CH. V] VARIATIONS IN FERMENTING POWER 55 



2. The influence of oxygen. Torrey (1905) found that 

 the power of a certain dysentery bacillus to ferment maltose 

 was augmented by alternate aerobic and anaerobic culture. 



Wilson describes an atypical B. typhosus which slowly 

 fermented lactose in a litmus-broth tube but produced 

 fermentation in 3 days when the same medium was poured 

 into a Petri dish. 



Andre wes and Horder (1906) mention a streptococcal 

 strain which failed to ferment lactose under ordinary con- 

 ditions but did so readily when grown anaerobically. 



3. The influence of atmospheric pressure. Harden (1901) 

 has shown that the amount of formic acid produced by B. 

 coli from glucose, at the ordinary pressure of the atmosphere, 

 is very small. Under greater pressure the yield of acid is 

 increased, while at the same time the amount of gas formed 

 is diminished. In other words, the final stage in the process 

 of fermentation, which consists in the conversion of the acid 

 into various gases, is inhibited. 



4. The age of the culture. Older cultures of B. diphtheriae 

 usually ferment both glycerine and lactose ; a young culture 

 of the same organism can attack neither of these substances 

 (Muir and Ritchie). 



6. The age of the medium. The streptococcus pyogenes 

 normally does not ferment saccharose, raflBnose or salicin, 

 but if old media be used this organism will ferment both the 

 first two substances, and even the last named in the course of 

 a week (Martin). On the other hand B. diphtheriae which 

 gives its characteristic " sugar reactions " on fresh beef serum, 

 fails to do so if this medium is old (Fisher, 1909). 



6. The composition of the medium. The addition of 

 carbolic acid in small quantities to the media' used destroys 

 the natural fermenting properties of many bacteria (Penfold, 

 1911 b). 



The presence of sodium benzoate inhibits the power of 

 B. coli to produce gas from dextrose, one of the most stable 

 and fundamental difiFerences separating the coli from the 

 typhoid-dysentery group (Herter, 1909). Penfold (1911 c) 

 found that many intestinal organisms (B. coli, B. enteritidis 



