Antimicrobial activity of plant extracts against pathogenic microorganisms / Puteri Juanita Zamri

Aim- Methanolic extracts of 15 traditional medicinal higher plants were selected and evaluated. Antibacterial screening test was done to evaluate their antimicrobial activities against Staphylococcus aureus (S. aureus), Escherichia coli (E. coli), methicillin-resistant Staphylococcus aureus (MRSA) and extended spectrum P- lactamase (ESBL) Escherichia coli (RESCO 1) strains.
Method- The ground dried leaves or seeds were extracted with 70 % aqueous methanolic solution for 48 hours. The extracts were then filtered and the whole cycle was performed for three times. The combined methanol extracts were then dried in vacuo using a rotary evaporator. The dried methanolic extracts were later dissolved in 4 % DMSO and subjected to antimicrobial evaluation using the microdilution plate method. The 50% Minimal Inhibitory Count (MIC50) of microorganisms against standard antibiotics was determined prior to antibacterial screening by using the microdilution plate and the agar­ well diffusion method. Imipinem and vancomycin were tested against Gram-negative and the Gram-positive bacteria respectively.
Conclusion- This study elucidated that most of the plants extracts have antimicrobial activities against at least one type of microorganism with the exception for Radix astragali and Antidesma stipularis (leaves) which have no antimicrobial activities. However, only six of them were able to show significant values of inhibition index. These medicinal plants inhibited the growth of selected bacterial strains with inhibition index ranging between 0.36 to 0.84 with reference to vancomycin at the concentration of 31.99 pg/ml for S aureus and 13.34 pg/ml for MRSA 2 or with reference to imipinem at the concentration of 0.98 pg/ml for E. coli and 3.24 pg/ml for RESCO 1. According to the results, Melastoma malabathricum (fruit) has the highest value of inhibition index towards S. aureus, with the value of 0.84 while Azadirachta indica has the lowest value against S. aureus which is only 0.36. The antimicrobial activity profile of all species of plants [except Radix astragali, Antidesma stipularis (leaves) and Centella asiatica] against the tested strains indicated that S. aureus was the most susceptible bacterium of all the bacterial test strains. On the other hand, E. coli was only inhibited by Cassia surattensis and Antidesma stipularis (seed). In general, it was observed that sensitive strains of Gram-negative bacteria are less susceptible to the plant extracts compared to sensitive strains of Gram-positive bacteria. However, for the case of the multi-resistant strains for both Gram positive and Gram negative bacteria, the antimicrobial activities of the plant extracts were almost similar. Besides that, the MIC50 determination showed that the microdilution plate method is more sensitive than the agar-well diffusion method and is able to produce reproducible results.