Bugs on Fingers (printer friendly)

These protocols are thought to be suitable for S1/S2 pupils but all schools should conduct the necessary risk assessments before performing the experiment.

Objective
The objective of the experiment is to observe the differences in bacterial/yeast colony number and colony morphology
between blood agar plates. One plate with unwashed fingerprints and another with washed hand fingerprints. Main
hypothesis is that there will be less overall growth and activity on the clean plates.

Context in real-life
Highlights both the importance of personal hygiene and the normal relationship our bodies have with micro-organisms.

Links with the curriculum
To demonstrate using two or three simple examples (e.g. S.aureus as a opportunistic pathogen) how bacteria can also harm us.

Kit needed
• Blood agar plates (A non-selective medium for the isolation and cultivation of many pathogenic and non-pathogenic
microorganisms. Available from Sigma. Cost unknown but should be cheap)
• Incubator
• Optional: Microscope/Camera/Pens and Papers

Description of experiment
1. Blood agar plates prepared by school lab technician.
2. Each student is provided with two plates each. One for clean finger and one for dirty finger. Students wash their hands
AFTER setting up ‘dirty’ plates.
3. Plates are labeled, taped and then incubated at 37oC for 24-48 hours.
4. Students then score what has grown with a provided key on the using the criteria.

Likely outcomes and what they mean
• Is the main hypothesis true? Students can possibly draw what they see on their plates.
• If a camera is available then maybe take pictures?
• More likely follow-up exercise would be looking at the plates under a light microscope. Drawings of individual cells?
• The teacher can explain the gram stain and other simple techniques that scientists use to identify different
microorganisms.
Scoring Plates
• Form – What is the basic shape of colony? (http://www.sciencebuddies.org/mentoring/project_ideas/MicroBio_
Interpreting_Plates.shtml)
• Elevation – What is the cross sectional shape of the colony? (see sciencebuddies website)
• Surface – How does the surface of the colony appear? e.g. smooth, glistening, rough, dull wrinkled??
• Opacity – Transparent? Opaque? Translucent (almost clear)? Iridescent (changing color in changing light?)
• Pigmentation – White, buff, red, purple, etc…

Opportunities for extension
• Cotton bud swabs can be taken and incubated from different parts of the body or school for a comparison.
• Maybe a ‘how clean is your school’ style experiment? This is relevant to modern day hospital working environments etc...


This activity sheet is one of several developed by early career scientists at the MRC Human Reproductive Science Unit in
Edinburgh. You can download all the activity sheets at www.hrsu.mrc.ac.uk/schools. If you experience problems
accessing the website, please email schools@hrsu.mrc.ac.uk.