Dear Jyoti Mayekar many thanks for your interesting technical question. In general, zinc is an essential element for the human body so that zinc is not significantly toxic to humans. However, ZnO nanoparticles are smaller than the cells of bacteria. Thus they are able to disrupt the cell membranes of bacteria and inhibit their growth. When we would swallow / eat ZnO nanoparticles, they would just dissolve in the stomach acid and do no harm.
For more general information about this issue please have a look at the following useful articles:
Properties of Zinc Oxide Nanoparticles and Their Activity Against Microbes
Article Properties of Zinc Oxide Nanoparticles and Their Activity Ag...
Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism
Article Review on Zinc Oxide Nanoparticles: Antibacterial Activity a...
Antibacterial Activity and Mechanism of Action of Zinc Oxide Nanoparticles against Campylobacter jejuni
https://aem.asm.org/content/77/7/2325
The first two articles are freely available as public full texts on RG.