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lar tiny microscopic creature causes a particular illness. Some scientists had come to believe the way to prove a germ caused a disease would be to find the germ in the body of someone who was sick, take the germ out and grow it on its own outside the body, and then show that infecting another animal with the germ would cause the disease. Koch developed the methods that made this process possible, and found a way to ferret out even the elusive tuberculosis.
Tuberculosis proved especially difficult from step one. The first problem in tracking down a microorganism was to find a germ that is too small to see with the naked eye - when one does not even know what to look for. Even with a microscope, one needs to have a way of distinguishing the germ from the stuff it is living in. Eventually, the trick of staining the germs with a chemical that makes them a different color from that of the other tissue around them was discovered. Although this method allowed scientists to see most bacteria, tuberculosis had one more trick up its sleeve: the germ was encased in a thick waxy coat that was impervious to any stain. It was not until 1882 that Robert Koch found that when he stained bits of TB-infected lung with one particular old bottle of methylene blue dye that he had lying around his laboratory, and then washed them with a second, brown, stain, tiny bright blue rods appeared.
Koch was brilliant but he was also lucky. It was his luck that ammonia vapors had gotten into his old bottle of methylene blue. The ammonia mixed into the dye could eat through the bacillus's unique protective coating and allow the dye to get inside. It was by accident that Koch was able to see for the first time the creature that had been killing human beings for centuries. But Koch had no way of knowing that these were the germs he sought and not just some other bugs that happened to be in this lung. Could the rods be the cause of tuberculosis?
The next step would be to grow the rods outside the body. How does one grow bacteria? Scientists had been growing the bugs in pools of a liquid nutrient. One of Koch's favorites was the liquid from inside an ox's eye. But in a world swarming with microbes, it was hard to keep other bacteria out of these pools. They became a crowded mess of bugs, making it very difficult to infect another animal with one specific germ. In solving this problem, Koch would prove his brilliance. He considered the merits of a method that had been tried by some other researchers, who had grown bugs on the cut surface of a potato.
If you have ever found mold growing on solid foods in your refrigerator, you may have noticed that it grows in distinct patches, each with its own color and texture. Koch observed about this phenomenon:
What can we conclude from these observations of colonies developing on potatoes?...Most often each colony is a pure culture [all one type of microorganism] and remains a pure culture until it enlarges to the point that it touches its neighbors. If instead of the potato, a liquid medium....were exposed to the air, then undoubtedly the same number and the same kinds of germs would develop as on the potato, but the development of these germs in the liquid would be different...Some of the organisms would sink to the bottom of the liquid, while others would rise to the top...In short, the whole liquid would reveal under the microscope...a tangled mixture of different shapes and sizes.
Koch concluded that if he could make one of his excellent liquid bacteria foods solid, he would have the perfect solution. It was actually Frau Hesse, the wife of one of Koch's co-workers, who came up with the perfect way to make a solid culture medium: agar-agar, a gelatin-like substance derived from seaweed and used