(20) The owners brought Pasteur beer of different quality. They wanted to see whether Pasteur was talking big. Pasteur sat in front of the microscope, and the nature of each kind of beer was correctly distinguished by him.

(21) Pasteur said, “We could kill harmful microorganisms at appropriate high temperature.” It really worked. The beer did not turn sour after being heated. The brewers, upon seeing this, couldn't admire Pasteur more. This is the “pasteurization” still in use today.

(22) In 1865, the silkworm industry in France suffered a great crisis. A large number of silkworms were covered with black spots and died. Pasteur discovered a new kind of oval bacteria on the dying silkworms with a microscope. He put forward proper methods to prevent the bacteria and thus saved the declining sericulture.

(23) Bacteria made the wine sour and were the chief cause of silkworm disease. But what about human infectious diseases? Did the bacteria play tricks too? Certainly, with repeated experiments, Pasteur made sure that those tiny bacteria were to blame!

(24) Pasteur announced the result of his experiments at a lecture of the Paris Medical College. The audience were in a shock: how could small bacteria cause various deadly infectious diseases? What a surprising finding!

(25) Pasteur's finding caused a sensation in European medical circles. Even Robert Koch, a young doctor in a remote German village, was also fascinated by the mysterious bacteria. In addition to providing medical services for the villagers every day, he immersed himself in bacterial study.

(26) On his 30th birthday, his wife used her savings to buy him a microscope as a birthday gift. Koch was so grateful to his wife, for nothing would be more precious to him.

(27) From then on, he devoted himself to studying various bacteria under the microscope day and night. In order to obtain bacteria specimens, he prepared delicious broths as a medium for better and faster bacteria growth and reproduction.

(28) However, all kinds of bacteria were mixed and bred together in the nutrient broths. How could he obtain a pure culture with a single kind of bacteria? Koch pondered day and night, racking his brain to find a solution to this question.

(29) It suddenly occurred to him that the bacteria would not move or mix together if they grew on a solid surface instead of in broths. But nutrition would be in shortage this way. Hum! If only there was something that existed as both liquid and solid simultaneously!