CH. II] THE CHARACTERS OF BACTERIA 21 



Washburn, 1906) and a non-virulent bacillus of Blackleg 

 rendered virulent by the addition of lactic acid {ibid.). In- 

 creased resistance to antiseptics may be developed by pro- 

 longed exposure to their action (Rettger and Sherrick, 1911 : 

 Penfold, 1911 c). 



Many other examples are given in later pages. 



9. Unusual fermenting properties on the part of bacteria 

 are frequently acquired after prolonged growth on special 

 sugar or peptone containing media. The length of time 

 required varies in difterent cases, within wide limits. For 

 example, B. typhosus can be "trained" to ferment dulcite in 

 less than two weeks (Penfold, 1910 a) but cannot be trained 

 to ferment lactose in less than two years (ibid.). The method 

 is not invariably successful, but this may be due to the fact 

 that the trial in many cases is not sufficiently prolonged. 



Unusual proteolytic powers may be developed in a strain 

 of organisms by an analogous process. Miss Peckham (1897) 

 induced the power to form indol on the part of many strains 

 ofB. typhosus by cultivating them in a medium rich in peptone. 



10. By artificial selection, through a number of genera- 

 tions, it is often possible to develop variation in a particular 

 direction. 



Goodman (1908) by this method obtained from a strain of 

 B. diphtheriae with moderate power of producing acid in 

 dextrose broth, two strains possessing respectively a greatly 

 augmented and a greatly diminished power of acid production. 



Rettger and Sherrick (1911) in the same manner obta,ined 

 from a slightly pigmented strain of B. prodigiosus two strains 

 showing in one case brilliant colouration and in the other 

 complete absence of colour. They also obtained by selection 

 a strain of staphylococcus aureus unusually resistant to the 

 action of corrosive sublimate. 



Conn (quoted Glenn, 1911) obtained by the same method 

 strains of a micrococcus with high and low powers of liquefy- 

 ing gelatin. 



This method, however, may prove unsuccessful. Rettger 

 and Sherrick failed to modify pigment production in B. ruber 

 balticus and they quote Buchanan and Traux as having been 



