The War Against Disease. Bacteriology

One couldn't expect to keep all deadly microorganisms away from all human beings at all times. Sooner or later, exposure to disease was certain. What then?

To be sure, the body had ways of fighting microorganisms, since it could recover from infections spontaneously. In 1884, the Russian-French biologist, Ilya Hitch Mechnikov (1845-1916), was to find a dramatic example of such "counterbacterial warfare." He was able to show that the white corpuscles of the blood, equipped with the capacity to leave the blood vessels if necessary, flocked to the site of infections or of bacterial invasion. What followed was very much like a pitched battle between bacteria and white corpuscles, with the latter not necessarily always winning, but winning often enough to do a great deal of good.

Yet there had to be more subtle antibacterial weapons, too, since in the case of many diseases, recovery from one attack meant immunity thereafter, although no visible changes in the body could be found. A logical explanation for this was that the body had developed some molecule (an "antibody") which could be used to kill an invading microorganism or neutralize its effect. This would explain the effect of vaccination, since the body might have developed an antibody against the cowpox microorganism and found it usable against the very similar smallpox microorganism.

Now at last that victory could be repeated not through an attack on the disease itself but on the microorganism that caused the disease. Pasteur showed the way in connection with anthrax, a deadly disease that ravaged herds of domestic animals. Pasteur searched for a microorganism that would cause the disease and found it in the form of a particular bacterium. He heated a preparation of such bacteria just long enough to destroy their ability to bring on the disease. These helpless "attenuated bacteria," by their mere presence would force a body to develop antibodies against them, antibodies that could be used against the fresh, deadly bacteria, too.

In 1881, Pasteur carried through a most dramatic experiment. Some sheep were inoculated with his attenuated bacteria while other sheep were not. After a time, all the sheep were exposed to deadly anthrax bacteria. Every sheep that had been first inoculated survived without ill effect; the others caught anthrax and died.

Similar methods were established by Pasteur in the fight against chicken cholera and, most dramatically of all, against rabies (or hydrophobia), the disease caused by the bite of a "mad dog." In effect, he was creating artificial cow poxes, so to speak, to protect men and animals against a whole variety of small poxes.

The success of Pasteur's germ theory created an intense new interest in bacteria. The German botanist, Ferdinand Julius Cohn (1828-98), had been interested in microscopic plant cells in his youth. He showed, for instance, that plant protoplasm was essentially identical with animal protoplasm. In the 1860s, however, he turned to bacteria and, in 1872, published a three-volume treatise on the little creatures in which the first systematic attempt was made to classify them into genera and species. For that reason, Cohn may be considered the founder of modem bacteriology.

Cohn's most important discovery, however, was of a young German doctor named Robert Koch (1843-1910). In 1876, Koch had isolated the bacterium causing anthrax and learned to cultivate it (as Pasteur was doing in France). Koch brought his work to Cohn's attention, and the enthusiastic Cohn sponsored him vigorously.

Koch learned to grow bacteria on a solid gel, such as gelatin (for which, later, agar-agar, a product of seaweed, was substituted), instead of in liquid. This made a great deal of difference. In liquid, bacteria of different varieties mix easily and it is difficult to tell which variety may be causing a particular disease.

If, however, a culture were smeared on a solid medium, an isolated bacterium would divide and redivide, producing many new cells that would not be able to move from the spot. Though the original culture might be a mixture of many species of bacteria, that one solid colony would have to be a pure variety. If it produced a disease, there could be no question as to which variety was responsible.

Originally, Koch placed his gel on a flat piece of glass, but an assistant, Julius Richard Petri (1852-1921), substituted a shallow dish with a glass cover. Such "Petri dishes" have been used in bacteriology ever since.

Working with pure cultures, Koch was able to evolve rules for the detection of the microorganism causing a particular disease. He and his assistants discovered many such, and the high point in Koch's career was his identification, in 1882, of the bacterium that caused tuberculosis.