It is also possible to include dyes, antibiotics and other substances that speed up or slow down the growth of certain types of bacteria to help us select and identify particular bacterial species. This photo, for example, shows a chromogenic agar plate, which contains special dyes that change colour in the presence of substances produced by the bacteria as part of their survival and growth. Pink colonies in this case, for example, contain enzymes that break down lactose, a type of sugar. - Nicole Stoesser