LABORATORY WORK IX DAIRY BACTERIOLOGY 2QI 



the large, acid, surface colonies and some of them will probably 

 prove to be the species desired. 



In the same way isolate and inoculate on agar slants one rapid 

 and one slow liquefier, and several of the neutral type of colo- 

 nies. 



After about two days' growth purify the cultures and reserve 

 for future study. 



*No. 22. Milk Agar. Add 1.5% agar to some skim milk. Af- 

 ter half an hour boil (better to heat in an autoclav) until the 

 milk curdles into a custard. Replace water of evaporation ad- 

 just reaction to 1.5% acid filter through absorbent cotton 

 place in tubes and sterilize in the usual way. After the third 

 sterilization slant the tubes and allow to harden. This milk 

 agar is not so transparent as ordinary agar, but the lactic bac- 

 teria grow upon it more readily. Inoculate the purified cultures 

 obtained .in No. 21 upon the milk agar slant, and reserve for 

 future use. 



No. 23. Oidium Lactis. Procure a little soft cheese from 

 market, preferably of the Camembert or Brie type. 1 Pour out 

 into a Petri dish a tube of plain gelatin, and into another a tube 

 of litmus gelatin. Allow to harden. With a platinum needle 

 scrape off a little of the growth on the rind of the cheese and 

 touch it upon the surface of the gelatin at several spots. It is 

 well to try several parts of the cheese rind in this way. Cover, 

 and after two days' growth it will usually be possible to find 

 spreading colonies of Oidium on the plates. They are thin, and 

 spread rapidly over the surface; they may be recognized under 

 the microscope because at first they are seen to be composed 

 of threads like a mold, which soon break up into short sections. 

 The colonies usually become about *4 inch in diameter. (See 

 Fig. 29.) 



*No. 24. Cheese Mold. On the same plates, after 2 or 3 days, 



1 If such soft cheese is not to be found conveniently, Oidium lactis can usu- 

 ally be found in samples of old tub butter. 



