Cambridge Secondary (7-9). Stage 7. MCE Science. Workbook - Unit 5: Food Chains and Food Webs - Question 4

Five nutrient-agar Petri dishes are labelled 1 Control (not opened), 2 Inside of mouth, 3 Unwashed fingertip, 4 Clean fingertip and 5 A chosen surface. Observe through the closed lids; do not reopen growing cultures. Your teacher handles safe disposal.

Why do you think microorganisms can grow on the agar?