DIRTY MILK 79 



bacillus, the streptococci, etc., do not grow well upon 

 the ordinary media used for counting the number of 

 bacteria in milk. It requires special methods to de- 

 termine whether milk contains these specific micro- 

 organisms. 



The number of fermenting organisms in a sample of milk 

 may be estimated by planting progressively smaller quan- 

 tities in fermentation tubes containing glucose or other 

 fermentable sugars. A very good test is known as the " Wis- 

 consin curd test." This consists in coagulating the milk 

 with rennet and allowing the curd to stand hi a warm place 

 for several hours. If gas-producing organisms are present, 

 the curd when opened will be found to be full of gas bub- 

 bles like Swiss cheese. The presence of many gas-produc- 

 ing organisms usually indicates dirty conditions of stables, 

 cows, or containers. A few gas-producing organisms may 

 be found in most milks. 



By sowing the milk upon gelatine plates the relative 

 number of liquefying bacteria may be determined. 



By the use of Endos media or lactose litmus agar the 

 number of acid-producing bacteria may be determined. 

 Other special groups may be differentiated by special 

 technic. 



It takes six weeks to determine whether tubercle bacilli 

 are in milk; from three days to a week to determine whether 

 it contains typhoid bacilli; two to three days or more to de- 

 termine whether the streptococci are virulent; and several 

 days to a week to recognize diphtheria bacilli in milk. 

 These examinations, therefore, are not practical as routine 

 tests, for the milk would be consumed long before the 

 knowledge was available. 



The streptococci in milk deserve a special word. There 

 are very many different kinds of streptococci. A few strep- 

 tococci will be found in the sediments from most milks. 

 They are seldom found to any great extent by direct micro- 

 scopic examination of a clean product. Occasionally a 



