46 Microbes and You 



microbes. Nature had provided the answer to the problem 

 of culturing microbes as separate species. If a single micro- 

 organism carried through dust in the air, as Pasteur had shown, 

 landed on a cooked potato, it was capable of multiplying into a 

 visible growth called a colony. All members of a colony represent 

 the progeny of the original organism and hence we have a pure 

 culture. If he could just work out a few of the details, he had the 

 answer to what might have become a perplexing problem. Finally, 

 with the assistance of two military doctors, Friedrich Loeffler 

 (1852-1915) and Georg Gaffky (1850-1918), he announced that by 

 streaking mixtures of microbes over the surface of a fresh boiled 

 potato free of other organisms, colonies of the bacteria contained 

 in the mixtures could be made to develop on the potato. When 

 these colonies were well-enough isolated, they could be picked 

 from the potato with the assurance that all microbes from such a 

 colonv were alike. This was revolutionary! 



Before too many organisms had been cultivated, however, Koch 

 began to appreciate that the food requirements of bacteria varied 

 tremendously, some being a great deal more exacting about the 

 diet set before them. Joseph Schroeter (1835-1895) first separated 

 chromogens from each other in 1872 bv growing them on such 

 solid substances as potatoes, coagulated egg white, starch paste, 

 and meat, but he ran into difficultv^ when he tried to cultivate non- 

 pigmented organisms. Koch came along and knew what he 

 wanted— a solid, transparent, sterile medium. Gelatin, as a solidi- 

 fying agent, seemed to best fulfill these requirements, and since 

 mycologists had been using this material for thirty years, Koch 

 adopted it in 1881, thus revolutionizing bacteriological technic. As 

 is so often true with new ideas, time modifies their original seem- 

 ingly wonderful characteristics. Gelatin has two major limitations. 

 Some bacteria can utilize the substance as a source of food, thus 

 turning it into a liquid. If one is seeking Ivtic ferments, gelatin 

 serves a useful purpose, but the enzyme also makes the medium 

 useless from the standpoint of trying to isolate pure cultures on a 

 solid medium. A second serious defect of grelatin is that it is a 



