INTRODUCTORY AND GENERAL 35 



attempt at a scientific explanation of the facts of recovery from, 

 and subsequent immunity to, the infectious diseases. Pasteur was 

 a chemist, and was only led to the study of bacteriology by the 

 pursuit of chemical investigations into examining reactions which 

 he proved to be due to micro-organisms. His theory was a 

 chemical one. A certain amount of food is necessary for each 

 bacterium, and when the total amount contained in a given solu- 

 tion is used up the growth of the bacteria must cease. For 

 example, if we take a dilute solution of sugar (containing the 

 necessary salts, etc.), and inoculate it with yeast, the cells will 

 begin to divide and multiply with great rapidity. After a time 

 the growth ceases, and it will not be resumed if we inoculate the 

 fluid with an additional amount of yeast. We may compare the 

 test-tube to the patient, the yeast to the pathogenic organism, and 

 the process of fermentation to the disease, and we may say that 

 the fluid has recovered from the disease and is now immune to it. 

 This immunity depends upon the absence of sugar, which was 

 used up by the yeast cells, and if more sugar be added the process 

 of fermentation may be restarted by a fresh inoculation, or by the 

 yeast still remaining. 



The theory can easily be disproved, from the fact that bacteria 

 may grow well enough in the dead tissues and fluids of immune 

 animals ; and, secondly, because immunity, as we have seen, may 

 be produced (in some cases) by the injection of the chemical pro- 

 ducts of the bacteria, substances which can hardly use up food 

 materials. The theory has, however, been recently revived in a 

 modified form by Ehrlich, who considers that there is sufficient 

 evidence for the occurrence of this form of immunity in certain 

 cases. He calls it atreptic immunity. 



The retention hypothesis of Chauveau is the exact opposite of 

 Pasteur's. Several observers showed that the growth of micro- 

 organisms in fluid media might cease spontaneously whilst 

 abundant food material remained unutilized. This was found to 

 be due to the presence of certain products of metabolism, which, 

 like carbon dioxide in the case of animals, act as poisons to the 

 organism which produces them. For instance, the fermentation 

 of sugar by yeast is found to cease when about 14 per cent, of 

 alcohol is present, and if a strong solution be taken the process 

 will stop at this point, but can be started again if the alcohol be 

 removed by distillation. Here the fermentation is stopped by 

 alcohol, a product of metabolism of the yeast cell, which acts as a 



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