Andra Harris, age 12, of Waynoka, Oklahoma, for her question:

What percentage of bacteria are harmful?

Scientists hesitate to give a specific answer to this question. These midgets are invisible to the naked eye and tracking them down requires tedious research. Naturally the most urgent targets are those that are potential enemies of people, plants and animals. We also know many beneficial types, but not a fair enough proportion of the neutral types.

Thousands of bacteria strains have been identified but nobody knows how many others are waiting to be discovered. Hence, we can only guess what percentage of the total bacteria population is harmful. Some bacteriologists assume that the 1 teeming majority is made up of neutral types that do no direct good or harm to living plants, animals or people. Certainly these far outnumber the known baddies.

Back in 1941, researchers at Cornell University tried to estimate the total bacteria population of the United States. The fascinating project was based on samples taken from the soil and water, from the surface of plants and the insides of animals and people. The astronomical total ran to more than ten septillion, each septillion being figure one with a tail of 24 zeros. Nobody claimed that this impossible census was anything but a rough estimate and nowadays most experts might say it was way too low.

However, the categories used to assemble this bacteria census provide some helpful information about our friends and enemies. The percentages were estimated on numbers of bacteria in different categories. Though the investigators warned that figures for harmful bacteria are likely to be higher than they really are.

Based on some 10 septillion bacteria we could encounter in the United States, about 100 quintillion could be harmful to people. Animals could be harmed by twice that number but only one tenth as many might injure the plant world. These figures do not give a clear or precise picture and show that estimating the number of our bacterial enemies is just about impossible.

A more general figure estimates that of every 30,000 bacteria, only one is potentially harmful to living people, animals and plants. However, this would be much higher during an epidemic, when disease bacteria tend to multiply far faster than helpful and harmless ones.

We must conclude that at present it is impossible to give a precise estimate of the harmful, helpful and harmless bacteria that share every corner of our world. But we do know that our enemies are a very small minority. They are way outnumbered by those that help us directly or indirectly and mostly likely the largest majority are neutral bacteria that neither help nor harm us.