterium, were found to be highly affected. This, in turn, proved the effect of the NPs harboring these chemical compounds, albeit, along with other probable compounds which demands future testing. Notably, the cytotoxicity assay of these two chemical compounds, AT and EG, proved them to safe enough with a proper limit of usage, which was confirmed against HaCat cell lines. Such assays for both revealed only around 8% cytotoxicity at a very high concentration of 100 μg/mL. Our work, thus, follows the objective to propose new antimicrobials from NPs capable of perforating the barrier of resistance posed by both, the Gram positives and -negative bacterial pathogens. Our work adds onto the ongoing effort to re-discover and repurpose biochemical compounds with an aim to combat the antimicrobial resistance offered by the Gram-positive and -negative bacterial species.