(30) “Agar it is!” His wife mumbled when she was awakened by him from dream. “Yes! It's marvelous agar!” Koch cried out in excitement, and immediately took his wife to the kitchen to prepare some agar.

(31) When the mixture of broth and agar cooled down and condensed into a flat frozen block, he divided it into many smaller blocks, and inoculated the bacteria carefully on them. This time, the bacteria could no longer swim freely.

(32) As bacteria multiplied into one family after another, the world's first culture medium for growing purebred bacteria was created. Nowadays, such “solid medium” is still in use in laboratory for breeding pure bacteria.

(33) However, to his dissatisfaction, Koch found the bacteria under the microscope too tiny and too transparent to see clearly. If only I could dress the bacteria in colorful clothes, he thought.

(34) Then he bought a load of dyes and tried over and over again to color the bacteria. Dozens of times though he tried, the bacteria never put the colorful clothes on easily.

(35) Refusing to lose heart, Koch looked for other dyes all around. Finally defeated by aniline, the stubborn bacteria put on an indelible blue suit. Their slender outlines were seen clearly under the microscope.

(36) “Solid medium” and “bacteria staining method” facilitated scientists all over the world to findmore and more bacteria. The first killer bacteria of human life were discovered by Loeffler, Koch's most outstanding pupil.

(37) Loeffler was German. In his time, the diphtheria was rampant and he had to watch many times the painful death of innocent children. He was deeply ashamed of his inability as a doctor.

(38) To save more innocent lives, he believed the first thing to do was finding the “killer”. Loeffler started to study the tunica albuginea growing in the throat of the sick children. By using the “bacteria staining method”, he finally found a kind of matchstick-shaped bacteria.

(39) With great joy, Loeffler told his teacher about what he had found. “Don't jump to conclusions.” Koch said calmly, “You should undergo animal experiments to prove that this kind of bacteria did cause diphtheria. Only then will your findings be scientific.”