106 LABORATORY EXERCISES IN BACTERIOLOGY. 



have been determined, although if it prove not above -j-2 it is not customary to 

 make any closer adjustment to the standard; if, however, it be more acid than this, 

 the proper correction should have been made before the milk is distributed to the 

 tubes. Having been tubed, it is then sterilized in the steam-bath at 100 C. by the 

 usual fractional method. 



Litmus milk is prepared by adding to the above sterilized tubes, after faintly 

 alkalinizing the milk with sodium hydroxide solution, sufficient sterile litmus solution 

 to give a distinct but pale blue tinge to the milk. If added prior to the sterilization, the 

 blue color of the litmus milk is liable to be changed to a dirty reddish-brown color. 

 The preparation is used in the study of acid production. 



PRESERVATION OF MEDIA AFTJER PREPARATION. 



In order to prevent drying of the media in tubes, which is likely to produce a 

 tough scum upon the surface unfavorable for the development of bacteria, and may 

 cause much shrinkage of the mass, it is well to close the mouths of the tubes over 

 the cotton stoppers with rubber caps, rubber stoppers, tinfoil, or some other device. 

 Before capping a tube the protruding portion of the cotton stopper should be 

 trimmed off with a scissors and the surface of the cut end of the stopper, as well as the 

 lip of the tube, flamed to destroy any infection which may possibly persist. * The cap, 

 or whatever else is employed as a cover, should have been disinfected in a carbolized 

 solution and rinsed in well-boiled water before application. Should these precautions 

 be omitted or the stopper not have been originally thoroughly sterilized, the moisture 

 evaporating from the medium in the tube is likely to collect in the stopper and favor 

 growth of any infectious elements which may not have been destroyed. Properly 

 applied, however, a rubber stopper or cap over the cotton plug is of much advantage 

 in the preservation of the medium. 



The same end may be attained by keeping the tubes of nutrient substance in 

 covered jars ; these should be well sterilized before the tubes are placed in them. More- 

 over, the tubes should be transferred to the jars directly from the sterilizer, before 

 mould-spores or bacteria have chanced to come in contact with the cotton stoppers ; 

 otherwise the moisture sure to be retained in the cotton will favor the most profuse 

 growth of contaminating organisms in the plugs and these will eventually penetrate the 

 latter and gain entrance to the nutrient substance in the tubes. If a small piece of gum 

 camphor be placed in such a jar, it will be found of considerable service in checking 

 the development of moulds, the small amount of camphor absorbed by the medium 

 from the air of the jar not being of consequence usually. 



In summer weather, especially in southern latitudes, it is best to keep gelatine 

 tubes, properly sealed and capped, in the refrigerator, as the prevailing temperature 

 is usually sufficient to liquefy the medium. 



