CULTURES IN SOLID MEDIA. 75 



tube. This is due to the fact that a little fluid is pressed out of the 

 jelly, probably by a slight contraction while cooling. If the tubes 

 are slightly inclined from the horizontal the film does not slip and 

 the fluid accumulates at the bottom. After a day or two they may 

 be placed in an upright position. 



These roll tubes possess several advantages. They are quickly 

 made and take but little space in the incubating oven, and the film 

 of jelly is protected from contamination by atmospheric germs. 

 When colonies have formed we may examine them through the thin 

 walls of the tube, either with a pocket lens or a low-power objective. 

 In making a stick culture from a single colony in one of these roll 

 tubes, we invert the tube, remove the cotton air filter, and pass the 

 point of a sterilized platinum needle up to the selected colony. In 

 the same way we obtain material for microscopical examination. 



Streak Cultures. In his earlier experiments with solid culture 

 media Koch made " streak cultures" by drawing the point of a plati- 

 num needle, charged with bacteria, over the surface of a gelatin or 

 agar plate ; and this method is still useful in certain cases. If we 

 draw the needle over the moist surface several times in succession 

 the greater number of bacteria will be deposited in the first streak, 

 and in the second or third single cells are likely to be left at such 

 intervals from each other that each will develop an independent 

 colony. If the streaks were made with impure stock we may thus 

 succeed in getting separate colonies of the several bacteria contained 

 in it, so that this method may be employed for obtaining pure cul- 

 tures. But for this purpose it is much inferior to the plate method, 

 and it is chiefly used for observing the growth of bacteria on the sur- 

 face of solid culture media. Thus we commonly make a streak upon 

 the surface of cooked potato or solidified blood serum in studying the 

 development of various bacteria on these culture media. 



Cultures upon Blood Serum. The use of blood serum as a 

 solid medium is practically restricted to stick cultures and streak 

 cultures, for we cannot substitute it for the gelatin and agar media 

 in making plates and roll tubes. This is because it only becomes solid 

 at a temperature which would be fatal to most bacteria (70 C.), and 

 when once made solid by heat cannot again be liquefied. Its use is, 

 therefore, restricted mainly to the cultivation of bacteria for which 

 it is an especially favorable medium. It may be used, however, in 

 combination with a gelatin or agar medium. For this purpose it is 

 most conveniently kept in a fluid condition in the little flasks hereto- 

 fore described (" Sternberg's bulbs "). 



The gelatin or agar jelly in test tubes is liquefied by heat and 

 cooled in a water bath to about 40 C. The desired amount of ste- 

 rile blood serum is then forced into each tube by passing the slender 



