Biofilms surround us in everyday life. Their abundance and growth has a huge impact on the well-being and health of humans. In medical devices, such as dental implants, biofilms can cause an infection of the gum and jawbone, which is known as mucositis. In the worst case, this infection can worsen, leading to peri-implantitis and a potential subsequent loss of the implant. Through better understanding of growth mechanisms of biofilms severe courses of these diseases could be prevented. Ultimately, in-vitro and in-vivo experiments are essential to understand biofilm formation. These types of experiments are, however, time consuming and expensive. A suitable numerical model which leads to sufficiently good results in an in-silico experiment can mitigate these shortcomings. In this talk, a biofilm growth model is presented [1] which derives itself directly from the Hamilton principle of stationary action. The first and second laws of thermodynamics are therefore directly included in the derivation of the model and thus automatically fulfilled. Additionally, the model has two growth mechanisms working in parallel, volumetric growth, the growth mechanism where the mass stays constant, leading to a change in density if no other conditions are implemented, is combined with density-based growth where, in the pure case, the density stays constant. The combination is the first of its kind in literature. The in-silico experiments performed with this model give good phenomenological accordance with the expected behavior of a biofilm. The model proves to be very reactive to small changes in geometry and input variables and thus able to capture a variety of growing behavior. Additional investigations, where the model is tested against in-vitro experiments, also show a good agreement between both experiments qualitatively. The presented model is therefore deemed to be very capable in the following investigations of biofilm formation on dental implants.