**4. Conclusions and Future Perspectives**

Nanotechnology and specifically, silver nanoparticles, have a promising future ahead in the field of food. Silver nanoparticles have demonstrated extensive antimicrobial activity against foodborne pathogens as well as great effectiveness when they are incorporated into different types of packaging. Today, most studies focusing on the use of Ag-NPs in packaging are at the laboratory level and in most countries, are not allowed. In the European Union, in particular, more data are necessary to define the regulation of their employment. Therefore, investigation of the use of nanoparticles as a food additive is needed, as well as the evaluation of their effect on consumer health, since there are no long-term studies that assess the real concerns of their consumption. Very few studies have focused on the relationships between nanoparticles and oral microbiota, and, in the same way, effects of silver nanoparticles on the composition of the intestinal microbiota and the consequences on their metabolic activity are largely unknown. The range of models and diverse experimental conditions, such as *in vitro*, *ex vivo* and *in vivo* approaches, animal models and control conditions, make it even more difficult to compare the results and draw final conclusions. A crucial aspect for *in vitro* studies is to take care to incorporate the changing physiochemical properties of silver nanoparticles during transit of the gastrointestinal tract in the study design. It is also necessary to continue studying the different types of silver nanoparticles including form, size distribution as well as dose and modes of administration/exposure of them to state detrimental effects on health. Finally, the difficulties involved in the evaluation *in vivo* of the effects of ingested nanoparticles in the gut, due to differences between species (rodents vs. humans), may also be highlighted. Probable variability between individuals, not only in terms of the composition, but also in terms of the functional metabolic properties of the microbiota, should also be taken into account along with host physiological characteristics and environmental factors. In conclusion, given their potential and wide properties against foodborne pathogens, research into silver nanoparticles is of great interest for the food industry but is not exempt from difficulties that must be resolved in order to certify the safety of their use.

**Author Contributions:** Investigation and evaluation of published studies, I.Z.-P., C.C. and M.V.M.-A., Contributing in writing the manuscript, I.Z.-P., C.C., B.B. and M.V.M.-A., Designing of figures and tables, I.Z.-P.; C.C.; Supervision of the study, M.V.M.-A. All authors have read and agreed to the published version of the manuscript.

**Funding:** Research in our lab is funded by Grants AGL2015-64522-C2-R (Spanish Ministry of Economy and Competitiveness) and ALIBIRD-CM 2020 P2018/BAA-4343 (Comunidad de Madrid). I.Z.-P. thanks BES-2016-077980 contract.

**Conflicts of Interest:** The authors declare no conflict of interest.
