226 MILK AND THE PUBLIC HEALTH CHAP. 



The writer, 1 examining milk samples obtained direct from 

 individual cows, found in a number of them bacilli which 

 morphologically, particularly when stained by methylene 

 blue, closely resembled diphtheria bacilli. They showed bead- 

 ing and granules, and frequently were distinctly clubbed. 

 They were non-pathogenic to mice. They grew upon blood 

 serum as minute white or yellow-white colonies, and differed 

 from B. diphtheriae in this and other cultural characters. 



III. ISOLATION OF B. TYPHOSUS FROM MILK 



The detection and isolation of this organism from milk is 

 still a matter of considerable difficulty, but by the use of 

 certain special media its isolation has been greatly facilitated. 

 The milk is thoroughly centrifugalised, the supernatant fluid 

 decanted, and the sediment mixed with a little sterile water. 

 Fractions of the emulsion so made are distributed over the 

 selected media, contained in a series of large Petri- dishes. 

 There are now a number of special media which have been 

 recommended for the purpose, and individual workers often 

 obtain the best results by using those with which they are 

 specially acquainted. Of these media the following may be men- 

 tioned : Drigalski-Conradi agar, lactose bile-salt neutral red agar 

 (L.B.A.), fuchsin agar, brilliant green agar, and malachite green 

 agar. As far as the writer has experimentally investigated 

 them he prefers L.B.A., but fuchsin agar is also useful. The 

 composition and preparation of L.B.A. is given on page 181. 



After the plates are inoculated they should be thoroughly 

 dried, uncovered, in the blood-heat incubator. They are 

 then covered, inverted, and incubated. After 18 to 24 hours' 

 growth the plates are examined. B. coli and other lactose 

 fermenters grow as red colonies, and frequently produce a haze 

 in the surrounding medium. B. typhosus and other non-lactose 

 fermenters produce white colonies. The number of white 

 colonies can be considerably reduced by the addition of 

 saccharose, dulcite, and salicin (in 1 per cent solution), as 

 well as lactose, to the medium. By their addition fermenters 

 of these substances, although they may not ferment lactose, 

 are also excluded, since their colonies will now be red. 



1 Report of the Medical Officer, Local Government Board, 1906-7, p. 205. 



