2. They are microscopic in size and hence the more difficult to deal with.
3. They are found everywhere that there is life—in the air, in water, in the soil.
4. They multiply very rapidly under favourable conditions.
5. Some bacteria are useful to the housekeeper; many kinds are her enemies.
6. Some of these enemies get into food and, growing there, cause a change in it—then we say the food is spoiled.
CONDITIONS OF BACTERIAL GROWTH
All plants have the same requirements. Any well-known plant may be put before the class to help them to think of these. They must be told that microscopic plants differ from other plants in one respect; they do not need light. Hence bacterial requirements are as follows: (1) water, (2) food, (3) air (oxygen), (4) heat.
The class should be led to see that if any one of these conditions is removed, the remaining ones are insufficient for the plant's activity.
MEANS OF OVERCOMING BACTERIA
To the housekeeper, preserving food means overcoming bacteria. There are only two ways of doing this, either of which may be chosen: