Tracie Buck, age 10, of South Casco, Maine, for her question:

How big  are bacteria?

Suppose you could scale your size down down down to the size of one bitsy bacterium. Let's decide to be an average bacterium    not a giant or a dwarf. Now let's take a look around at the neighboring bacteria in our miniature world. Some seem to be as big as dogs and cats, others are as large as horses. Over there is a whopping bacterium that seems to be as big as an elephant. If we hunt around we will find others as small as tiny mice. Or so it seems.

Actually, bacteria come in very different shapes and sizes. There are midgets and monsters, just as in our world we have whopping whales and skinny little mosquitos. But even the biggest bacteria are too small for our eyes to see. For this we need a microscope, perhaps strong enough to magnify 1,000 times. With its help, an average sized bacterium looks about as big as the head of a pin. The smallest bacteria look smaller than pinpoints. The largest ones look as wide as grains of sand. Our microscope, remember, makes all these tiny creatures appear 1,000 times larger than they really are.

It reveals that bacteria come in assorted sizes    but it is very difficult to compare their true sizes with things in our world. For this we need to use our imaginations. One trick is to measure off a line one inch long. Now imagine a one inch row of bacteria, standing side by side. If you count them one by one, the answer would be about 25,000. That is if this row of midgets happens to be average¬sized bacteria. If they are smaller than average, there may be tens of thousands more in a one inch row.

It is hard to imagine their tiny sizes. But we can compare their shapes with ordinary things in our everyday world. Many types are shaped like .little sausages, either short and fat or long and skinny. Other types are round balls or curved shapes like the new moon. Many look like long wriggly worms. Some have curly grooves and some have tiny tails. We need very powerful microscopes to see these differences and to tell one bacterium type from another.

Naturally bacteria do not have bodies like the visible creatures that populate our world. Scientists tell us that they are not really animals at all and some suspect that they are not even plants. They seem to be simplified living things with a miniature kingdom of their own. Like other living things, they need food, moisture and other items. Each bacterium is merely one tiny cell. It soaks in the things it needs from outside and uses its nourishment to thrive and multiply.

Inches are too clumsy to measure the sizes of bacteria. Scientists use a small measuring unit called a micron. There are 25,000 tiny microns in an inch so the average sized bacterium measures about one micron. Many of the smaller types measure only half a micron and some of the giants measure ten microns.

Some people think that all bacteria are enemy germs. It is true that some types do cause diseases and infections in plants, animals and people. But other types help to digest our food and others enrich the soil for our crops., Most bacteria types do not help us directly or harm us at all.