230 LABORATORY EXERCISES IN BACTERIOLOGY. 



(b) Diastasic Ferments. A number of bacteria possess the power of converting 

 starch into glucose by means of an elaborated diastasic enzyme. 



Exercise 60. Inoculate several tubes of sugar-free bouillon (or peptone 

 solution) with Bacillus subtilis and incubate for eight or ten days. Prepare 

 a thin starch paste, adding two per cent, of thymol (which should be tested 

 for sugar previously, to insure the absence 'of the latter), and mix equal 

 parts of the culture and paste. Put aside for six or eight hours in the in- 

 cubator and then test with Fehling's solution for the presence of glucose. 



(c) Invertin Ferments. Such ferments are developed by a few bacteria, and accom- 

 plish the transformation of cane-sugar into glucose. 



Exercise 61 . Make a ten days old culture of Microspira comma in sugar- 

 free bouillon. Prepare a two per cent, solution of cane-sugar, adding two 

 per cent, of strong carbolic acid. Mix equal parts of the culture and sugar 

 solution. The carbolic acid should restrict the bacterial activities, but will 

 not prevent the action of the enzyme; and if the mixture be allowed to 

 stand for several hours and then tested with Fehling's solution, the glucose 

 reaction will be obtained. As a control, test in the- same manner the cane- 

 sugar and carbolic acid solution, noting the failure of the copper reduction. 



(d) Rennet Ferments. The phenomenon of milk coagulation may be dependent 

 upon the lactic acid produced from the milk-sugar in a culture of a given organism, 

 or to an enzyme. To demonstrate the latter a culture of some bacterium incapable 

 of such acid formation is to be selected, as Bacillus prodigiosus. 



Exercise 62. A culture of Bacillus prodigiosus upon milk is prepared 

 and placed in the incubator for twenty-four hours, when without change in 

 color the milk will be found solidly coagulated. What has been the influ- 

 ence as to reaction of the medium? 



Or, a sugar-free bouillon culture of the organism is prepared (ten days 

 old) and filtered through a porcelain filter, equal parts of the filtrate and of 

 a milk tube mixed and placed in incubator temperature for a few hours, 

 when the same result is reached. 



Plant in milk from known cultures of Micrococcus pyogenes, Bacillus 

 typhosus, Bacillus coli, Mycobacterium diphtheria, and Microspira comma. 

 Note results in each case. 



A number of other types of fermentation are commonly met with, as the alcoholic 

 fermentation of sugar, that of the production of acids, as lactic acid or acetic acid 

 from sugar and alcohol; the production of alkaline substances, as ammonia, from 

 proteids free from sugar (urea-fermentation) by various bacteria. It is claimed that 

 free acid is formed only in sugar-containing media; and upon the supposition of the 

 early splitting of the small amount of meat-sugar in ordinary media or in media to 



