122 BACTEKIOLOGICAL AND ENZYME CHEMISTRY 



By means of further observations, using starch gelatine as 

 a cultivation medium, it can be shown that various micro- 

 organisms are capable of secreting amylase. Thus, e.g., an 

 ordinary Petri dish may be taken and a thin layer of melted 

 starch gelatine poured into it and allowed to set. A few drops 

 of ordinary sewage diluted ten times with water can then be 

 run over the surface of the jelly, any excess being poured off ; 

 at the end of twenty-four hours a number of colonies will 

 probably have appeared. On pouring a dilute solution of 

 iodine on and off the plate, a number of colonies will be found to 

 be surrounded with white rings, showing that the starch has 

 been saccharified in their immediate neighbourhood, i.e., that 

 the particular organism forming the colony has the power 

 of secreting amylase. It is possible, of course, to take out 

 such colonies with a sterile platinum wire and prepare streak 

 cultures in starch gelatine tubes. 



In order to be sure that the white ring observable on addi- 

 tion of iodine is not simply due to the production of alkalinity 

 in the medium at that point, the plate may be treated with 

 dilute hydrochloric acid prior to the addition of iodine, but 

 in this case there is danger that the colonies may be sterilised. 



Among the bacteria which produce amylase Koch's cholera 

 bacillus may be mentioned, also B. anihracis, B. megatherium, 

 and B. lactis aerogenes, which is a characteristic sewage 

 organism. B. coli communis does not, however, secrete amy- 

 lase ; in fact, this organism can be used as an elegant test for 

 the production of sugar by an amylase-secreting organism, 

 such as the bacillus of cholera or anthrax, by growing the 

 latter in starch gelatine and then incubating with B. coli, when 

 the characteristic gas formation due to the fermentation of 

 sugar by this organism will be noted. 



That the saccharification of the starch is really due to the 

 formation of amylase by the organism, and that it is not due 

 simply to its ordinary developmental activity, may be proved 

 by taking a little of the converted starch gelatine, melting 



