A one-step simple synthesis of silver colloid nanoparticles with controllable sizes is presented. In this synthesis, reduction of [Ag(NH(3))(2)](+) complex cation by four saccharides was performed. Four saccharides were used: two monosaccharides (glucose and galactose) and two disaccharides (maltose and lactose). The syntheses performed at various ammonia concentrations (0.005-0.20 mol L(-1)) and pH conditions (11.5-13.0) produced a wide range of particle sizes (25-450 nm) with narrow size distributions, especially at the lowest ammonia concentrations. The average size, size distribution, morphology, and structure of particles were determined by dynamic light scattering (DLS), transmission electron microscopy (TEM), and UV/Visible absorption spectrophotometry. The influence of the saccharide structure (monosacharides versus disaccharides) on the size of silver particles is briefly discussed. The reduction of [Ag(NH(3))(2)](+) by maltose produced silver particles with a narrow size distribution with an average size of 25 nm, which showed high antimicrobial and bactericidal activity against Gram-positive and Gram-negative bacteria, including highly multiresistant strains such as methicillin-resistant Staphylococcus aureus. Antibacterial activity of silver nanoparticles was found to be dependent on the size of silver particles. A very low concentration of silver (as low as 1.69 mug/mL Ag) gave antibacterial performance.

In this first systematic study, we describe the influence of various surfactants and polymers on aggregation stability and antibacterial activity of silver nanoparticles (NPs) prepared by a modified Tollens process. The surfactant/polymer stabilizing effect was monitored using the newly established method based on a titration of the aqueous dispersion of the silver NPs by aqueous solution of poly(diallyldimethylammonium) chloride (PDDA). The aggregation process was evaluated by the dynamic light scattering (DLS) and UV/vis spectra measurements and finally confirmed by TEM. Among all of the investigated modifiers, two surfactants (sodium dodecyl sulfate-SDS and polyoxyethylenesorbitane monooleate-Tween 80) and one polymer (polyvinylpyrrolidone-PVP 360) exhibit superior stabilization of the silver NP dispersions against the process of aggregation. The differences in the stabilization ability of various tested substances are discussed with respect to their structure and possible mechanism of the surface interaction with the NPs. The antibacterial activity of the modified silver NPs was significantly enhanced especially when modified by SDS where the minimum inhibition concentration (MIC) decreased under the "magical value" of 1 µg‚mL -1 . A correlation was found between the aggregation stability and enhanced antibacterial activity in the system of the silver NPs modified by SDS, Tween 80, and high molecular polymer PVP 360.

Silver nanoparticles have already been successfully applied in various biomedical and antimicrobial technologies and products used in everyday life. Although bacterial resistance to antibiotics has been extensively discussed in the literature, the possible development of resistance to silver nanoparticles has not been fully explored. We report that the Gram-negative bacteria Escherichia coli 013, Pseudomonas aeruginosa CCM 3955 and E. coli CCM 3954 can develop resistance to silver nanoparticles after repeated exposure. The resistance stems from the production of the adhesive flagellum protein flagellin, which triggers the aggregation of the nanoparticles. This resistance evolves without any genetic changes; only phenotypic change is needed to reduce the nanoparticles' colloidal stability and thus eliminate their antibacterial activity. The resistance mechanism cannot be overcome by additional stabilization of silver nanoparticles using surfactants or polymers. It is, however, strongly suppressed by inhibiting flagellin production with pomegranate rind extract.