CHAPTER II 



THE BAGTERIOLOGICAL EXAMINATION OF MILK : METHODS AND 



TECHNIQUE 



Culture Media. Quantitative Examination of Milk and Method of Milk Dilution. 

 The Making and Staining of Preparations for the Microscope. Qualitative 

 Examination of Milk : — Aerobic Examinations. Method of Examination of 

 Special Organisms. Special Methods. Routine Procedure in Examina- 

 tion of Milk. 



Culture Media 



Although for the culture of micro-organisms finding their 

 natural habitat therein milk itself may be looked upon as the most 

 favourable medium, yet for differentiation, comparative, and even 

 cultural purposes, the ordinary media of the laboratory are essential, 

 and too much stress cannot be laid upon the necessity for care and 

 uniformity in their preparation. In the course of a long investiga- 

 tion extending over some years, which has been complicated by a 

 series of irreconcilable discrepancies in the description by various 

 workers of evidently identical organisms, the conviction has been 

 gradually forced upon us that until some general standard of 

 media has been adopted by bacteriologists, results obtained will 

 never be really comparable. It is in point of reaction especially 

 that the greatest divergence would appear to occur, and it must be 

 remembered that Alkalinity, Neutrality, and Acidity, are terms 

 that have no practical meaning as applied to culture media unless 

 it is stated at the same time in reference to what special indicator 

 the terms are used. A medium may be alkaline to litmus, yet at 

 the same time acid to phenolphthalein, or acid to both of these and 

 yet alkaline to fluorescine. To a medium which is just neutral to 

 sensitive litmus paper a quantity of caustic soda solution corre- 

 sponding to o-o68 per cent, of NaOH must be added before the 

 same medium becomes neutral to phenolphthalein. Media neutral- 

 ised with the latter, therefore, are more alkaline by 0068 per cent, of 



