10 MAX LEVINE 



variability is indicated by the much larger standard deviation and 

 coefficient of variability. MacConkey types, therefore, cannot be dif- 

 ferentiated on the basis of quantitative acid-production in galactose 

 as indicated by titration with phenolphthalein. 



There does not seem to be any correlation between the amount of 

 acid formed from galactose and the source of the organisms. One 

 organism from the cow formed less than 0.4% acid. It was omitted 

 in calculating acid-production by the group. If included, the mean for 

 the cow strains becomes 1.30%, with a coefficient of variability of 

 19.2%. 



The Voges-Proskauer-positive strains form somewhat less acid 

 (1.21%) than do the Voges-Proskauer-negative strains (1.36%). The 

 difference (0.15% normal) is slight, but it is greater than the differ- 

 ences observed with the MacConkey types or with the strains from 

 different sources. The difference is of some interest, moreover; for, 

 as will appear later, whereas the Voges-Proskauer-positive strains 

 form less acid from the monosaccharids than do the Voges-Proskauer- 

 negative strains, the reverse is true when more complex substances 

 (except lactose) are fermented. 



C. MANNITOL 



The hexite, mannitol, is attacked about as readily as galactose. 

 The average amount of acid formed by all strains is 1.32%, with a 

 sharp mode at 1.30% normal. (Acid-production was not determined 

 in 2 cultures, 1 from horse and 1 from man.) The frequency dis- 

 tributions and the relation of the Voges-Proskauer reaction, the 

 source, and the MacConkey types to the amount of acid formed from 

 mannitol are shown in Table 3. 



The mode for each of the MacConkey types is at 1.30%, and the 

 means are 1.32, 1.30, 1.33, and 1.31% normal acid respectively. The 

 MacConkey types are therefore indistinguishable on the basis of quan- 

 titative acid-production from mannitol. 



A comparison of the amounts of acid formed by Voges-Proskauer- 

 negative and Voges-Proskauer-positive strains indicates that the latter 

 attack mannitol somewhat more readily, but the difference is not 

 appreciable. 



Except for the horse strains, which have a mode at 1.50%, the 

 organisms from all the other sources group themselves around 1.30% 

 normal acid as a mode. In general, the differences observed between 

 the means are too slight to be of any significance. The average of the 



