After a week, old media must be removed and new media must be added. Why?
Growth of bacterial population in the lab is known as culture. Pure culture contains only one type of bacteria.
If bacterial culture is kept in the media for a week means in old media then the cells use all the available nutrients and excrete toxic metabolites and when these nutrients are depleted, or wastes are accumulated or some other factor then bacterial population will die. Products that are made by bacteria are harvested from the culture.
But if old media will not removed then wastes are accumulated and nutrients are depleted that results in death or decline of bacterial population.
Therefore, new media must be added to make continuous growth the bacterial population and the products made by the organisms are harvested from the culture