ARS | Publication request: Antimicrobial activity of spherical silver nanoparticles: evidence for induction of a prolonged bacterial lag phase

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: February 28, 2011
Publication Date: N/A

Technical Abstract: Background: Recently, there has been a great deal of interest surrounding the discovery that Ag[0] nanoparticles (Np) are more effective antimicrobial agents in terms of the minimum effective concentration than their Ag[+] counterparts. Methods: Both solid and liquid phase experiments were performed using BHI medium. For liquid phase experiments a 96-well plate reader was employed to characterize microbial growth (OD at 590 nm, 37deg C) in the presence of different Np concentrations. Growth parameters were extracted by fitting OD-time data to a common sigmoidal mathematical expression, which is a function of beginning and final OD values, a first-order rate constant (k), as well as the time to calculated half-maximal OD (tm). The parameter tm is also a function of initial cell concentration (Ci), k, as well as the lag time (T). Results: For solid state assays we found that 1e-2 to 2e-2 mL of 0.3e-6 to 3e-6 molar solution of a keratin-stabilized Np completely inhibited the growth of an equivalent volume of ca. 1e3-1e4 CFU/mL of Staphylococcus aureus, Salmonella Typhimurium, or E. coli O157:H7 on solid surfaces. At lower Np concentrations, only small colonies were observed. In liquid phase assays, we found that growth always occurred, but tm varied linearly with Np levels from ca. 7 to greater than 20 h. We also found that there was little effect of Nps on k (ave k = 1.1 ± 0.075 per h). To test whether the Nps were changing Ci via cell death, we performed probabilistic calculations assuming that the perturbations in tm were due to Ci alone and found that the Np-induced change in tm could only be due to a prolonged T (ca. 1 to 20 h). Conclusion: We propose that the difference between the solid and liquid system relates to the obvious difference in the residence time of the Nps since when small, Np-inhibited colonies were selected and streaked on fresh medium, growth proceeded normally. We also propose that the lag time effect is occurring on solid surfaces but, because the of long T values, the bacteria eventually die or become non-culturable.