224 Milk Inspection. 



lished by incubation, since the milk may contain from the feed, etc., 

 only harmless, acid fast rods. 



The inoculation is made into guinea pigs by injecting them 

 either subcutaneously or intramuscularly in the hind leg with either 

 1 c. c. of the full milk, or better, with the centrifugal sediment mixed 

 with a small amount of the skimmed milk with or without cream. 

 The sediment should be obtained by using rapidly revolving elec- 

 tric centrifuges. 



If the samples have to be transported long distances, they 

 should be mixed before transportation, and if possible immediately 

 after drawing the milk, with 1:2000 to 1:3000 of formalin (boric 

 acid 1 :50 or 1 :100 is also satisfactory). The tubercle bacilli wliich 

 are protected by a waxy capsule from the effects of the preserving- 

 agents, are not harmed by such preservation to such an extent that 

 they could no longer be demonstrated by inoculations. 



For each milk injection at least two guinea pigs should be 

 used, since occasionally the inoculated animals die as a result of 

 some other intercurrent disease. 



In the presence of tubercle bacilli the regional lymph glands 

 swell after several days or a few weeks. Such animals, 

 if they do not die before, should be killed on the appearance of 

 these swellings, and they as well as those which died should be 

 examined for the presence of tuberculosis. The surviving guinea 

 pigs should be kept under observation for several months. 



In examining entire herds, it is advisable to group the cows ; 

 for instance, five animals may form one group and the mixed milk 

 of this group should be separately inoculated. 



The counting of bacteria is carried out either by the ordinary 

 method of plating which is made with certain dilutions of milk on 

 agar or gelatin or also by direct counting in smears which should 

 be prepared according to Olav Skar. 



The method consists in mixing in a reagent glass 4/10 c. c. of a 

 2% solution of carbol-methylene blue (for animal cells and bac- 

 teria) and 3.5 c. c. carbol-methylene blue, with 0.5 c. c. of a 3% 

 sodium hydrate solution (for bacteria alone). Then 10 c. c. of milk 

 is added to the stain with a pipette and heated for about 10 minutes 

 at 70 C. Of the mixture, 1/50 of a c. c. is uniformly smeared 

 upon a certain sized field (24 X 20 m. m.) of a special slide, and 

 dried in the air. Without any further fixation or other treatment, a 

 number of fields in the smear are counted in their entire length and 

 width, and with the aid of the ocular micrometer the number of 

 bacteria in the counted fields is calculated to the c. c. of milk, ac- 

 cording to the standard given below. When chains and clumps are 

 encountered each bacillus must be counted. The ocular micrometer 

 of Zeiss in Jena as applied by Skar has a determined field capacity 

 so that one bacterium, with the above technique and with a certain 

 tube length of the microscope, viewed with a 1/12 oil immersion 



