Counting bacterial colonies on an agar plate

This worksheet is aimed at either GCSE or A-Level students studying microbiology. The worksheet is an ideal lesson starter to use following preparation and incubation of agar plates that have been inoculated with bacteria (from unpasteurised milk in this example, but easily adaptable for any other source of bacteria). The worksheet introduces the idea of binary fission and the concept of a visible colony being made of many thousands of individual bacterial cells. The worksheet also includes an activity on counting bacterial colonies and estimating numbers based on dilutions that would set students up well for a subsequent lesson where they did this activity using their own agar plates.

Key words: Agar, bacteria, colony, estimate, milk, pasteurisation, binary fission