131 



dreds of strains and have yet to find an exception". My finding 

 of Pfeiffer's bacilli which did not give the symbiosis reaction 

 is not however in agreement with Davis' experience. These 

 strains cannot be excluded from the Pfeiffer's bacillus group, 

 firstly because all possible transitions from marked to absent 

 symbiosis exist; secondly, because the same strain may give a 

 different result on a repetition of the test; and thirdly, because 

 the microscopic and macroscopic morphology and mode of 

 growth on different media of various strains decidedly creates 

 the impression that they belong to the same group. In the 

 following investigations of the remaining characters of Pfeif- 

 fer's bacillus, — included in which there are strains giving 

 marked, as well as weak, symbiosis, — it will also be shown 

 that no dividing line can be drawn between the first and 

 the last. 



Nevertheless the symbiosis phenomenon is a very valuable 

 recognition mark of Pfeiffer's bacillus, when it is present; 

 and it was found to be very pronounced in at least 95<>/o of 

 the 800 or so, strains I investigated. The worth of the test 

 as a means of distinction from other species of bacteria 

 (whooping-cough bacillus, Bacillus haemoglobinophilus canis) 

 will be discussed in connection with the other biological cha- 

 racters of these bacterial species. 



It will now be interesting to investigate the growth-pro- 

 moting power of different species of bacteria, and whether 

 any connection can be established between it and the other 

 biological characters of the different species. 



A comparison of the symbiosis test was made (Sept. 1920) between 

 the bacteria mentioned below (the majority were old laboratory 

 strains), and the Pfeiffer's bacillus strain „I 6". Cultures were made 

 at the same time on blood agar and on plates of the medium used 

 in the test itself (ordinary agar with a little haemolysed blood) the 

 latter containing a very sensitive indicator (phenol red — china 

 blue, see p. 162). The blood agar plates were observed for 21/2 

 days. If there was no clear area around the colonies the strain was 

 considered to be non-haemolytic („0"). Haemolysis which could not 

 be observed after IV2 days' incubation was called „weak", even if 

 very clear areas subsequently appeared. In the next column the 



9* 



