A method of inoculating a culture medium including providing a droplet including a single cell type on a droplet actuator and inoculating a culture medium with the droplet. A method of providing a metabolically useful substance to a cell culture, including providing a droplet actuator including a cell culture droplet loaded thereon, the sample droplet including cells and a cell culture medium, and a second droplet comprising a metabolically useful substance. The method also includes conducting one or more droplet operations to combine the cell culture droplet with the second droplet on the droplet actuator. Related methods, droplet actuators, and systems are also provided.