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absent when there is intimate contact between the two species 

 of bacteria, there might possibly be cases where the converse holds 

 and that the symbiosis test should therefore be carried out not only 

 with the separated cultures but also with mixed ones. M. Neisser's 

 cultivation of Pfeiffer's bacillus in mixed culture — even in mixed 

 colonies — on ordinary agar might seem to favour this view. 

 Neisser regarded this as a symbiosis in the more restricted sense, 

 as he could not produce any increased growth of Pfeiffer's bacillus 

 with dead xerosis bacilli. It may be pointed out that according to 

 my experience the increased growth is produced, at any rate when 

 a medium which is suitable is used, by a very diffusible sub- 

 stance, and as far as we can see, the distance at which the effect 

 can be delected depends only upon the amount of „V" formed. 

 Vigorously acting microbes can induce increased growth of Pfeif- 

 fer's bacillus at a rather (though not much) greater distance than 

 weakly acting ones, suggesting that the growth-promoting substance 

 is always the same qualitatively but is manufactured in different 

 amount. But if, in certain cases, „symbiosis" could only be brought 

 about by immediate contact or at a very short distance between 

 the two species of bacteria, it would then be a case of the action 

 of substances with a very slight power of diffusion and would 

 therefore be a phenomenon of quite a different kind from the 

 usual one, and it would only create confusion to associate the two 

 phenomena. 11 must also be remembered that when the two species 

 of bacteria are brought so near to one another that competition 

 comes into play their growth will be determined by the interplay 

 between different factors which can only be controlled and estimated 

 very inadequately. The characteristic of the „sym bio- 

 sis" of Pfeiffer's bacillus is just the marked di- 

 stance effect". 



Besides the necessity of inoculating Pfeiffer's bacillus and 

 the air coccus at a suitable distance from 1 one another, it is 

 of importance that the former be inoculated in streaks and 

 the latter at points between them. We thus ensure that 

 all the different distances between the air coccus and Pfeiffer's 

 bacillus which have any value, will be represented and in 

 such a way that the proximate portion of the Pfeiffer's bacillus 

 culture does not prevent the effect at a greater distance. Fur- 

 ther, those parts of the Pfeiffer's bacillus streaks which are 

 outside the zone of activity of the air coccus, will show the 

 nature of the growth out of reach of the symbiosis effect. 



With a view to accuracy in judging the phenomenon it 

 has been found to be convenient to inoculate Pfeiffer's ba- 

 cillus in two streaks, and not be content with only one. 



