166 BACTERIOLOGICAL EXAMINATION OF WATER. 



parent; if the reaction is alkaline, the growth acquires a 

 brownish colour. This coloured growth is never so marked as 

 that obtained when B. coli is grown on the same potato. 

 Some varieties of B. coli, however, produce a colourless growth 

 exactly like that of B. typhosus. The best way to employ 

 the test is to inoculate the same potato on one side with B. 

 tvphosus and on the other side with the microbe under inves- 

 tigation. 



(7) Proskauer and Capaldi's Media. These media are of the 

 greatest value as a means of diagnosis. No. I. medium has the 

 following composition : 



Asparagin ....... O20 per cent. 



Mannite . * . ... . . 0-20 



Sodium chloride .' . . .'..." . 0-02 



Magrnesium sulphate . ... . . 0*01 



Calcium chloride . . . 0'02 



Potassium mono-phosphate .... 0'20 



The reagents are dissolved in distilled water and sterilised 

 for an hour and a half in the steam steriliser. The medium, 

 which has a slightly acid reaction, is then carefully neutralised 

 with caustic soda. No. II. medium contains : 



Witte's peptone . . . . . 2 - per cent. 



Mannite .... . . . .01., 



The reagents, as in No. I. medium, are dissolved in distilled 

 water and sterilised in the steam steriliser for an hour and a half. 

 The medium will then be found to have an alkaline reaction ; 

 it must be carefully neutralised with citric acid. A solution of 

 litmus is then added until the media acquire a deep-red colour. 

 They are then sterilised again for half an hour, filtered and 

 placed in sterile test-tubes, which are plugged with cotton wool 

 and covered with a cap of parchment paper, which is fastened to 

 the tube by an india-rubber ring. 



If these media are inoculated with the B. typhosus and B. 

 coli respectively and incubated at 37 C. for twenty hours, the 

 following changes will be noticed : 



No. I. medium. No. II. medium. 



/No growth or change hO 

 J3. lyphosus-5 reac tion j Growth, reaction strongly acid. 



/"Growth, reaction neutral or 

 B. Coli . Growth, reaction acid x fahit] a]k;iljm ._ 



