i88 MILK AND THE PUBLIC HEALTH ci.m-. 



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 in 

 at present known all such organisms indicate outside pollution 

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

 pollution. 



III. DETEJaiIXATrf)N OF THE NUMBEK OF StKEFTOOOCCI 



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. Or(hnary Ijroth will do, 

 but the neutral n/d broth is preferable and gives better results. 

 The tubes are incubated for two days at 'iT' G. 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. 



T(j isolate the streptococci, brush diluted loopfuls of tlie 

 positive tubes ovw' plates containing nutrient agar. Inculiate 

 for 24 hours, and if necessary for 2 days, at 37" 0. Hub- 

 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 arc lik('ly 

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

 to inoculate the agar plates. 



The tests recommended to differentiate the streptococcus 

 strains isolated arc the following : moi-phology, growth upon 

 sloped nutrient agar, growth in nuti'ient broth, growth vqion 

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

 in lactose, saccharose, salicin, mannite, raffinosc, and inulin. 



These sugar - ;dcohol media for the differentiation of 

 streptococci were introduced by Gordon, and may conveniently 

 be prepared as follows ; 



A stock solution is made up cuutaining lemco 10 giammes, 

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

 ii/|ueous litmus- solution 100 c.c., distilled water to 1 litre. 'I'his 



