Phytochemical Investigation of the Leaves Of polygonum Senegalense Meisn for Antimicrobial Activity

Abstract

Antimicrobial resistance has been named by the WHO as one of the three paramount public health challenges of the present- day. This resistance to antimicrobial agents means that simple bacterial infections such as pneumonia could soon be a death sentence. Medical procedures associated with immunosuppression such as cancer chemotherapy, advanced surgical procedures and transplants would be risky to perform. Out of the total antibacterial agents in use today, 65% are β–lactams. Bacteria have developed resistance to all the antibiotics in this class, including the antibacterials of final recourse: carbapenems. The key mechanism of resistance to β-lactams is their hydrolysis by β-lactamases. Development of inhibitors for β-lactamases would be effective as it would lead to a continued use of these drugs. Therefore, there is a great need to develop new antibacterials to address this emerging problem of antibacterial resistance. Natural products from medicinal plants hold great promise as sources of new antibacterials. Since Polygonum species have been used in the treatment bacterial infections such as urinary tract infections and gonorrhea, there was need to establish their antibacterial properties. In this study, the leaves of Polygonum senegalense Meisn were extracted with acetone. The extract obtained was concentrated over a rotary evaporator to obtain a crude sample. Column chromatography of the crude extract was carried out over silica gel, the eluent being n-n-hexane containing increasing proportions of ethyl acetate. Further purifications were carried out using Sephadex LH-20 and PREP-TLC. This led to the isolation of 10 compounds (1 -10). The compounds were characterized using 1H NMR, 13C NMR, H-H-COSY, HMBC and NOESY and mass spectrometry. The isolated compounds include flavones, flavanones, chalcones and a diastereomeric pair of 5,7,9-Trihydroxy-6-methoxyhomoisoflavones (9 and 10). The major compounds were evaluated in vitro for antimicrobial activity, and molecular docking to the β-lactamase carbapenemase, KPC-2. Among the pure compounds tested, 2',6'-Dihydroxy-4'-methoxydihydrochalcone (3) was the most active (MIC 0.22 μM) against Bacillus subtilis (ATCC 6051) and Aeromonas hydrophila (ATCC 7966) (MIC 1.68 μM). This compound also showed good activity (MIC 4.42 μM) against Neisseria gonorrhoeae (ATCC 19424). 5-Hydroxy-7-methoxyflavone (pinostrobin, 2) was the most active compound (MIC 1.28 μM) against Neisseria gonorrhoeae (ATCC 19424), while 2',4'-Dihydroxy-6'-methoxychalcone (4) was the most active against Porphyromonas gingivalis (ATCC 33277) (MIC 1.77 μM). All the compounds were inactive (MIC > 200 μM) against Streptococcus pneumoniae (ATCC 33400), Nocardia transvalensis (ATCC 6865), Mycobacterium smegmatis (ATCC 19420), and Cutibacterium acnes (ATCC 33179). Among the compounds docked compound (2) had the lowest binding energy ( -7.9 kcal/mol) and showed bonding interactions with active site residues Ser 70, Ser 130 and Trp 105. The study justifies the traditional application of the plant to treat microbial infection.