THE PARTS PLAYED BY BACTERIA IN NATURE 21 



repelled by the fluid in the tube. The effect of a given fluid 

 differs in different organisms, and a fluid chemiotactic for one 

 organism may not act on another. Degree of concentration is 

 important, but the nature of the fluid is more so. Of inorganic 

 bodies salts of potassium are the most powerfully attracting 

 bodies, and in comparing organic bodies the important factor 

 is the molecular constitution. These observations have been 

 confirmed by Ali-Cohen, who found that while the vibrio of 

 cholera and the typhoid bacillus were scarcely attracted by 

 chloride of potassium, they were powerfully influenced by 

 potato juice. Further, the filtered products of the growth of 

 many bacteria have been found to have powerful chemiotactic 

 properties. It is evident that all these observations have a 

 most important bearing on the action of bacteria, though we 

 do not yet know their true significance. Corresponding chemio- 

 tactic phenomena are shown also by certain animal cells, e.g. 

 leucocytes, to which reference is made below. 



The Parts played by Bacteria in Nature. As has been said, 

 tlic chief effect of bacterial action in nature is to break up into 

 more simple combinations the. complex molecules of the organic 

 substances which form the bodies of plants and animals, or 

 which are. excreted by them. In some cases we know some of 

 the stages of disintegration, but in most cases we know only 

 general principles and sometimes only results. In the case of 

 milk, for instance, we know that lastjc acid is produced from 

 t,frfi lap.f-Qfip, by the action of the bacillus acidi lactici and of 

 other bacteria, and that from urea ammonium carbonate is 

 produced by the micrococcus ureae. That the very complicated 

 process of putrefaction is due to bacteria is absolutely proved, 

 for any organic substance . can be preserved indefinitely from 

 ordinary putrefaction by the adoption of some method of 

 killing all bacteria present in it, as will be afterwards described. 

 This statement, however, does not exclude the fact that 

 molecular changes take place spontaneously in the passing of 

 tlit- organic body from life to death. Many processes not 

 usually referred to as putrefactive are also bacterial in their 

 origin. The souring of milk, already referred to, the becoming 

 rancid of butter, the ripening of cream and of cheese, are all 

 due to bacteria. 



A certain comparatively small number of bacteria have been 

 I -loved to be the causal agents in some disease processes 

 occurring in man, animals, and plants. This means that the 

 fluids and tissues of living bodies are, under certain circum- 

 >tauces, a suitable pabulum for the bacteria involved. The 



