Immunity in Unicellular Organisms 25 



It is really acquired as the result of a direct and gradual chemical 

 influence on the protoplasm of the Stentors which, once adapted, 

 all survive doses which are lethal for the unacclimatised control 

 organisms. 



The vegetable micro-organisms, which are much more easily 

 cultivated than are the Protozoa, frequently manifest most charac- 

 teristic phenomena of acclimatisation. The first systematic researches 

 in this direction were carried out by Kossiakoff l in the laboratory of 

 Duclaux. He studied the antiseptic action of borax, of boracic acid, 

 and of corrosive sublimate on the anthrax microbe and several other 

 bacilli (Bacillus subtilis, Thyrotrix scaber and T. tennis). He found 

 that all these micro-organisms can be gradually accustomed to doses 

 which are absolutely bactericidal to the same species when not so 

 acclimatised. The acclimatised Thyrotrix tennis withstands almost 

 double the amount of bichloride of mercury that the non-acclimatised 

 bacillus will resist. The ordinary anthrax bacillus will not develop at all 

 if the culture medium contains more than 0*005 of boracic acid whilst 

 the same organism, when accustomed by passage through successive 

 cultures in which this substance is present in gradually increasing 

 proportions, grows well in spite of the presence of 0'007 of the same 

 antiseptic. Since these observations were made similar facts have 

 been demonstrated by several other observers, and the ready accli- 

 matisation of Bacteria to poisons is now generally admitted. Danysz [28] 

 (loc. cit.\ with the object of elucidating the mechanism of this adapta- 

 tion, has studied the action of arsenic acid on the Bacillus anthracis. 

 He has demonstrated that this bacillus will gradually accustom 

 itself to grow in broth containing a quantity of arsenic acid which at 

 first inhibited all development. During this phenomenon of adapta- 

 tion, which is acquired after a series of passages through media more 

 and more highly arsenicated, the bacillus secretes a coating of mucous 

 substance which protects the sensitive parts of the microbial cell. 

 Here, therefore, is formed something exactly corresponding to what 

 the same observer has demonstrated in anthrax bacilli that have 

 acquired a tolerance for rat's serum. This analogy extends even 

 to the throwing out of the protective substance into the culture 

 fluid. When one sows an ordinary unadapted bacillus in arsenicated 

 broth to which has been added some of the fluid from a culture of 

 the adapted bacillus, development takes place in a marked fashion. 

 On the contrary when the same material is " seeded " into arsenicated 

 1 Ann. de VInst. Pasteur, Paris, 1887, 1. 1, p. 465. 



