Action of silver nanoparticles on microbes
The exact mechanism which silver nanoparticles employ
to cause antimicrobial effect is not clearly known and is
a debated topic. There are however various theories on
the action of silver nanoparticles on microbes to cause
the microbicidal effect.
Silver nanoparticles have the ability to anchor to the bacterial
cell wall and subsequently penetrate it, thereby causing
structural changes in the cell membrane like the
permeability of the cell membrane and death of the cell.
There is formation of ‘pits’ on the cell surface, and there is
accumulation of the nanoparticles on the cell surface [3].
The formation of free radicals by the silver nanoparticles
may be considered to be another mechanism by which the
cells die. There have been electron spin resonance spectroscopy
studies that suggested that there is formation of
free radicals by the silver nanoparticles when in contact
with the bacteria, and these free radicals have the ability to
damage the cell membrane and make it porous which can
ultimately lead to cell death [4,5].
It has also been proposed that there can be release of
silver ions by the nanoparticles [6], and these ions can
interact with the thiol groups of many vital enzymes and
inactivate them [7]. The bacterial cells in contact with
silver take in silver ions, which inhibit several functions
in the cell and damage the cells. Then, there is the generation
of reactive oxygen species, which are produced
possibly through the inhibition of a respiratory enzyme
by silver ions and attack the cell itself. Silver is a soft
acid, and there is a natural tendency of an acid to react
with a base, in this case, a soft acid to react with a soft
base [8]. The cells are majorly made up of sulfur and
phosphorus which are soft bases. The action of these
nanoparticles on the cell can cause the reaction to take
place and subsequently lead to cell death. Another fact
is that the DNA has sulfur and phosphorus as its major
components; the nanoparticles can act on these soft
bases and destroy the DNA which would definitely lead
to cell death [9]. The interaction of the silver nanoparticles
with the sulfur and phosphorus of the DNA can
lead to problems in the DNA replication of the bacteria
and thus terminate the microbes.
It has also been found that the nanoparticles can
modulate the signal transduction in bacteria. It is a wellestablished
fact that phosphorylation of protein substrates
in bacteria influences bacterial signal transduction.
Dephosphorylation is noted only in the tyrosine
residues of gram-negative bacteria. The phosphotyrosine
profile of bacterial peptides is altered by the nanoparticles.
It was found that the nanoparticles dephosphorylate
the peptide substrates on tyrosine residues, which leads
to signal transduction inhibition and thus the stoppage
of growth. It is however necessary to understand that
further research is required on the topic to thoroughly
establish the claims