36 LABORATORY MANUAL OF MICROBIOLOGY 



Solution B is made by mixing two parts of potassium silicate 

 solution (specific gravity, 1.085)^ with one part of a 1 per cent 

 KOH solution. This solution will gel on standing. The alkali 

 solution (1 per cent) should be mixed with the silicate solution 

 to make Solution B only a short time before it is to be used. The 

 solution will keep for about 2 weeks to 1 month. 



These two solutions are then titrated against each other with 

 an indicator, which shows the reaction desired. 



The solutions can be sterilized. Solution A is sterilized in an 

 autoclave for 10 minutes at 110°C. Solution B, however, will 

 solidify if treated in a like manner. It is necessary, therefore, to 

 put it in a sterile flask and boil for 5 to 10 minutes. 



For growth of bacteria, a medium is prepared as follows: 



1. 100 cubic centimeters of A, and 50 cubic centimeters 

 distilled water. 



2. 200 cubic centimeters of nutrient liquid (preferably steri- 

 lized longer than the silicate solution). 



3. Amount of B required to bring medium to desired reac- 

 tion. These three solutions are cooled as rapidly as possible 

 after sterilization, and are then poured together. Plates can 

 then be poured without danger of the gel solidifying before 

 pouring is completed. Time required for setting of the gel is 1 

 to 6 hours. 



Five grams of thoroughly ground cellulose is suspended in 

 100 cubic centimeters of the following solution. 



Diammonium phosphate ((NH4)2HP04) 5.0 gm. 



Magnesium sulphate (MgS04-7H20) 1.0 gm. 



Ferrous sulphate (FeS04-7H20) 0.02 gm. 



Potassium chloride (KCl) 1.0 gm. 



Distilled water 100.0 cc. 



About 2 cubic centimeters of the suspension of the cellulose in 

 the medium is poured upon the surface of each sihca plate, in 

 such a manner as to have the cellulose uniformly distributed 

 over the surface; some CaCOs is then powdered on over the 

 whole surface of each plate. The plates are placed in an incu- 

 bator at 60 to 65°C., until the excess of water has evaporated, 



1 11.6 grams potassium silicate made up to 100-cubic centimeters solution 

 gives specific gravity = 1.085. 



