The antibacterial activity and mechanism of action of silver nanoparticles on Escherichia coli were investigated in a study by Wen-Ru Li and colleagues. The authors analyzed the growth, permeability, and morphology of bacterial cells following treatment with nanoparticle silver. They found that a concentration of 10 μg/ml inhibited the growth of E. coli cells, while also affecting the permeability of bacterial membranes. When exposed to 50 μg/ml of silver nanoparticles, E. coli cells developed pits and gaps and cell membranes became fragmentary, showing severe damage. Membrane vesicles were also dissolved and dispersed, and their membrane components disorganized and scattered. Ultimately, the study results suggest that silver nanoparticles may damage the structure of bacterial cell membranes and depress the activity of some membranous enzymes, eventually killing E. coli bacteria. Sondi and Salopek-Sondi reported similar findings.
Antibiotics can cause symptoms in patients to temporarily disappear, yet leaving behind a host of resistant organisms in the system. These resistant organisms reappear at a later time, straining the immune system. Silver solutions show marked activity against resistant strains, and are potentially synergistic or additive to antibiotics. In a study by Souza and colleagues, a silver-water dispersion in combination with nineteen antibiotics was tested against seven bacterial strains for synergism. Out of 96 tests, five were synergistic, 89 additive, and only two antagonistic, showing that the combination of silver-water dispersion with antibiotics allows a more complete clearing of the pathological organism.
Lara and colleagues exposed a range of drug-resistant pathogens of clinical importance, including multidrug-resistant Pseudomonas aeruginosa, ampicillin-resistant Escherichia coli O157:H7 and erythromycin-resistant Streptococcus pyogenes, to a suspension of silver nanoparticles. They determined that silver nanoparticles i) inactivate a range of drug-resistant and drug-susceptible bacteria, both Gram positive and Gram negative, ii) exert their antibacterial activity through a bactericidal rather than bacteriostatic mechanism, and iii) inhibit the bacterial growth rate from initial contact with the bacteria. Through a Kirby–Bauer test, they demonstrated that the general mechanism of bactericidal action of nanoparticle silver is by inhibition of cell wall synthesis, protein synthesis, and nucleic acid synthesis. Their data suggests that silver nanoparticles are effective broad-spectrum biocides against a variety of drug-resistant bacteria and a potential candidate for use in pharmaceutical products and medical devices to help prevent the transmission of drug-resistant pathogens.
Li, Wen-ru; Xie, Xiao-bao; Shi, Qing-shan; Zeng, Hai-yan; Ou-yang, You-sheng; et al. Antibacterial activity and mechanism of silver nanoparticles on Escherichia coli. Applied Microbiology and Biotechnology. 85.4 (Jan 2010): 1115-22.
De Souza, A., Mehta, D. and Leavitt, R.W. (2006) Bactericidal activity of combinations of Silver-Water DispersionTM with 19 antibiotics against seven microbial strains. Current science.