The focus of the summer school lies on host-pathogen and pathogen-pathogen interactions. In this module, we will start our training to learn some basic tools to investigate bacterial strains such as growth rate and virulence factors. To study how the pathogen-pathogen interaction takes place, we will learn to measure the formation of biofilm. The biofilm can be formed on different surfaces and is the main bacterial strategy to survive and avoid the elimination by antimicrobial treatment. Later on, we will investigate how the bacteria interact with the host cells. As soon as the bacteria interact with host cells, they can induce a cell death (cytoxicity) effect which allows the microorganisms to spread to other tissues in the host. Furthermore, the cell death produced by different microorganisms is correlated with bacterial virulence. By this assay, we can study how virulent are different bacterial strains and/or species. Finally, the pathogens can interact with host cells and be internalized. The intracellular bacteria are protected from host immune system as well as antimicrobial treatment. Thus, we will investigate the bacterial invasiveness which is correlated with the capacity of each microorganism to survive in host cells. In summary, this module will provide the main tools to investigate in vitro bacterial pathogenesis.