Experimental evolution of bacteria exposed to sub-inhibitory concentrations of silver nanoparticles

The rise of antibiotic resistant pathogens emphasises the need to develop new antimicrobials to treat bacterial infections. Silver nanoparticles are a new antimicrobial which harness the intrinsic toxicity of silver and make it more effective as an antimicrobial agent. Silver nanoparticles pollution is increasing because nanosilver is commonly added to many consumer goods. Pollution will generate sub-inhibitory concentrations in some environmental compartments which may facilitate the appearance and spread of resistance to silver, through mutation, recombination and lateral gene transfer events. The frequency of each of these events is increased by the SOS response. Therefore, if silver nanoparticles induce the SOS response, widespread resistance to silver nanoparticles becomes likely. If resistance becomes widespread, we will have compromised an antimicrobial that could prove useful in a post-antibiotic era. In this series of experiments, I aimed to induce the SOS response in Escherichia coli and Acinetobacter baumannii by exposing each species to sub-inhibitory concentrations of silver nanoparticles over a prolonged period. There was no evidence that the SOS response was induced, but the assay did not seem to work, as ciprofloxacin, a known inducer of the SOS response, also induced no change. However, several lines exhibited silver resistance. This resistance cannot be due to acquisition of known resistance mechanisms, such as the sil operon, since there was no opportunity for lateral gene transfer during these experiments. Consequently, it seems likely that mutations that confer resistance have occurred within some lineages. Identification of the loci involved will require genome sequencing of the resistant isolates.