Insects and Diseases / A Popular Account of the Way in Which Insects may Spread or Cause some of our Common Diseases

by Rennie Wilbur Doane

BACTERIA

n the border line between the plant and the animal worlds are many forms which possess some of the characteristics of both. Indeed when an attempt is made to separate all known organisms into two groups one is immediately confronted with difficulties. In looking over the text-books of Botany we will find that certain low forms are discussed there as belonging with the plants, and on turning to the manuals of Zoölogy we will find that the same organisms are placed among the lowest forms of animals. The question is of course of little actual importance from certain points of view. It serves, however, to show the close relation of all forms of life, and from a medical standpoint it may be of very great importance owing to the difference in the life-habits, methods of reproduction and methods of transmission of many of the forms that cause disease. We have already seen that none of the diseases that are caused by animal parasites is contagious, while many of those caused by bacteria are both contagious and infectious.

Just over on the plant side of this indefinite border line are the minute organisms known as bacteria. Their numbers are infinite and they are found everywhere. The majority of them are beneficial to mankind in one way or another, but some of them cause certain of the diseases that we will have to discuss later so attention may be called here to a few of the important facts in regard to their organization and life-history in order that we may better understand how they may be so easily transferred from one host to another.

Although these bacilli are so extremely minute ([Fig. 7]), some of them so small that they cannot be seen with the most powerful microscopes, they differ in size, shape, methods of division and spore-formation. Each species makes a characteristic growth on gelatin, agar or other media upon which it may be cultivated. In this way as well as by the inoculation of animals the presence of the ultramicroscopic kinds may be demonstrated.

The method of reproduction is very simple. They increase to a certain point in size, then divide. This growth and division takes place very rapidly. Twenty to thirty minutes is sufficient time in some cases for a just-divided cell to attain full size and divide again. Thus in a few days time the number of bacteria resulting from a single individual would be inconceivable if they should all develop.

Fortunately for us, however, they do not all multiply so rapidly as this and besides there are natural checks, not the least of which are the substances given off by the bacteria themselves in their growth and development. Such excretions often serve to inhibit further multiplication. Sometimes, though not often, they form spores which not only provide for a more rapid multiplication, but enable the organism to live under conditions that would otherwise prove fatal to it.

Bacteria may be conveniently grouped under two heads: those that live upon dead organic matter, known as the saprophytic forms, and those that are found in living plants or animals, the true parasites. Such a grouping is not always entirely satisfactory, for many of the kinds that live saprophytically under normal conditions may become parasitic if opportunity offers, and also many of those that are usually regarded as parasitic may be grown in cultures of agar or other media, under which conditions they may be regarded as living saprophytically.

It is this power of easily adapting themselves to different conditions that makes many of the kinds dangerous. The bacillus which causes tetanus or lockjaw will illustrate this. It is a rather common bacillus in soil in many localities. As long as it remains there it is of no special importance, but if it is introduced into the body through a scratch or any other wound it becomes a very serious matter.

We may say, then, that the effect the bacillus has on the host depends largely on the host. Not only does it depend on what the host is, but the particular condition of the host at the time of infection is of importance. Children are subject to many diseases that adults seldom have. Hunger, thirst, fatigue, exposure and other factors may make a person susceptible to the actions of certain bacteria that would be harmless under other conditions.