Evaluation of behavior, growth, and swarming formation of Escherichia coli and Staphylococcus aureus in culture medium modified with silver nanoparticles

Silver nanoparticles (AgNPs), as well as silver ions, are described as toxic to a broad spectrum of microorganisms, especially bacteria. In contrast to this, a current trend is to develop and carry out the in vitro cultivation of microorganisms, facilitating the study of interactions between populations of cells and species. Thus, the goal of this study was to evaluate the behavior, growth, and swarming formation of bacteria under conditions of co-culture in solid medium modified with AgNPs. The aqueous extract from the leaves of Handroanthus serratifolius was used to chemically reduce silver nitrate (AgNO₃) solution, forming AgNPs. This synthesis route was performed in an aqueous medium at 50 °C for 3 h. The hydrodynamic diameter (HD) and polydispersity index (PdI) were obtained by dynamic light scattering (DLS), and Zeta potential (ZP) of the AgNPs were measured by electrophoretic mobility. Atomic force microscopy (AFM) was used to evaluate the shape of the AgNPs. Luria Bertani (LB) medium was used for the liquid culture steps and for the solid medium, bacterial agar was added. Solutions containing AgNPs or AgNO₃ were added at final concentrations of 256, 128, or 64 μM. Subsequently, microorganism Escherichia coli ATCC® 8739 and Staphylococcus aureus ATCC® 25923 were plated with AgNPs, AgNO₃, and control media. Analyses of the AgNPs showed an average HD of 76.02 ± 3.08 nm, PdI of 0.461 ± 0.012, and ZP of -21.5 ± 2.2 mV; in addition, AgNPs were nearly spherical. The solid culture medium elaborated and modified with AgNPs at the concentrations of 256 and 128 μM inhibited the growth of the tested microorganisms and decreased the swarming formation. However, those media modified at a concentration of 64 μM did not induce any alteration in the growth and proliferation of the microorganisms. Furthermore, it was observed that plates containing modified culture media with 128 μM, increased proximity between both co-cultured bacteria occurred. Thus, the application of AgNPs in solid culture media becomes a promising and potentially reproducible strategy for evaluating the behavior, swarming formation, and toxicity of AgNPs, making the understanding of possible bactericidal or bacteriostatic effects, and also colonizing strategies.

Keywords: Green synthesis; Modified culture medium; Silver nanoparticles; Swarming; Toxicity.