Among the inorganic antibacterial agents, silver (Ag) has
been known most extensively since ancient times to fight
infections and control spoilage. The antibacterial and
antiviral actions of Ag, Ag+ and Ag compounds have been
thoroughly investigated30-32. It is well known that silver ion
and silver-based compounds are highly toxic to
microorganisms33, showing strong biocidal effect against as
many as 16 species of bacteria, including Escherichia coli34.
Silver nanoparticles are highly reactive because they
generate Ag+ ions, whereas metallic silver is relatively
unreactive35. It has also been shown that nanoparticles
efficiently penetrate microbial cells, suggesting that lower
concentrations of nanosized silver particles would be
sufficient for microbial control. This approach could be
more efficient than existing treatments, especially for
certain organisms that are less sensitive to antibiotics
because of their resistance to cell penetration36.
The higher resistance of Gram-negative bacteria to plant
extracts has previously been documented and related to
thick murein layer in their outer membrane, which prevents
the entry of inhibitor substances into the cell37. Gram
negative bacteria contain a lipopolysaccharide layer at the
exterior, followed underneath by a thin layer of
peptidoglycan38. Lipopolysaccharides contain negative
charge39 and attract the weak, positively charged Ag
nanoparticles40. On the other hand, Gram-positive bacteria
are principally composed of a thick layer of peptidoglycan,
consisting of linear polysaccharide chains cross-linked by
short peptides to form a three dimensional rigid structure41.
The rigid and extended cross-linking not only endows the
cell wall with fewer anchoring sites for the Ag nanoparticle,
but also makes it difficult to penetrate. This may account
for less effect of the Ag nanoparticles on Gram positive
bacteria when compared to Gram negative bacteria in this
study. Ag nanoparticle was shown to cause antibacterial
effect by rupturing the membrane of bacterial cells at low
concentration. The concentration of nanoparticles was
responsible for biocidal effect along with the treatment
time. TEM image showed the particle binding and damage
in cell wall of bacteria. Synergistic effect of nanoparticles
along with ultrasonic treatment showed an enhanced
antibacterial effect 42.