However, the extent of this effectiveness has remained a puzzle. Silver ions are known to damage both the external membrane and the internal components of bacteria. However, while many things can be damaging to bacteria, silver's potency puzzled professor David Avnir of the Hebrew University, Jerusalem.
The effect on the bacterium Pseudomonas aeruginosa was dramatic. Previously healthy bacteria died en masse; when sufficient silver was applied, initially 99.999% of the bacteria were killed through this second-hand exposure. The authors argue that the result “strongly indicates that not only does silver persist within the dead cells, but that it is an available source for further biocidal activity on viable cells." Avnir and colleagues dubbed the process the “zombies effect.”
To confirm that the silver was responsible, Avnir used a control where a P. aeruginosasample was killed by heat instead. While high temperatures proved equally lethal to the original sample, it did not turn them into zombies. The heat-killed bacteria had no effect on other members of its own species, demonstrating that it is the silver that turns the bacteria into zombies.
Avnir explains the process as the bacteria soaking up the silver, and then releasing it after death when living bacteria act as even more attractive sponges.
This theory was supported by testing the effect of the leftover water that had been used to create the zombies. At high initial silver concentrations, this wastewater was similarly lethal. However, when low concentrations of silver were used, the leftover water was less likely to kill bacteria than zombies. Avnir attributes this to the zombies absorbing so much silver that, at low concentrations, not enough was left to kill all the bacteria in the second round.
Credit: Ben-Knaz Washlak et al. via Scientific Reports. The leftover water used to kill the original sample was tested along with the zombies.
One of silver's advantages as an antiseptic lies in its persistence, continuing to kill bacteria for a substantial period of time. This feature is put to use with the infusion of silver into medical products such as bandages. “It follows,” the authors argue, “that prolonged action of silver ion releasing formulations benefit, at least in part, from the phenomenon we describe here.”
Avnir's work may lead to new ways to apply silver's antiseptic properties more effectively, controlling the release for maximum potency. Whether thirty ions of silver are sufficient to make a bacterium destroy other members of its species remains unknown.