Contribution to optimization and standardization of antibacterial assays with silver nanoparticles: the culture medium and their aggregation.

The antimicrobial activity of silver nanoparticles is determined by their size and specific properties, as well as by the chemical composition of the exposure medium in which the nanoparticles are suspended. When the antibacterial tests are carried out in a culture medium, aggregation of the nanoparticles is produced, decreasing their effectiveness. This study proposes the addition of surfactants to the culture medium to prevent the aggregation of silver nanoparticles and optimizes the concentrations of these surfactants. The aggregation of silver nanoparticles was studied by dynamic light scattering (DLS) after dispersion in three liquid culture media (Mueller-Hinton (MH), Luria-Bertani (LB) and Brain Heart Infusion) in which four different surfactants (SDS, Triton X100, Tween 80 and CTAB) were added at concentrations of 0, 0.1, 0.5, 1, 1.5 and 2%. Results showed that, the optimal culture media to prevent aggregation of silver nanoparticles were MH and LB with higher concentrations of Tween 80 and Triton X100 surfactants; being MH + 2% of Tween 80 and MH + 1% Triton X100 the best combinations obtained because the results obtained were closest to the sizes of nanoparticles in ultrapure water. In addition, it has been verified that the optimal medium + surfactant combinations chosen did not affect the viability of Escherichia coli bacteria. Nanoparticle aggregation was not observed by single particle inductively coupled plasma mass spectrometry (SP-ICP-MS) when nanoparticles were incubated for long incubations periods (24 h) in the optimal medium chosen.