56 



SOLID MEDIA 



Starch jelly. 



To 180 grams of water add 10 grams of potato meal and 5 grams pre- 

 cipitated calcium carbonate. Distribute in Erlenmeyer flasks or Petri 

 dishes and sterilize at 115 C. When the starch cools it forms an homogeneous 

 whitish layer on the bottom of the vessel. 



Heinemann's jelly. Heinemann recommends the following jelly in place of 

 potato. The artificial medium has the advantage of being of constant composition 

 and reaction. 



1. Prepare the following solution : 



5 grams. 



2 



2 



Asparagin, 



Di-potassium phosphate, 

 Di-sodium phosphate, 

 Magnesium sulphate, 

 Calcium chloride, - 

 Ammonium lactate, 

 Water, - 



2 



2 



2 



200 



2. Dissolve 15 grams of agar and 10 grams of peptone in the warm in 600 grams 

 of water. 



3. Mix the two solutions. Make neutral to phenol-phthalein. Filter. 



4. After filtering, and while still hot, add 30 grams of starch made into an homo- 

 geneous suspension with a little water. Boil the mixture for several minutes. 



5. Distribute in tubes. Sterilize at 120 C. for 5 minutes. Slope the tubes 

 and allow them to cool. 



Bread. 



A. Soak some slices of white bread in distilled water, place them in covered 

 glass dishes, and sterilize at 115 C. for 20 minutes. 



B. 1. Crumble some bread and dry it in the air between sheets of filter 

 paper. 



2. When dry, grind it up in a coffee mill. 



3. Put the powder in layers 1-2 cm. thick in Petri dishes or in Erlenmeyer 

 flasks, and add sufficient distilled water to soak all the bread (about 2J parts 

 of water to 1 part of bread by weight). 



4. Sterilize at 115 C. for 20 minutes. 



Rice milk. 



1. Mix intimately 



Milk, - - 150 grams. 



Peptone broth, - 50 ,, 



Powdered rice, - - - 100 ,, 



2. Distribute the mixture in Petri dishes in layers 1-2 cm. deep. 



3. Heat to 115 C. for 20 minutes. The mixture solidifies and forms an 

 opaque white layer. 



5. Coloured media. 



Coloured media are used for the recognition of particular micro-organisms 

 which produce changes of colour in them. Only a few formulae are given here, 

 the majority being reserved for description later (vide the typhoid bacillus, 

 the colon bacillus, etc.). 



Media tinted with blue litmus [or neutral-red] and containing a carbo- 

 hydrate are the most generally used : organisms which ferment carbohydrates 

 with formation of acid, when grown in a litmus medium change the colour 

 of the litmus from blue to red [and in a neutral red-medium produce a bright 

 red colour], 



Preparation of litmus solution. Granulated litmus is ground up, 85 per cent, 

 alcohol is poured on to the powder, and the mixture boiled. 6 to 8 parts of 

 water are added to the residue, and the resulting liquid mixture is heated and 



