CH. Ill] A CONSIDERATION OF THE EVIDENCE 35 



of free acid is really formed. Clark (1910) states thatHofmann's 

 bacillus produces slight but definite acidity in dextrose broth 

 if phenol-phthalein be used as an indicator, whereas if litmus 

 is used the reaction always appears alkaline. 



(6) The age of the culture is also a factor of importance. 

 MacConkey (1909) finds that the indol reaction in the case 

 of B. coli is not given by a 2 or 3 days' culture ; the latter 

 should be nearly a week old in order to give a positive result. 

 A young culture of B. diphtheriae is unable to ferment 

 glycerine and lactose though an older culture will usually do 

 so (Muir and Ritchie). An old culture of cholera will not 

 liquefy gelatin (Wood, 1889). Graham Smith (1906) has 

 pointed out that many strains of diphtheria bacilli do not 

 grow well in broth when first isolated from the throat and 

 therefore do not produce acidity at once. 



(c) The time allowcmce. In the case of many sugar 

 fermenters an incubation period of 48 or even 72 hours is 

 required before acidity becomes apparent, and in the case of 

 other organisms a similar " latent period " may elapse before 

 the appearance of pigment. 



Still longer observation is sometimes necessary. Klotz 

 (1906) describes a coliform organism which did not produce 

 indol until the 20th day. Petrusky (1889-90) showed that in 

 the case of B. typhosus a certain slow fermentation of lactose 

 does take place in litmus whey although the organism is 

 regarded as a non-fermenter of lactose. Penfold (1910 b) 

 states that the same organism ferments dulcite — a property 

 usually denied to it — ^if the experiment is prolonged for 2 or 

 3 weeks. Bahr (1912) describes a dysentery bacillus of the 

 "Flexner" type which, after 4 days incubation, only fer- 

 mented mannite but fermented maltose and saccharose also, 

 after 15 days' incubation. Wilson (1910) states that he has fre- 

 quently isolated bacilli from the intestine which required from 

 9 to 21 days to produce acidity in lactose litmus broth and 

 several more days to produce gas. 



In other cases the change in the reaction is reversed after 

 an interval. Thus, Bahr mentions another "Flexner" bacillus 

 which produced a feeble acid reaction in mannite at the end 



3—2 



