Antimicrobial Activities of Silver Nanoparticles-Strobilanthes Crispus (Agnp-Sc) Against Selected Bacteria

The emerging of resistant bacteria strains against current drugs has raised worldwide
concern. This situation has led to invention of many alternative plant-based treatments
which is effective, safer, and economical. One of it is through biosynthesis of silver
nanoparticles combined with plant extract. This study is aimed to determine the ability
of silver nanoparticle-Strobilanthes crispus (AgNP-SC) as an antimicrobial agent
against selected bacteria. S. crispus extract was chosen based on the previous studies
that have shown various medicinal benefits. Effectiveness of AgNP-SC at various
concentrations have been tested on seven selected bacteria namely Escherichia coli
O157, Pseudomonas aeruginosa, Streptococcus mutans, Staphylococcus aureus,
Streptococcus pyogenes, Bacillus cereus and Salmonella enterica newport. Preliminary
tests have been conducted to determine the susceptibility of the bacteria against AgNPSC
through disc diffusion assay (DDA), followed by minimum inhibitory concentration
assay (MIC) and minimum bactericidal concentration assay (MBC). Results from these
three susceptibility tests have shown that AgNP-SC have highest antibacterial effects
on E. coli, P. aeruginosa and S. mutans at lower concentration compared to silver
nanoparticles (AgNPs) and S. crispus aqueous (SC) extract. Thus, AgNP-SC was
further tested using time-kill assay to confirm bactericidal effect against all three
bacteria. In time-kill assay, AgNP-SC was found to has better effect on antibacterial
activity compared to AgNPs and SC only. AgNP-SC was found to show bactericidal
properties within four hours incubation period on E. coli and P. aeruginosa, whilst
showing bacteriostatic property against S. mutans. Morphological changes of treated
bacteria had been determined by scanning electron microscopy (SEM). From SEM
analysis E. coli, P. aeruginosa and S. mutans morphologies have been distorted by
AgNP-SC. Gene expression of treated bacteria had been studied by Real-time PCR
analysis. In gene expression analysis, AgNP-SC affected the gene that encodes for
adhesin protein in E. coli and genes that involve in biofilm formation in P. aeruginosa
and S. mutans. As conclusion, AgNP-SC has great potential as antimicrobial agent
against all three bacteria by disturbing the cell membrane and interrupting the genes
that regulating virulence factors in bacteria.