Handbook of Nature-Study · Lesson CLXXXVII · Topic 27: Mushrooms, molds and bacteria
187. Bacteria
Goal: Your child sees for themselves that washing hands with soap removes germs, and can explain the book's everyday rules for cleanliness.
You'll need
- 1 packet of plain, unflavored gelatin and 2 clean small plates or shallow dishes
- Plastic wrap and tape
- A slice of boiled potato in a zip bag
- A permanent marker
- A hand lens (magnifying glass)
- A nature notebook and a pencil (colored pencils if you have them)
What's in the book
The lesson gives three experiments (bacteria from moldy bread grown in boiled milk, a soiled and a washed finger on gelatin plates, and a slice of potato left open to dust), then thirteen questions about cleanliness and health.
The lesson, step by step
The leading thought
Bacteria are such small plants that we cannot see them without the aid of a microscope, but they can be planted and will grow. The object of this lesson is to enforce cleanliness.
A safety change
The book's Experiment A grows bacteria from colored spots on moldy bread in bottles of milk. We skip it: we don't know what those bacteria are, and growing millions of them at home isn't safe. We do the other two experiments, sealed, and throw them away without opening.
The finger test (Experiment B)
An adult makes the gelatin with boiling water as the box says, unsweetened, and pours a thin layer into two clean dishes. When cool and set:
let one of the children touch lightly the gelatin in one saucer for a few seconds with his soiled finger.
Ask him to wash his hands thoroughly with soap and then apply a finger to the surface of the gelatin in the other plate.
Cover each dish tightly with plastic wrap, tape it, label it and keep both in a dark cupboard.
The potato (Experiment C)
Leave a slice of boiled potato on a plate uncovered for an hour, then seal it in a zip bag with the date. Keep it in a cool, darkish place.
Look each day
Can you see that the gelatin is becoming clouded where the soiled finger touched it? This is a growth of the bacteria which were on the soiled finger.
What change has taken place in the appearance of the slice of potato? Are there any spots growing upon it?
Look through the plastic only. After a week, throw everything away sealed.
Talk it over
Ask a few of the book's questions and let your child explain each from what they saw:
Why should the hands always be washed before eating?
Why should the finger nails be kept clean?
Why should wounds be carefully cleansed and dressed at once?
Why is a damp cloth better than a feather duster for removing dust?
What we know now
Bacteria are not plants; they are tiny one-celled living things in a group of their own. Most bacteria are harmless, and many are helpful, like the ones that make yogurt and cheese and the ones living in our own bodies. Only some cause sickness, and soap and water wash most of them away.
Nature notebook
Draw the two gelatin dishes on day 1 and day 4, labeled "dirty finger" and "washed finger." Write the rule you learned in your own words.
Answers to the book's problems
The gelatin touched by the unwashed finger grows cloudy spots of bacteria; the washed finger leaves few or none. Hands carry germs to the mouth, so we wash before eating; dirty nails hide germs; wounds are cleaned so germs can't grow in them; a damp cloth picks dust up instead of throwing it into the air.
Worked out for this site from the scan. If a number in your copy differs, trust the book.