48 THE BACTERIOLOGICAL EXAMINATION OF MILK 



steriliser for half an hour, filter again, and run into test tubes. 

 Sterilise in the autoclave at two atmospheres in order to ensure 

 destruction of the spores of bacillus subtilis. 



Choice of medium— 



This must be left very largely to individual experience and the 

 objects of the investigation. In a general way the constituents 

 of the various media described indicate the purposes to be 

 obtained. The general standard liquid media are bouillon and 

 milk, the solid media are gelatine (for room temperature cultiva- 

 tion) and agar (for blood heat). In tropical countries a combination 

 of the two may be used. Further, just as gelatine is a solid 

 bouillon, so gelatinised milk may be used when a solid milk 

 medium is required. For anaerobes glucose and formate media 

 are commonly used. There are, of course, various media used 

 for different species of organisms. For the streptothrix group, 

 including B. tuberculosis, glycerine media and potato are used. To 

 isolate the B. typhosus carbolised media and Eisner are taken. 

 Chromogenic bacteria nearly always grow well on potato. The use 

 o{ litmus milk, beer wort, wort gelatine, milk agar, etc., is sufficiently 

 designated in the names of the media. 



Preservation of media. — Media may be kept in good condition 

 for months if a few simple precautions are borne in mind. The 

 tubes or flasks containing the medium must be effectually sealed, 

 either with caps, corks, or paraffin. The store of media must then 

 be kept in a closed metal box, and in a cool dark place. 



Quantitative Examination of Milk and Method 

 OF Milk Dilution 



In the many experiments we have carried out with fresh milk 

 we have not found it necessary as a rule to carry dilution further^ 

 than 500 times. But in some cases it will be found necessary to ; 

 dilute up to ICXDO or even 5000 times if a satisfactory result is to be; 

 obtained. The procedure we have adopted for this dilution, though] 

 simple, is likely to become involved unless undertaken with care : 

 and proceeded with a step at a time. The method is as follows 



Apparatus required {for six samples) — 



1. Six ordinary i c.c. pipettes. 



2. ^ Six dropping pipettes specially calibrated to drop exactly] 

 twenty drops to every cubic centimetre. 



1 Very accurate calibration is required in the pipettes, and those used by usj 

 have been supplied by Ruelle of 10 Rue Chouin, Paris. 



