Identifying bacterial cultures. Technician using Analytical Profile Index (API) test kits to identify bacterial cultures. Small samples of a bacterial culture (several cultures seen in the petri dishes at lower centre and centre right) are added to the wells on the test kit, each of which contains a different chemical substrate. Reactions between the bacteria and the substrates produce colour changes in the wells. The sequence of colour changes produced is characteristic and can be used to identify the bacteria in a culture.