16 BULLETIlsr 782, V. S. DEPAETMENT OF AGRICULTURE. 



The results of the fermentation of dextrose in these simple synthetic 

 media were so different from the fermentation in the complex extract- 

 broth medium that the results are compared in Table 7. In the 

 dextrose-broth medium incubated for 7 days, 15 cultures showed a 

 production of at least 1 per cent normal acid and on this basis 10 

 cultures showed a dextrose fermentation in the same medium after 

 21 days' incubation, yet in the synthetic medium 44 of the 68 cultures 

 fermented dextrose. The results indicate very clearly the inac- 

 curacies which may occur in the study of the fermentation of test 

 substances in complex media. 



GALACTOSE. 



The alkali-forming bacteria fermented galactose in a manner nearly 

 identical with dextrose. Practically all the cultures which fermented 

 dextrose fermented galactose and, in fact, reached about the same 

 hydrogen-ion concentration in the synthetic medium. 



LACTOSE, SACCHAROSE, AND RAFFINOSE. 



Lactose is- not fermented in the ordinary beef-extract medium 

 when measured by the usual titration methods or by the change in 

 hydrogen-ion concentration. The assumption that the alkali-forming 

 bacteria are nonlactose fermenters based on results of the fermenta- 

 tion of the sugar in ordinary media is quite incorrect, as is shown in 

 Table 8. It was found that 11 of the cultures showed a slight change 

 in hydrogen-ion concentration in synthetic medium A. Medium A 

 and medium C were the same as previously described except that 

 lactose was substituted for dextrose. The slight fermentation of 

 lactose was more clearly indicated by the results obtained with 

 medium C. There is included also in the table the reaction of skim- 

 milk cultures of the organisms after 7, 14, and 30 days' incubation 

 at 30° C. In every case the acidity decreased during the first 7 days 

 and then increased. After 14 days the acidity in aU but 3 cultures 

 was higher than on the seventh day, while in 30 days aU the cultures 

 showed a marked increase in acidity compared with that of the 

 seventh day. 



The acid formation, which is probably secondary in milk, correlates 

 perfectly with the cultures which showed a slight fermentation of 

 lactose in the synthetic media. All the cultures which showed no 

 indication of fermentation of lactose in the sodium-ammonium-phos- 

 phate medium failed to show any secondary acid formation in milk 

 during a period of 30 days. 



Saccharose was found to be fermented by only 2 of the cultures, 

 Nos. 31 and 72. In a synthetic medium these change the reaction 

 from Pg 7.1 to Pg 6.5 and 6.6, respectively. This indicates only a 

 slight fermentation. 



Rafhnose was not fermented by any of the cultures even when 

 several of the different media were used. 



