THE PREPARATION OF CULTURE MEDIA 23 



Potato cultures used to be considered as capable of 

 assisting very materially in the differentiation of very 

 similar micro-organisms ; for example, in many text- 

 books the growth of the typhoid bacillus on potato is 

 given as one of the principal means of distinguishing 

 it from other closely allied forms. Eecent research 

 has shown, however, that for such purposes no reliance 

 can be placed on the potato test. 



Of great interest and importance in connection with 

 potato culture is a paper by Krannhals, 1 in which in- 

 vestigations are recorded on the growth of Koch'& 

 cholera bacillus on potatoes, showing that, contrary 

 to the accepted idea, this organism not only grows on 

 potatoes at 35 C., but that it will also develop at 

 from 15-35 C. on potatoes artificially rendered alkaline, 

 or which have spontaneously become alkaline, no 

 growths appearing on acid potatoes at all. Krannhals 

 found that some potatoes which when originally pre- 

 pared gave an acid reaction, on being tested subsequently 

 exhibited an alkaline reaction, and in this manner he 

 accounts for the impression that the cholera organism 

 will grow on acid potatoes, the change from acidity to 

 alkalinity in the latter having taken place unknown 

 to the investigator. Whether any particular kind of 

 potato is specially liable to exhibit this peculiar be- 

 haviour Krannhals is not able to say, but he is of 

 opinion that in all descriptions of growths on potatoes 

 it should be clearly stated whether the medium was 

 acid or alkaline, and that the potato should be tested 

 during the growth of the organism ; and, moreover, the 

 appearance of the latter compared with that of its growth 

 on alkalised potatoes. Slices of prepared potato may be 

 rendered alkaline either by saturating them with a 1 to 



1 ' Zur Kenntniss des, Wachsthums der Kommabacillen auf Kar- 

 toffeln,' Centralblatt fur Bakteriologie, vol. xiii. 1893, p, 33. 



