578 S. HENRY AYERS AND COURTLAND S. MUDGE 



remain on the plate for five minutes or until it has penetrated 

 through the agar. All the acid-forming colonies show a yellow 

 color against a purple background. Of course when the plate 

 is crowded the whole plate may be yellow (acid). If desired, 

 brom-cresol purple can be added to the medium at the time of 

 preparation. To get the proper color, add 8.0 cc. of stock solu- 

 tion to each liter of medium; the preparation of the stock solution 

 is described on page 573. 



The peptonizers should not be counted until the plate has 

 been flowed with a 5 per cent solution of acetic acid, because 

 the medium is often rendered sufficiently acid to cause a pre- 

 cipitation of the casein except about colonies which produce a 

 strong alkaline reaction. The zone about such colonies under 

 this condition may be clear and resemble peptonization. Flowing 

 with acid will cloud these zones unless there is true peptonization. 

 After flowing the plate with acid allow the acid to stand on the 

 milk-powder medium for ten minutes, then make the count of 

 peptonizers. The acid precipitates the dissolved casein of the 

 milk powder except about colonies where it has been digested. 

 Evidence of this casein peptonization is shown by a clear zone, 

 and colonies should only be counted as peptonizers when there 

 is a definite, clear zone. The activity of the bacteria in the 

 colony can be further estimated by the diameter of the zone. 

 In case no peptonizers show on the plate which is being counted, 

 they should be counted on plates with a lower dilution. They 

 can be easily counted even though the plate is crowded. 



When the plate has been flowed with brom-cresol purple, 

 then with acetic acid, the plate is yellow instead of white. This 

 does not interfere in any way with the counting of the peptonizers. 



The milk-powder medium, therefore, gives a picture which 

 varies greatly with different samples of milk. Sometimes there 

 are numerous colonies of strong acid-forming bacteria, sometimes 

 none, sometimes many peptonizers and again only a few. While 

 the relation between these various colonies and the total count 

 has not been fully worked out, we believe that there are certain 

 correlations which will be found to have a definite relation to 

 the history of the milk. 



