LABORATORY WORK IN DAIRY BACTERIOLOGY 289 



When the plates are ready, count as follows: 



(7. The number of liquefying colonies. These may roughly 

 be separated into those that liquefy rapidly and those that liquefy 

 sloivly, giving two easily distinguishable types. 



b. Count the number of Bad. lactis acidi (see page 30). 



r. Count the number of B. aero genes (including B. coli). 

 These can be recognized by their brilliant acid, large surface 

 colonies and frequent gas bubbles. 



d. Count the number of colonies that neither produce acid, nor 

 liquefy gelatin, and group them together as neutral species. 

 This last group will contain many different kinds of bacteria, 

 and may, with experience, be differentiated into several other 

 types; but it is hopeless for the beginner to do much more than 

 is indicated above in qualitative analysis. 



*No. 21. Separation of the Common Species of Bacteria from 

 Milk. Allow some milk to stand at about 70. As soon as it 

 begins to sour, but before it curdles, dilute I c.c. 500,000 times 

 and make litmus gelatin plates. Allow to grow four days at 70 

 unless liquefiers make it impossible. 



a. Determine the total number of bacteria in I c.c. of milk. 



b. Determine the number of acid bacteria per c.c. 



c. Determine the number of liquefiers per c.c. 



d. Determine the percentages in each case. 



e. Isolate a colony of Bact. lactis acidi. This may be recog- 

 nized as follows: It is an intensely acid colony, rather opaque, 

 always below and never on the surface. It is small, only just 

 visible to the naked eye and when examined under a low power 

 microscope it frequently, though not always, shows a slight 

 roughness, looking like minute spines around its edge. (Fig. 16.) 

 With a platinum needle lift out one of these colonies and inoc- 

 ulate into a gelatin tube by stabbing the needle directly into the 

 gelatin as shown in figure 82. After growth purify as described 

 in experiment 13, label and set aside for future use. 



/. Look over the plates made in this and the last experiment 



