844 Mutation in Micro- Organisms 



and carefully studied in cultures, in which their behaviour was similar 

 to that observed in the bladder. They fermented glucose, lactose, and 

 saccharose with the production of much gas. After about two years, 

 the patient recovered spontaneously from the pneumaturia, but the 

 bacteria were still present in the bladder. Both here and in the 

 cultures, however, they were found to have lost completely the power 

 of forming gas by the fermentation of sugars. The cultures were kept 

 for a long time subsequently, and repeatedly tested to see whether they 

 would regain their gas-forming power. After about a year, these 

 organisms suddenly reacquired the power of forming gas from lactose 

 and glucose ; and shortly after, the patient began to suffer once more 

 from pneumaturia. Examination of the bacteria from the bladder, 

 showed that they had — like the organisms in the cultures — reacquired 

 their gas-forming powers 



The behaviour of the organisms in the patient was therefore closely 

 parallel to their behaviour in the artificial cultures. Moreover, the 

 experiments seem to shew that the same bacteria were present through- 

 out — that is, it was not a case of mixed cultures or a reinfection. 

 It seems justifiable, therefore, to conclude that in this case at least 

 a physiological character was lost and reacquired by the bacilli not only 

 in artificial cultures, but also under *' natural " conditions in the living 

 organism. 



B. Morphological Mutations. 



Concerning structural variation in bacteria very little indeed is 

 known. This is not because structural differences between members 

 of the same species are uncommon, but because the normal structure 

 and life-history of nearly every species is still largely a matter of 

 conjecture. Many species are probably polymorphic, the various forms 

 depending partly upon the particular stage which has been reached in 

 the specific life-cycle and partly upon external conditions. Many 

 bacteria, moreover, continue to exist and multiply after they have 

 assumed a degenerate or abnormal form (so-called involution forms). 

 The modifications dependent upon these various factors are not 

 variations in the ordinary sense of the word. One would be equally 

 justified in classifying a spermatozoon, a foetus, a leper, and a man 

 with both his legs cut off, as structural variations of the human species. 



^ Both the organisms in the cultures and those in the bladder had also acquired the 

 new power of clotting milk. 



