TRANSPARENT MILK AS BACTERIOLOGICAL MEDIUM 513 



calcium oxalate should be removed by centrifugation. For most 

 purposes the citrated milk appears to be the more satisfactory 

 medium, since the citrate gives changes which correspond with 

 those which take place in milk without further treatment. 

 Citrates are a normal constituent of milk. 



We have used these media for the cultivation of a number of 

 organisms — streptococci, anaerobes, and members of the colon- 

 typhoid group. A few organisms grow better in the citrated 

 milk than in oxalated milk, though the latter is useful for special 

 purposes because of the removal from it of the calcium. The 

 citrated milk shows all the cultural reactions which may be ob- 

 served in untreated milk of the same dilution. Some organisms, 

 notably the paratyphoids, produce a reaction not to be observed 

 in untreated milk. These produce in citrated milk a milky 

 translucence or opacity which we attribute to the decomposition 

 of the citrate with liberation of calcium in the ionized state, an 

 appearance which may be produced artificially by the addition 

 of a small amount of calcium chloride to the sterile medium. This 

 reaction is not produced by cultures in oxalated milk from which 

 the calcium has been removed. 



The cultural reactions which we have observed in transparent 

 citrated milk may be summarized as follows: 



I. Neither acid production nor digestion of casein. Reaction alkaline or neutral 



a. The medium remains clear except for the clouding due to visible growth. 

 The addition of a few drops of CaCU solution causes it to become 

 milky. Example: Bact. t)-phosum. Bact. alkaligenes 



6. The medium becomes milky, probably due to a release of ionized calcium. 

 Example: Bact. paratyphosum, Bact. cholerae-suis 



II. Acid production 



a. A small amount of acid may do nothing more than change the color of 



the indicator which may be added to the medium. Example: Strep. 



pyogenes 

 6. A larger amount of acid may produce translucence. Often observed as 



a transitory reaction. Example: Bact. cloacae 

 c. Large amounts of acid produce coagulation or precipitation of casein. 



Example: Strep, lacticus, Lactobacillus bulgaricus, Bact. coli, 



Clostridium welchii 



III. Rennin production 



a. Without release of calcium from citrate should give precipitate when 

 calcium chloride is added unless the casein has been digested 



