In vitro evaluation of the effects of silver nanoparticles on Enterococcus faecalis cells’ viability

Enterococcus faecalis is a component of the gut microbiota of healthy chickens. It is also an opportunistic pathogen that has a serious economic impact on poultry production. The growing presence of multidrug-resistant (MDR) enterococci in poultry environments and products represents a major public health concern, therefore finding new methods to combat MDR bacteria has become urgent. The study aimed to determine the effect of silver nanoparticles (AgNPs) on E. faecalis viability. Three E. faecalis strains were treated with AgNPs at different concentrations. One experimental group was treated with 1% Virkon™ S disinfectant instead of AgNPs. The results revealed no significant decrease in the viability of E. faecalis due to AgNPs in contrast to the control. The largest reduction was 10-11 p.p. Virkon™ S only slightly reduced the viability of bacterial cells: to 67% (33 p.p.) in E. faecalis ATCC 51299 and 61% (39 p.p.) in E. faecalis 1D (isolate from one-day-old chicks), compared to the control (cell viability, 100%). The results showed that AgNPs exhibited little activity against E. faecalis. Surprisingly, in some cases, viability increased with increasing AgNP concentrations. Further analyses are needed (e.g., metabolic, proteomic) to explain the mechanisms that allow E. faecalis to resist AgNPs.