Germ Farm - Growing microbes from the environment

Two weeks ago we demonstrated how germs move between people when they touch each other and play.  Still, it is very difficult for children to understand that germs can be present even when they do not see them.  In this activity, we will be able to grow the tiny microbes in the environment so that they can be seen with the naked eye.  My children had the opportunity to choose the places that they thought these invisible (to the naked eye) germs might be and then to see how they grew. 

Parent Background Guide

We all know that microbes are all around us.  Some are harmful, but many many more are either beneficial or do not affect us in any way.    The goal of this experiment is to grow some of those microbes.  Most likely we will be growing yeasts and molds whose spores are part of every non-sterile environment.  These are the living things that make leftovers into science experiments in our refrigerators and that cause laundry left wet to long to mildew, but they are also the source of important things like antibiotics, medicines, cheese, and the ever popular beer.

Why potatoes for growing the microbes?  Professional microbiologists make a variety of different types of growth media using potatoes.  In most cases they have cooked or ground the potatoes and combined them with other ingredients.  However, even raw potatoes like we will use in the lab have a high carbohydrate content and are slightly acidic.  This provides a good basis for the growth of fungus, yeasts, and some bacteria infecting plants and plant decomposition.  This is very useful for growing environmental pathogens, but for our purposes it also helps to add a bit of safety because bacteria that would be dangerous to people cannot typically grow on potato based media. 

This week’s activity

Materials
One large potatoe or 2 small ones.
Knife and freshly cleaned cutting board (improve your results by pouring boiling water over the board before you start)
4 clean ziplock plastic bags (not recycled, or use one recycled to see if they really get clean)
A sharpie
Either a dirty house of the courage to look like a bit of a freak rubbing your potato on surfaces in public
Latex gloves (optional, but they improve accuracy)

Preparation:   Rinse the outside of your potato.  Wash your hands carefully.  For the most accurate results sterile your cutting board and wear gloves when cutting the potato.  Probably not necessary, but it’s nice to start with good techniques early.  Slice your potatoes into four pieces maximizing the cut surface area.  Put one potato into each plastic bag and seal. 

Discovery time: Label one bag “control” and explain to your child that you will be leaving that potato alone to see what happens when you try to keep the potato clean.  Let your child help you decide what types of places they would like to test for germs.  We did unwashed hands after school, the dirt outside, a public restroom, the door to a public place, and a toy in a public area.  Label the remaining 3 bags with the item you would like to test.  Have your child wash their hands very carefully and/or wear gloves.  Then help them to rub the open part of the potato on the item they want to test.  Place each slice back in the plastic bag and seal.  Put all bags in a dark warm area (I had mine in a paper bag in the laundry room).  The warmer the area, the faster you will see results and the more likely you will get colonies of bacteria as well as fungus. 

Tying it together:  Keep the potato slices still in their bags without opening them.  If the room is reasonably warm you should see some good growth after one week, in colder areas you might need to wait up to three weeks.  Place the bags next to each other and allow your children to draw conclusions about the dirtiest and cleanest areas they tested.  Help them to understand that the control bag shows what bacteria and fungus are present all around then in the air, in the kitchen, and on the potatoes themselves.  Ask questions like, where was the dirtiest area the potato touched?  What would you want to do after touching that area? 

DISPOSAL:  Seal the bags in a larger plastic or ziplock bag and then throw it away.  Do not open just in case you grew something nasty.



Toys in a public play area. (a little bit of growth)



Doors entering a public area -heavy growth in one area, very little on the rest of the surface.



The Control - very little growth, concentrated where the knife first sliced (I did not sterilize my tools).

A public restroom - lots and lots of yeasts and molds.



Dirt outside our house - very little growth.

Max's hands afterschool.  Growth over most of the potato.