452 REPORT — 1893. 



imparted to bacteria by more purely artificial means, e.g.,hj subjecting- 

 them to chemical treatment, nor has it yet been found possible to convert 

 a perfectly harmless micro-organism into a pathogenic one. 



Instances of artificially induced changes of bacterial function, other 

 than pathogeneity, are far more easy to study with accuracy and precision 

 as the complicating differences of animal organisation are eliminated. 

 Thus, whilst any two animals selected for experiment must necessarily 

 differ in more or less important respects, any number of test-tubes con- 

 taining a culture medium of precisely the same composition can be 

 prepared. 



Changes of Function. — Everyone who has cultivated bacteria over long 

 periods of time will probably have noticed more or less conspicuous 

 changes in some of their functional activities, e.g., that the power of 

 liqtiefying gelatin, possessed by some, has become diminished, or that 

 the power of producing pigments has become impaired, or perhaps has. 

 actually disappeared altogether. Or, again, it may frequently be observed 

 that an organism which had originally the power of fermenting some 

 particular substance has lost this power through prolonged culture, and, 

 indeed, even a single passage through gelatin may sometimes apparently 

 destroy the capacity to exercise this function. Thus, I have in my 

 possession a bacillus which has the power of fermenting calcium citrate, 

 and this function it continues to exercise for years if grown in suitable 

 media. On submitting such a fermentins solution of calcium citrate to 

 plate cultivation, colonies make their appearance in due course ; but on 

 transferring one of the colonies to a sterile solution of calcium citrate it 

 invariably fails to set up a fermentation, the bacillus having by mere 

 passage through the gelatin-medium lost its fermenting power. If, 

 however, a similar colony be put into broth containing calcium citrate 

 the latter is readily fermented ; on now inoculating from this to a weaker 

 broth containing calcium citrate this also is put into fermentation, and 

 by successively passing in this manner to weaker and weaker broths con- 

 taining calcium citrate we may ultimately set up fermentation in a 

 calcium citrate solution which was absolutely unfermentable when the 

 bacilli were taken directly from the gelatin plate (' Micro-organisms in 

 their Relation to Chemical Change,' Royal Institution, 1892). 



A striking example of permanent loss of function is described by 

 Laurent (' Ann. de I'lnst. Pasteur,' iv. [1890], p. 465) in the case of the 

 Bacillus ruber of Kiel ; an organism which, as its name implies, pro- 

 duces a red pigment. Laurent found that if cultures of this bacillus were 

 exposed to bright sunlight for a period of three hours the subsequent 

 cultures were almost invariably colourless, and so permanent was this, 

 loss of pigment-producing power that thirty-two successive cultures, 

 carried on over a period of a year, failed to restore it. 



If such numerous bacterial varieties can be artificially induced in the 

 laboratory, it is surely highly probable, in fact all but certain, that 

 similar modifications have been, and are still, continually arising amongst 

 the bacteria growing amidst natural surroundings This anticipation is 

 fully borne out by the direct examination of the bacterial forms occurring 

 in nature. It is a most striking and significant fact that in the case of 

 almost any micro-organism which has received special attention on 

 account of some particular property which it possesses, e.g., pathogenic 

 power, a careful examination of the natural habitat of such an organism 

 has almost invariably led to the discovery of one and often many other 



