Electron microscopy is useful in studying the interactions between S. aureus and polyurethane nanoparticles as good models for bacteria-polymer relations. A valuable ensemble of investigation tools allows not only to understand the cellular dynamics, but provides information about nanoparticles delivery (to host cells and consequently to tissues and organs) as well. Analysis of the electron images can bring better comprehension of processes such as adhesion, in response to the reciprocal attraction between nanoparticles and cells, and endocytosis. Understanding the course of nanoparticles, we can suppose the existence of reversible mechanisms (exocytosis), and clear up how bacteria-host cells interactions work. Electron microscopy gives helpful answers for the research in toxicology and raises new questions about the relations between bacteria and polymeric materials