Fluids the Favorite Home of Bacteria.--Tap water, standing water, milk, vinegar, wine, cider all can be proven to contain bacteria by experiments similar to those quoted above. Spring or artesian well water would have very few, if any, bacteria, while the same quantity of river water, if it held any sewage, might contain untold millions of these little organisms.
Foods preferred by Bacteria.--If bacteria are living and contain no chlorophyll, we should expect them to obtain protein food in order to grow. Such is not always the case, for some bacteria seem to be able to build up protein out of simple inorganic nitrogenous substances. If, however, we take several food substances, some containing much protein and others not so much, we will find that the bacteria cause decay in the proteins almost at once, while other food substances are not always attacked by them.
[Illustration: Growth of bacteria in a drop of impure water allowed to run down a sterilized culture in a dish.]
What Bacteria do to Foods.--When bacteria feed upon a protein they use part of the materials in the food so that it falls to pieces and eventually rots. The material left behind after the bacteria have finished their meal is quite different from its original form. It is broken down by the action of the bacteria into gases, fluids, and some solids. It has a characteristic "rotten" odor and it has in it poisons which come as a result of the work of the bacteria. These poisonous wastes, called _ptomaines_, we shall learn more about later.
Conditions Favorable and Unfavorable to the Growth of Bacteria.--Moisture and Dryness.--_Experiment_.--Take two beans, remove the skins, crush one, soak the second bean overnight and then crush it. Place in test tubes, one dry, the second with water. Leave in a warm place two or three days, then smell each tube. In which is decay taking place? In which tube are bacteria at work? How do you know?
Moisture.--Moisture is an absolute need for bacterial growth, consequently keeping material dry will prevent the growth of germs upon its surface. Foods, in order to decay, must contain enough water to make them moist. Bacteria grow most freely in fluids.
Light.--If we cover one half of a petri dish in which bacteria are growing with black paper and then place the dish in a light warm place for a few days, the growth of bacteria in the light part of the dish will be found to be checked, while growth continues in the covered part. It is a matter of common knowledge that disease germs thrive where dirt and darkness exist and are killed by any long exposure to sunlight. This shows us the need of light in our homes, especially in our bedrooms.