Formates by B. coli communis. 



463 



Experimental. 



Tltr Examination of the Behaviour of Non-gas-producing Organisms towards 

 Formates as a means of Deciding whether the Organism has been Derived 

 from an Original Gas-producing Strain.' 



It has been mentioned above that by artificial selection of B. coli comvmnis 

 it is possible to obtain strains which do not produce gas from glucose, 

 and that this phenomenon consists in part, in some cases, in a lessened 

 power to decompose formic acid possessed by the selected organism. 

 In the case of the strains examined by Penfold and Harden (1912) the 

 power of decomposing formic acid was in all cases retained by the 

 selected strains, and certain strains examined in the course of this work 

 were found likewise to have retained this power. In the case of one strain, 

 however, the power to decompose formic acid had been entirely lost. It 

 may, therefore, be considered as probable that the strain incapable of 

 decomposing formic acid represents a more advanced stage in the process 

 of selection, and that, therefore, this type would be more permanent in 

 character. Such indeed has proved to be the case, for while the strain 

 which retains the power to decompose formic acid tends to revert in its 

 properties to the parent organism as regards the production of gas from 

 glucose, the other strain, which cannot decompose formic acid, shows no 

 such tendency, although it has been frequently sub-cultured during the course 

 of seven months. 



In view of the fact that the more permanent non-gas-producing type of 

 artificially selected strain is unable to decompose formic acid, it may be 

 suggested that the same phenomenon might be exhibited by naturally 

 occurring non-gas-producing organisms, and that in order to decide whether 

 a strain which, at any particular time, does not produce gas has been 

 recently derived from a gas-producing strain, an examination of its behaviour 

 towards formic acid might be of crucial importance. 



It frequently happens that organisms isolated from natural sources differ 

 apparently only as regards the power to produce gas from carbohydrates and 

 allied substances, and the question arises as to whether the one organism 

 may have recently been derived from the other. Arkwright (1913), for 

 example, has obtained varieties of B. acidi lactici differing in the aforesaid 

 respect, both strains occurring in the same sample of urine, and he was 

 also able to show that in certain cases the non-gas-producing strain could 

 be trained to decompose sodium formate if grown for some time on a peptone 

 water medium containing this salt. The writer has found that the power to 

 produce gas from mannitol may, in some instances, be made to disappear by 



