VII. 



CULTURES IN LIQUID MEDIA. 



to the introduction of gelatinous media by Koch in 1881, 

 cultures were made in various organic liquids, and these are still 

 largely used, being for certain purposes preferable to solid media. 

 The method of preparing and sterilizing the flesh infusions and 

 other organic liquids commonly used has already been given. We 

 are here concerned with the various modes of using these nutritive 

 liquids in cultivating bacteria. 



Flasks and tubes of various forms have been employed by differ- 

 ent investigators, but the most useful receptacle for liquid as well as 

 for solid culture media is the ordinary test tube. These are care- 

 full}' cleaned, plugged with a cotton air filter, sterilized in the hot-air 

 oven at 150 C., and are then ready to receive the filtered liquid. 

 Usually the tube should not be filled to more than one-third to one- 

 half of its capacity. Sterilization of the culture liquid is then effected 

 by placing the tubes in the steam sterilizer for half an hour on three 

 successive days. Before using, the tubes should be placed for a few 

 days in an incubating oven at 30 to 35 C. to test the sterilization. 

 This is especially important with liquid media, for if a single living 

 spore is present it may give rise to an abundant progeny, which will 

 be distributed through the liquid in association with the species 

 which has been planted. In solid cultures, on the contrary, such a 

 spore would give rise to a colony, which by its locality and characters 

 of growth would probably be recognized as different from the species 

 planted, and consequently accidental. This is the great danger in 

 the use of liquid media ; imperfect sterilization, or accidental contami- 

 nation by atmospheric germs, may lead the inexperienced student 

 into serious errors resulting from the assumption that the micro- 

 organisms present in his cultures are all derived from the seed he 

 planted. 



On the other hand, liquid media are more convenient than solid 

 when it is the intention to isolate by filtration the soluble products of 

 bacterial growth; for injection into animals to test pathogenic power; 

 for experiments on the germicidal or antiseptic power of chemical 

 agents, etc. 



