138 MANUAL OF BACTERIOLOGY. 



cheese. Moulds are also important in the ripening of some 

 kinds of cheese.f 



In examining milk for bacteria the number may be esti- 

 mated by precisely the same technique as is used for the es- 

 timation of bacteria in water, except that the milk must be 

 diluted; otherwise the plates are rendered opaque by the fat. 

 It may be diluted one hundred times with sterilized water; 

 when the number of bacteria is very great a second dilution 

 may be required. Estimations based upon such high dilu- 

 tions can only be approximate. The quantity taken for ex- 

 amination may be o.i to i c.c. Plates should be made im- 

 mediately upon collection of the sample. If the milk stands 

 for a few hours at the room temperature in the laboratory, 

 the number of bacteria becomes enormously increased. 



The detection of a particular kind of pathogenic bacteria 

 in milk or butter involves very great difficulties. Staining 

 of bacteria in milk may be done by the usual methods, but 

 the results are rendered unsatisfactory by the oil in the milk. 

 The demonstration of tubercle bacilli by staining methods is 

 hkely to involve many difficulties. In this connection it is 

 necessary to remember the group of bacilli which resemble 

 the tubercle bacillus in resisting decolorization with acids after 

 staining. (See p. 32.) The procedure of injecting milk into 

 guinea-pigs has been resorted to largely, but the results are 

 only obtained after the lapse of weeks, when the development 

 of tuberculosis in the guinea-pigs indicate that the milk was 

 tuberculous, provided that control guinea-pigs remain healthy. 

 Furthermore, the other acid-proof bacilli which may occur in 

 milk or butter are capable of producing nodules resembling 

 tubercles. t (See .Bacillus tuberculosis, Part IV.) The most 

 satisfactory plan is to apply the tuberculin- test to the cow from 

 which the milk is derived. 



Among the other articles of food, those are to be most care- 



* Conn. Agricultural Bacteriology. 



t Rabinowitsch. Zeitschrijt j. Hygiene. Bd. XXXVII., p. 439. 



