Pathogenic Gram-Negative Bacilli Susceptibility to Silver Nanoparticles (AgNPs)

Silver nanoparticles (AgNPs) are used in everyday consumer products because of their bactericidal properties. However, their introduction into the environment has raised ecological concerns. This experiment set out to determine the effect of AgNPs on opportunistic and pathogenic gram-negative bacilli: Shigella flexneri, Acinetobacter baumannii, and Salmonella typhimurium. A modified Creighton method of AgNP synthesis was used by reducing AgNO₃ with NaBH₄. A standard inoculum of each bacteria was grown in Trypticase Soy Broth and also in broth with the addition of 1 mL or 5 mL of 616.8 mg/dL AgNP solution. In addition, silver nitrate (AgNO₃) and sodium borohydride (NaBH₄) control treatments were established because both were used in the synthesis of the AgNP solution. Bacterial growth was determined by reading absorbance at 419 nm against appropriate blanks. A second experiment was conducted growing the same organisms on Mueller Hinton II agar using a Kirby-Bauer technique and standard bacterial inoculum. One-centimeter Whatman paper disks were saturated with 20 µL of the AgNPs solution, placed on plates, incubated overnight, and zones of inhibition were measured.  The results showed that Acinetobacter baumannii had the greatest susceptibility to treatment with AgNPs and parental compounds, followed by Salmonella typhimurium. Shigella flexneri showed minor susceptibility. The most sensitive to AgNP exposure was Acinetobacter baumannii. Overall, AgNPs were shown to be effective as an antibacterial agent.