Antibacterial and Ultra Microscopic Studies of Salmonella Inhibited by Silver Coated Carbon Nanotubes

Abstract

There is an urgent need for newer antimicrobial agents to prevent or treat Salmonella enterica serovar Typhimurium foodborne illness. Silver coated carbon nanotubes (AgCNTs) are attractive candidates as these nanocomposites may have tremendous mechanical strength, nanometer diameter, high aspect ratios of length to diameter, and potential antimicrobial properties of silver. To investigate this hypothesis, we conducted a time line analysis of the antibacterial activity of AgCNTs against Salmonella enterica serovar Typhimurium using SEM, TEM, and AFM. Antibacterial studies were also conducted using the Kirby-Bauer disc diffusion assay, the bacterial growth curve assay, the standard plate count assay, and quantitative real-time PCR. In the Kirby-Bauer assay, AgCNTs at a concentration of 50 µg/mL showed an inhibition zone of 24.1 + 4.14 mm for Salmonella, while the growth curve assay showed that the lag phase was prolonged, and bacterial growth was reduced by 29.5%, as compared to the negative control. The standard plate count assays showed that the Minimum Inhibitory concentration of AgCNTs was between 50 - 100 µg/mL. Quantitative real-time PCR analysis showed that there was a dose-dependent reduction in Salmonella ttrRSBCA locus DNA concentration after exposure to AgCNTs. By comparison to AgCNTs, commercial silver nanoparticles or CNTs alone did not inhibit bacterial growth or gene expression significantly as tested by the assays mentioned above. SEM studies showed that the AgCNTs damaged the bacterial membranes with marked changes in morphology; TEM studies also confirmed the presence of fewer bacterial cells with damaged membranes. AFM studies are currently being conducted to further assess the mechanism by which AgCNTs inhibit Salmonella enterica serovar Typhimurium.