THE BACTERIOLOGIC EXAMINATION OF MILK 415 



than beef extract agar, or a medium with 1.5 per cent. agar. 

 Conn, on the other hand, believes that the difference in com- 

 position and reaction of media is not of great importance. Ayers 

 recommends casein agar as the most suitable medium for milk 

 examination, especially when it is desirable to determine the 

 number of liquefiers in milk. The author states that casein 

 agar is not as favorable for lactic acid bacteria as other media, but 

 more so for liquefiers. Ayers' casein agar is prepared as follows: 

 "To 300 c.c. of water (distilled) add 10 grams of casein (Eimer 



6 Amend C. P. casein prepared according to Hammarsten) and 



7 c.c. normal sodium hydroxid. Dissolve casein by heating to 

 boiling. It is desirable to let this stand for several hours to get 

 a perfect solution. This is not necessary, however. Make up 

 volume to 500 c.c. and bring the reaction of the solution to be- 

 tween +0.1 and +0.2 Fuller's scale. Do not allow solution 

 to become alkaline to phenolphthalein or over +0.2. If the casein 

 is weighed accurately and the normal solution is accurate the re- 

 action will be about +0.2. 



"The agar solution is prepared by dissolving 10 grams of agar 

 in 500 c.c. of water. 



"Both casein and agar solutions should be filtered, then mixed. 

 Tube and sterilize in autoclave under pressure for twenty minutes; 

 then cool the tubes quickly in cold water or ice water. The final 

 reaction of the medium will be about +0.1, Fuller's scale. If the 

 medium is alkaline the bacterial growth will be restricted. If the 

 medium is more than + O.-l some of the casein may be precipitated 

 during sterilization. The casein agar should be clear and almost 

 colorless when poured into Petri dishes." 



The time and period of incubation of the plates has to be 

 considered. Bacteria in milk are chiefly of fecal origin, but there 

 are others which may multiply with difficulty at high tempera- 

 ture. It is hopeless to attempt to account for all types of bac- 

 teria in milk by using one kind of medium, one temperature of 

 incubation, and only aerobic cultivation. However, other things 

 being equal, the best method is the one which gives the largest 

 number of colonies and the largest number of varieties in a rela- 

 tively short time. If the plates are incubated at 37 C. the fecal 

 bacteria will multiply at a great rate, the colonies will be rela- 

 tively large, and small colonies of other bacteria may be obscured. 

 At lower temperature fecal bacteria grow quite well, as a rule, 

 but the colonies are not so large, and those of other bacteria have 

 a chance to assert themselves. At room temperature, therefore, 

 we may expect not only a larger count but also a fairer repre- 

 sentation of groups or types of bacteria than at 37 C. Colonies 

 form more slowly at lower temperature than at 37 C., and this 



