i88 MILK AND THE PUBLIC HEALTH CHAP. 



accurately determined. The problem is, however, neither 

 so difficult nor so important as the same question applied to 

 water supplies. Lactose fermenters 'of coli type are absent 

 from pure milk drawn without contamination, and as far as is 

 at present known all such organisms indicate outside pollution 

 of the milk, and in general (directly or indirectly) manurial 

 pollution. 



III. DETERMINATION OF THE NUMBER OF STREPTOCOCCI 



To estimate the number of streptococci in milk the 

 method recommended as the simplest and most reliable is to 

 add diluted fractions of the milk, 1-0, O'l, O'Ol, O'OOl c.c. etc., 

 to tubes of glucose neutral red broth. Ordinary broth will do, 

 but the neutral red broth is preferable and gives better results. 

 The tubes are incubated for two days at 37 C. and then 

 examined, in hanging - drop preparation, for streptococcus 

 chains. The deposit should be selected for examination, and 

 several hanging-drop preparations made. A positive result 

 should be recorded only when quite definite chains of cocci 

 are detected, or, in doubtful cases, when stained preparations 

 show such definite chains. 



To isolate the streptococci, brush diluted loopfuls of the 

 positive tubes over plates containing nutrient agar. Incubate 

 for 24 hours, and if necessary for 2 days, at 37 C. Sub- 

 cultivate the colonies having the characters of streptococcus 

 colonies into broth or upon sloped agar in tubes containing 

 condensation water. In cases in which streptococci are likely 

 to be scanty, part of the centrifugalised deposit may be used 

 to inoculate the agar plates. 



The tests recommended to differentiate the streptococcus 

 strains isolated are the following : morphology, growth upon 

 sloped nutrient agar, growth in nutrient broth, growth upon 

 gelatine slope, action upon litmus milk, the production of acid 

 in lactose, saccharose, salicin, mannite, ram nose, and inulin. 



These sugar - alcohol media for the differentiation of 

 streptococci were introduced by Gordon, and may conveniently 

 be prepared as follows : 



A stock solution is made up containing lemco 10 grammes, 

 peptone 10 grammes, sodium bicarbonate 1 gramme, 10 per cent 

 aqueous litmus- solution 100 c.c., distilled water to 1 litre. This 



