LABORATORY WORK 297 



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

 if any large (usually about the size of a pinhead) colonies 

 appear on the surface, white in color and intensely acid. If so, 

 isolate one, preferably one that shows a gas bubble beneath or 

 beside it. Inoculate upon an agar slant, labeling it Bad. aerog- 

 ■ enes(f). Whether it is really that species will be determined later. 

 If no gas bubbles appear, isolate several of 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 colonies. 



6. Milk Agar. — Add 1.5% agar to some skim milk. Af- 

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

 curdles into a custard. Replace water of evaporation; adjust re- 

 Fig. 70. 



Platinum Needles. 



action 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 bacteria grow upon it more readily. 

 7. Oidium Lactis. — Procure a little soft cheese from market, 

 preferably of the Camembert or Brie type.* Pour out into a 

 petri dish a tube of plain gelatin, and into another a tube of lit- 

 mus 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 



* If such soft cheese is not to be found conveniently, oidium lactis can usually be 

 found in samples of old tub butter. 



