Antibacterial Effect of Silver Nanoparticles on Intestinal Bacteria

Introduction: The application of engineered nanoparticles (NPs) for food safety is increasingly being explored.  Silver is a non-toxic, inorganic antibacterial agent used in the pharmaceutical and medical industries.  Silver (Ag) NPs have a significant potential for a wide range of biological applications, including as an antifungal agent, antibacterial agent for antibiotic resistant bacteria and for preventing infections.  However, not much is known about the behavior of Ag NPs upon ingestion and whether they inhibit natural gut microflora.

Purpose: The objective of this study was to investigate the effects of Ag NPs on the intestinal bacteria, Escherichia coli, Lactobacillus acidophilus, and Bifidobacterium animalis.  

Methods: Ag NPs at concentrations of 0, 100, 200, and 300 mg/ml were used in tube dilution tests.  Tryptic soy broth or Lactobacilli MRS broth without NPs and containing NP-free solution were used as controls.  Cells were exposed to the different Ag NP concentrations in broth for 12 h, before plating on respective agar media.  

Results: At the end of 12 h, numbers of all three bacterial strains treated with 200 and 300 mg/ml of Ag NPs were significantly different from those of the controls. Ag NPs had no significant effect on E. coli up to 6 h of exposure, but after a 9-h exposure, the numbers were significantly lower than the controls (P ≤ 0.0001). Numbers of L. acidophilus and B. animalis treated with 200 and 300 mg/ml of Ag NPs were also significantly lower than those of the controls (P ≤ 0.05) at 9 h of exposure.

Significance: This research provides evidence that Ag NPs have effects on E. coli, L. acidophilus and B. animalis.  Further research is needed to examine effects of other NPs on intestinal bacteria, and effects of Ag NPs on human epithelial cells.