246 CLEAN MILK 



milk bacteriologist engaged in daily examinations must learn to 

 deduce for himself the more intricate inter-relations which arise- 



The determination of species is difficult and intricate work, to 

 be attempted on a large scale only by those who have much time and 

 energy to devote to it. The required tests are given on page 282.. 

 The careful working out of a culture isolated from a plate on the 

 various media, by the methods there outlined, and comparison of 

 the results with some text-book giving the various species in 

 groups for identification, is the only wholly satisfactory way. 



Streptococci may be recognized by the method outlined in. 

 standard methods of milk analysis.* Tubercle bacilli will not be 

 found in plates, being slow growers at best and plain nutrient agar 

 being a very poor medium for their growth. They may be found 

 by staining the cream or sediment of centrifugalized milk by the 

 Ziehl-Nielsen method, which shows them red on a blue ground. 

 Failing this, inoculations into guinea pigs from the same sources- 

 should be tried. 



Objections to the Numerical Standard. — I. That the sample- 

 examined can never be condemned in time to prevent use of the 

 milk by the consumer, since the results are not available for at least 

 24 hours, hence only the dairy which supplied that sample can be 

 beld responsible in a general way for putting it on the market. Of 

 course this objection applies to any system of analysis which takes 

 more than a few minutes — such as the mere testing of temperature 

 (as advocated in New York) or the microscopic count advocated 

 by Slack. 



It has been shown conclusively (Boston Board of Health Re- 

 ports) that the temperature test is absolutely fallacious in distin- 

 guishing high and low count milk, since it indicates only the tem- 

 perature at the moment of the test — not how long that temperature: 



* Very small white colonies, on agar plates after 24 hours, which yield diplococcii 

 in smears, but which, transferred to broth at 37° C, yield chains of cocci. 



